193
ROBERT T. HILL AND T. WAYLAND VAUGHAN.
18 GEOL, PT 2-----13
194
195
CONTENTS
196
197
ILLUSTRATIONS
198
199
GEOLOGY OF THE EDWARDS PLATEAU AND 11,10 GRANDE PLAIN ADJACENT TO AUSTIN AND SAN ANTONIO, TEXAS. By ROBERT T. HILL and T. WAYLAND VAUGHAN.
INTRODUCTION
The artesian systems of the eastern half of Texas are numerous and underlie several large areas. The productive areas extend from near Red River,Denton County,to Del Rio,on the Rio Grande, and from near the center of the State to the coastal islands of the Gulf. Collectively they comprise a district 450 miles in length and averaging 300 miles in width. It is doubtful if there has been anywhere a more remarkable development of artesian wells in the last tell years than in this region. At numerous places copious flows of water have been obtained, and districts which a few years ago had only inadequate supplies now possess artesian wells furnishing water in large quantities to cities,ranches, and farms,improving the hygienic conditions, and yielding water for the stock raising,agricultural,manufacturing, and transportation industries
The artesian wells of the eastern half of Texas belong to several distinct systems, the terns "system" including all wells having their source in the same set of rock sheets or strata. It would require a large volume to describe each of these various artesian systems. In the Cretaceous formations alone there are no fewer than five, and two of these the Travis Peak, or Waco, and the Edwards receive consideration in this paper. It is especially proposed to explain as well as possible the principles of the supply of the artesian belt supplied by the Edwards system and the probability of success or failure of wells sunk in different parts of its area.
Within the last two decades numerous artesian wells have been drilled in the vicinity of San Antonio, Texas, and for many years the source of the water has been a matter of perplexity. Unfortunately it has not been within our power to secure accurate logs of these wells, accompanied by specimens, or even to ascertain with accuracy the number and location of the wells, or to collect other essential data of a statistical "
The terms "Travis Peak" and "Edwards" are formation names used in this paper in place of "Trinity Sands" and "Caprina limestone," hitherto employed. The reasons for the employment of these new names are given on later pages (pp. 216, 227).
200
nature, such as the Government has deemed expedient to collect in other artesian regions. Such facts as are herein presented have been collected incidentally by the writers while making private reconnaissances or official geologic surveys in the region, and through correspondence with citizens. Accordingly, this report is not to be considered as a statistical or engineering paper, but rather one which deals with the geologic side of the underground water question.
The source of the San Antonio water supply has been ascertained by detailed studies of the structure and outcrop of the water bearing beds in the adjacent regions. A detailed map of the Austin quadrangle was made, and the thickness, sequence, paleontology, mineral composition, and water capacity of every portion of the Cretaceous section along the Colorado River were ascertained. A reconnaissance was next made southwestward from Austin via Fredericksburg, Kerrville, and the headwaters of the Frio and Nueces to Fort Clark, Texas, for the purpose of studying minutely the variation of the rock sheets outcropping in that direction, which, owing to the direction of their dip, must necessarily underlie the city of San Antonio. At the southeastern end of this line detailed studies and mapping were resumed upon the Nueces, Brackett, and Uvalde quadrangles. By this method a base line of geologic sections, so to speak, was established along the strike of the outcrop of the Lower Cretaceous formations, the sections at the extremities of which were determined with the greatest accuracy possible, while check sections have been made at numerous intervening points along the line.
The result of this work was the discovery that, while these well waters come from the same series of beds that supply the artesian wells of the Waco, Fort Worth, and Dallas regions north of the Colorado, their occurrence presents some important differences of detail. Instead of having their immediate source in beds of porous sands, like the wells about Waco, they are derived largely from the Edwards limestone, hitherto supposed to be one of the most impervious formations of the whole Cretaceous section.
It became apparent that this hitherto unappreciated water-bearing formation had great possibilities for supplying with either flowing or nonflowing wells a large area of country lying between Austin and San Antonio, extending west of the San Antonio River along the northern margin of the Rio Grande Plain toward the Pecos Piver, and even comprising the extensive summit region of the Edwards Plateau.
There are also many remarkable springs in that portion of the Rio Grande Plains and Edwards Plateau lying between the Colorado and Pecos rivers, which will be discussed in this paper. These belong to two distinct classes, each characterizing one of the geographic subdivisions mentioned, aid each exhibiting a method of escape of the underground water of the region. One line of these springs follows approximately the margin of the Rio Grande Plain, close to the line of
201
the railroad from Austin to Del Rio. The other class embraces the springs of the canyons of the Edwards Plateau.
In order to understand the occurrences of water of the San Antonio system, it is necessary to carry in mind
The general map (Pl. XXI) will illustrate the geographic relations of
GEOGRAPHY OF THE REGION.
The traveler by rail from Austin via San Antonio to Del Rio, on the Rio Grande, sees from the car window two conspicuous regions, each having its peculiar geographic features. On the left is a gently undulating plain whose margin the railroad follows. This is here termed
202
the Rio Grande Plain. On the right is constantly visible a line of low, circular, flat topped hills, the Balcones scarp line, which represents the jagged southeastward front of a higher region which has been called the Edwards Plateau. The Rio Grande Plain, the Edwards Plateau, and the Balcones scarp line are the chief geographic features of the region. Broadly considered, they are a lowland plain inclining gently southeastward to the Gulf of Mexico, an upland plain rising gradually toward the northwest, and a rugged zone of separation which includes a quick ascent from plain to plain.
The Atlantic and Gulf coasts of the United States from the Hudson to the Rio Grande are margined by a broad lowland called the Coastal Plain. The portion of it lying farthest to the southwest is called the Rio Grande Plain. One of the more important geographic divisions of the interior of the continent is the Great Plains. Its most southerly division is the Edwards Plateau. Farther north the Coastal Plain and the Great Plains lie far apart, the Mississippi Valley and the Appalachian belt and other geographic provinces being included between them; but southward they converge, finally meeting in southern Texas, so that the Rio Grande Plain and the Edwards Plateau lie side by side.
RIO GRANDE PLAIN.
In shape this plain is an irregular quadrilateral with angles turned toward the four cardinal points. On the northwest, as just stated, the Balcones scarp line separates it from the Edwards Plateau. On the southeast it is bounded by the Gulf of Mexico, and on the northeast it is arbitrarily limited by the Colorado River. On the southwest it is limited by the folded mountains of Mexico, which lie beyond the valley of the Rio Grande. It also sends a tongue for many miles up the Rio Grande.
The plains drainage, which follows the general slope from northwest to southeast, includes the Nueces, San Antonio, and Guadalupe rivers and their branches, besides various minor streams which join the Rio Grande and Colorado or enter the Gulf direct.
As compared with the adjacent plateau and mountain regions, its characteristic topographic feature is a low relief, but its surface is broken by occasional undulations and in places by hills of considerable height. Some of these are the low scarp lines of drainage valleys; others, like the Dos Hermanos Hills of Webb County, are buttes capped by limestone; others, like Pilot Knob, in Travis County, are old volcanic necks; Pinto, Las Moras, Turkey, and Elm mountains, in Kinney County, are buttes composed below of sedimentary rocks and above of caps of igneous rock; Sulphur Peak and Fort Inge, in Uvalde "
The indentation of this plain up the Rio Grande has been called the "Rio Grande Embayment" by Hill: Bull. Geol. Soc. America, Vol. III, 1891, p. 93.
Hill, ibid., p. 90.
203
County, are masses of solid basalt (see P1. XXII); the Anacacho Hills, extending east and west in southern Kinney County and constituting the most rugose part of the plain, are of still another type, consisting of a monoclinal plateau, or cuesta, sloping southward and presenting a steep scarp to the north. (See geologic profile, fig. 66, p. 260.)
The eastern part of the plain belongs climatically to the humid and subhumid regions; the western part, to the arid. The fertile "black lands" occupy large areas as far southwest as San Antonio; but contlnuous cultivation is limited by increasing aridity west of Bexar County. The plain as a whole is mostly grazing land, and supports one of the greatest stock raising industries of the United States. San Antonio is its commercial center.
In passing westward from comparative humidity to aridity, there is a corresponding decrease in the depth and richness of the residual soils. Soils derived from certain geologic formations which in the Black Prairie and eastern Texas timber regions are fertile, here become progressively more sterile and barren, and are impregnated or coated with a peculiar calcareous encrustation known in the Southwest as "tierra blanca" or "tepetate."
Much of the region is covered by the peculiar flora known as chaparral, consisting of a thorny growth of many species of scrubby acacias-mesquite, guaxillo, and huisatche-between which is an undergrowth of cactus, especially the large Opuntia known as nopal. For this reason the region is sometimes called the "chaparral country." There are also stretches of open prairies covered by nutritious grasses.
It is not within the province of the present paper to deal with the plain in its entirety, but only with a strip along its western margin not exceeding 30 miles wide, a belt underlain by the Cretaceous formations.
BALCONES SCARP LINE.
The Balcones scarp line is the frayed and ragged coastward border of the Edwards Plateau. From the more open and level lower country it appears as a sharp line of timber covered hills, and these are universally called "mountains" by the people of the region. It commences near the northern line of Travis County and continues a little south of west, through Travis, Rays, Comal, Bexar, Medina, Uvalde, and Kinney, to Valverde County, where it meets the Rio Grande. Near Austin its highest summits are about 400 feet above the margin of the lower plain; in Uvalde County, nearly 1,000 feet.
"The "Black Prairie" region is that underlain by the soft Upper Cretaceous chalky limestones and marly clays, where there is considerable rainfall. It comprises the richest agricultural lands of the State. The cities of Austin and Dallas are situated near its western margin; Terrell and Corsicana are near its eastern margin.
A singular fact in the progressive cartography of our country is that notwithstanding the conspicuousness of the Balcones scarp line as a topographic feature and that it was shown upon all the mother maps of the region, such as those made by Bartless, Roemer, and J. De Cordovas (J. H. Colton & Co., New York), it has been omitted from the more recent maps.
204
The drainage flows directly across the scarp line and has cut great canyons backward into the Edwards Plateau. The depth and precipitous character of these increase in the streams successively encountered as one goes westward. The portions of these streams lying within the area of the plateau before they cross the fault line have cut their channels approximately down to the level of the Rio Grande Plain. Their bocas in many places cut the scarp line into a series of tongue like salients projecting toward the plain, and the line is further diversified by plateau remnants in the form of outlying buttes. The position of the scarp is determined by a complex dislocation of the rocks, the Balcones fault, which will be described in a subsequent section (p. 258).
EDWARDS PLATEAU.
The Llano Estacado and the Edwards Plateau together constitute in Texas the Plateau of the Plains. This lies within the area inclosed by the Canadian on the north, the Pecos River on the west, the Balcones escarpment on the south and southeast, and an irregular line of scarps along the headwaters of the eastward flowing drainage of the Colorado, Brazos, and Red rivers of Texas. The general outline of this area is shown in the accompanying photograph of a model, P1. XXIII. It is over 500 miles in length and in places 280 miles broad. It is a vast quadrangular mesa, surrounded on all sides by descending escarpments. In its horizontal geologic structure and its relations to the surrounding region it may be broadly compared to a book laid upon a table and very slightly tilted or raised at one end. Its comonent strata are almost as parallel and regular as the leaves of the volume. While its central portion still presents a general level, its borders are cut by headwater erosion into a fringe of projecting drainage divides, accompanied by many remnantal buttes and mesas, showing the great erosion by which the plateau has been and is being gradually etched away. Its eastern margin in particular has been greatly reduced by this process.
The Llano Estacado and Edwards Plateau merge into each other along the central portion of the summit of the greater plateau, and there is no known line of demarcation between them. There is a great difference between the characters of their surface, their soils, and the underlying geologic strata, which collectively gives to each of the two regions a distinct character. The soil and underlying rocks of the Llano Estacado consist of unindurated loams, marls, and sands. Hardly a stone of building size can be found in its broad extent. The Edwards Plateau is in part a rough limestone country, resembling in some respects the western margin of the limestone country of the Grand "
Boca, a Spanish topographic term, used to indicate the mouth of a canyon valley where it debouches on a plain. Example, Boca del Agna, Jamaica, corrupted by the English into "Bog walk."
205
Prairie region (see Pl. XXIII), with which its, rocks are geologically identical. The hard Cretaceous rocks of the plateau partially underlie the Tertiary formations of the Llano Estacado, and it is probable that the latter have been eroded from the plateau.
It is in the latitude of Upton and Midland counties, or a little north of the thirty first parallel, that the hard rocks of the Edwards Plateau become covered by the marls and sands of the Llano Estacado, and it is here that the line between them is provisionally drawn while awaiting further exploration. In these counties the narrowest width of the whole plateau is found.
The Edwards Plateau occupies nearly the whole of the counties of Crockett,Valverde,Edwards,Sutton,Schleicher,Kimble,Kerr,Bandera,Gillespie,Kendall, and Blanco, and about one-half of the counties of Crane,Upton,Tom Green,Irion,Concho,Menard,Travis,Hays, Comal,Bexar,Medina,Uvalde, and Kinney. The northeastern and western boundaries are cliffs due to erosion. The northeastern overlooks a broad denuded area traversed by the Colorado River and its tributaries. The western margins the Pecos Valley, which at the north is an open plain and at the south a canyon. Beyond the Pecos are folded mountains, but in the region of the canyon these are separated from the river by a broad plateau similar in type to the Edwards.
The southeastern boundary is also a cliff, the Balcones scarp, but, as already stated, was primarily determined by dislocation rather than erosion. The main water parting lies near the western edge, and the greater part of the plateau is drained by streams running eastward and southeastward. These have cut deep canyons, dividing its marginal portions into long, narrow tongues. The drainage westward to the Pecos has accomplished only a moderate amount of erosion, so that the western cliff is comparatively simple in contour.
The part of the plateau which has been most thoroughly studied and with which this report is more especially concerned is the eastern and southern, the portion adjoining the Colorado Valley and the Rio Grande Plain. The characteristics of the plateau are most strongly impressed on the observer who enters it from the Rio Grande Plain, for in crossing the Balcones line he experiences a sudden and complete change of scenery, with accompanying changes in floral, geologic, and cultural conditions. Instead of long, wide sweeps of prairie, void of sharp relief, he finds a region of steep canyons and sloping hillsides. The monotony of deep and dusty soils is replaced by alternate outcrops of cream colored rocks and mils, occurring in long, continuous, and horizontal lines of stratification. A deciduous flora suddenly appears in the canyon valleys, replacing the semiarid chaparral. Rivers of flowing water, fringed by forests, replace the dry and stony stream ways of the plain, the mere trace of which is often lost in times of drought, and they now become fixed features of the landscape, boxed in with steep walled canyons. Rugged evergreen hills succeed the long stretches of low
206
undulating land with the yellow brown adobe soils. Some of the beds of stratification composing the canyon walls are barren of foliage; others are occupied by the dark evergreen shrubs, juniper and Sophora, extending like garlands around the brows of the circular hills. With the increasing altitude the air becomes more exhilarating and the heat of the day is tempered by cooler nights.
The Edwards Plateau, like the whole of the great plateau of which it is a part, presents three simple topographic elements-
The summit of the plateau is reached by ascending the long canyons of the streams and passing out upon it through their "draws" or caletas. Like that of the Llano Estacado, it is flat and void of constant-running streams. There are a few shallow, pondlike depressions or "sinks," which occasionally contain water immediately after rainfall. In general it is covered with a thick growth of nutritious grass and is without forest. Here and there, however, may be seen thick patches of scrub live oak, known as "shin oaks," growing in dense patches called "shinneries." For miles and miles' the level, grass covered plain stretches before the eye like a great sea, the view broken only at long intervals by the tall shaft of a ranchman's windmill, rising like the sail of a lonely vessel on the level sea. This summit region has an altitude of about 2,400 feet in the northern edge of Edwards County, gradually rising, with an ascent of about 4.5feet per mile, to the west and northwest and falling at the same rate toward the east.
In the dissected border of the plateau are hundreds of remnantal buttes or hills, like Mount Bonnel and Lone Tree Mountain, hear Austin, and bound Mountain, in northwestern Uvalde County (P1. XXIV). Standing upon one of these hills, one can see that its horizontal layers of rock once extended across the valleys to the opposing bluffs and summits and realize that the valleys have been cut out of these once continuous rock sheets. The major stream ways have eroded their wide valleys below the summit level, so that the surface of the old plateau is preserved only in the divides.
Ultimately the headwaters of the streams will all meet, and the general summit level will be cut into innumerable buttes and mesas, like "
This expressive term, denoting the cliff margin of an upland plain where it breaks away to a lower plain or valley, has not yet found its way into the dictionaries, but is in common use in many districts of the far West.
From the Spanish. This is a useful word for the ultimate and smallest headwater ramification of a lateral stream. It is synonymous with the term "draw," used in the middle plains region of the United States, the "coulee " of Montana, and "drain" as used in Colorado. In this paper the local topographic nomenclature of Spanish-America will frequently be used. This has been set forth in a paper entitled Descriptive topographic terns of Spanish-America: Nat. Geog. Mag., September, 1896, Vol. VII.
207
those now found along its border. These in turn will be planed down to lower and lower levels. By this process many layers of rock have been stripped from the plateau in the past, as others will be in the future.
The present plateau summit exists because of the superior hardness and the capacity for resisting erosion of the rock sheet that caps it. This cap rock is the Edwards limestone. It not only everywhere constitutes the summits, but gives character to all the surrounding scarps and hills. The beds which originally overlay it were comparatively soft and yielding, and have almost completely disappeared.
The streams of the plateau have important bearing upon the question of underground water, and as they constitute a group of rivers characteristic of a large region in Texas, they are well worth the serious attention of those interested in the economic and geographic problems discussed in this paper.
The Rio Frio will be described as a type of the rivers flowing through the southern margin of the plateau. From where it passes out of the plateau onto the Rio Grande Plain, just north of the Southern Pacific Railroad, for fully 50 miles toward its mouth its bed is a shallow, usually waterless stream way, meandering across the gravel covered country. Flowing water is usually found in this portion of its
North of the railroad the stream emerges from a boca (like that seen in Pl. XXV) of the Balcones escarpment, and still farther northward it traverses a canyon of the plateau. In this canyon the stream way is in the bottom of a flat valley filled with ancient gravel deposits and bordered on each side by high bluffs of stratified limestone. In its lower portion the valley averages 2 miles in width, and contains some fairly good agricultural lands. Entering the canyon, the road follows the dry, stony stream bed a short distance before flowing water is encountered. Farther up the stream running water again ceases and the dry bed again appears. In this manner the continuity of the river is interrupted and its bed is broken up into alternations of dry and "
The geologic formations here mentioned are described more fully on page 227.
208
watered segments. In the flowing segments are clear and swift bodies of water, supplying as much as 1,000 gallons per minute and competent to irrigate many acres of land.
Wide, flat bottomed canyons of this type continue from the Balcones scarp line via Van Pelts and Leakey to the "water hole" near the head of the West Fork, and the number of springs and quantity of living water progressively increase in that direction. Above the headwater "hole" a different type of canyon sets in and permanent water ceases. The fall or gradient of the meandering stream way in the canyon is about 23 feet per mile in the Nueces, and is probably the same in the Frio. The fall of the bottom of the canyon, without considering the stream meanderings, is about 60 feet per mile.
The scarps which make the outer borders of these flat bottomed valleys are nearly always steep, usually consisting of alternations of bluffs, benches, and slopes corresponding with the horizontal lines of stratification and giving to the landscape a terraced topography. Between the widely separated escarpment walls are the broad, level second bottoms, standing some 50 feet above the level of the present stream ways. These bottoms consist of an alluvial deposit (the Uvalde formation) and record a peculiar event in the history of the region an event to be discussed in subsequent pages. Nearly all the living or perennial water of the stream ways occurs in the flat bottomed canyons, and they are the seat of the chief agricultural population of the region. The beds of their stream ways are usually composed either of the smooth surface of some horizontal limestone stratum, over which the water may flow for a great distance, or of clean washed flints and limestone bowlders, often bleached to a chalky white color in the glaring sunshine.
In places the waters make wide and deep pools having the peculiar light sea green color characteristic of all the spring rivers breaking from the Cretaceous limestone of southwest Texas. Elsewhere they flow for long distances over a single stratum of horizontal limestone. In the latter case the water spreads out in a thin sheet only an inch or two in depth and sometimes 100 feet wide.
It is almost impossible for the traveler who has seen the continuation of this same stream in the dry region of the Rio Grande Plain to recognize it in the beautiful flowing river now before him. Forests of ash, pecan, and elm fill the valley, while gigantic cypresses border the water. If he should chance upon one of these water holes without having traced the continuity of the stream course, he would believe that lie stood upon the banks of a large and continuously flowing river. He would soon find, however, that after flowing a short distance the water would disappear, either by absorption into the bed of the gravelfilled stream way or through fissures in the solid underbed. These running water holes are constant, and do not depend upon the local rains, but are supplied by perennial springs draining the rocks underlying the plateau.
209
Above the springs at the head of the flat bottomed valleys the limestone bluffs become more vertical and close in toward each other, changing the cross profile of the canyon from U shape to V shape. In this, which may be called the cataract portion of the stream way, the gradient is much steeper, ranging from 100 to 300 feet per mile. here the stream way carries no water except in time of flood, but then it is a series of cataracts leaping from one stratification plane clown to another. The cataract portion of the stream way is usually through the upper part of the Edwards limestone, the rock which makes the scarp rock or mesa edge of the plateau. The flat summit, or "divide," is finally reached after climbing the rough and rocky slopes; and here the drainage is represented only by a few long, shallow caletas.
In resume, each of these stream courses may be said to present four well marked aspects:
The caletas and upper canyons are usually dry and waterless arroyos except in time of storm. The flat bottomed canyons contain permanent pools of flowing water, fed by springs, and on the lower plain the running water disappears entirely or for a considerable distance.
The chief laterals of these major canyons of the plateau are also peculiar and consist entirely of dry arroyos, carving the marginal canyon walls into thousands of entrant and reentrant curves, producing circular, flat topped hills, with slopes composed of alternations of small scarps and minor slopes. Toward their heads these laterals sometimes expand into amphitheatral basins, from which minor laterals ramify in palmate arrangement. These basins are covered with the wash soil of the adjacent hillsides, and are locally known as "grass valleys." So evenly is the debris scattered over them that it is often difficult to trace the stream way. Water is found in these laterals for only a few minutes or hours after each ordinary storm, and is evaporated or imbibed before reaching the main stream, but in time of sudden and very heavy cloudburst rainfall they carry gigantic and dangerous torrents. Each torrent moves the " wash" of the land a step nearer the major stream ways, and rolls the gravel in the beds of the latter onward toward the lower country of the Rio Grande Plain. Thus it is that an intermittent stream of gravel is and has been for a long time gradually flowing away from the Edwards Plateau and spreading over the Rio Grande Plain.
There are many interesting caverns in the Edwards Plateau, and inasmuch as their occurrence, together with the general question of limestone solution, has great bearing upon the distribution of underground water, it is essential that they be briefly mentioned. They are "
A V-shaped canyon, from the Spanish for scissors.
210
of three general types:
The flora of the Edwards Plateau presents many peculiar variations from that of the adjacent regions, especially the Rio Grande Plain. It shows three distinct phases, viz: the phase of the stream bottom, that of the breaks, and that of the summit. The low, alluvium filled valleys of the rivers present conditions of loose soil and constant moisture favorable to the growth of trees; hence narrow ribbons of forest are found along the streams and extending up into the semiarid region, far west of the limits of the upland forests of the humid region and dissociated from them. These embrace many species, such as the elm, chestnut oak,walnut,sycamore,cypress,live oak, and pecan. The live oaks and pecans attain great size and beauty. The occurrence of the cypress is a peculiar anomaly. This tree, which ordinarily grows only in the swamps and bayous of the low subcoastal regions, attains an enormous size at the edge of the deeper holes near the heads of permanent water of the Pedernales,Blanco,San Marcos,Guadalupe, Cypress,Onion Creek, and other streams. These localities are at altitudes from 1,000 to 1,750 feet above the sea, hundreds of miles west of the great cypress swamps of the eastern tier of Texan counties, with which they have no possible continuity. We have not noticed the cypress, however, in the Nueces, which flows front the still more arid western edge of the south side of the plateau, although the other trees usually accompanying it grow at Kickapoo water hole as large and as luxuriantly as elsewhere. These ribbons of valley forest near the headwaters of the Nueces and Frio terminate with the southern edge of the Edwards Plateau, practically ceasing at the margin of the Rio Grande Plain, and heave no connections with any other forest region whatever. Accompanying them are numerous ferns, maidenhair and
211
other delicate species, all plants requiring continuous moisture, while even the familiar mullein and Jamestown weed occur. This flora of the valleys is of interest, inasmuch as it is a modified representative of that of the great Atlantic timber belt, occurring as an isolated outlier in the semiarid region, preserved and nurtured in these valleys by the presence (due to geologic causes) of water and soil.
While the valleys support this modified flora of the humid region, the rocky slopes of the breaks between the streams and the summit present another group of vegetation shrubby trees which prefer the crevices of rocks, or small scrubby plants which develop large, strong, and hearty roots, out of proportion to the size of the growth above ground. These plants are found along the ledges of limestone wherever their roots can find a hold, or on the almost soilless outcrop of the interstratified chalky marls. Among them are dwarf oaks, the pinon Sopliora, and mountain juniper, which seem to prefer to follow the ledges of loosely jointed rock, besides many coriaceous perennials, including the agarita (Berberis) and the eastern yucca. There are also many small species of Compositor, Liliaceae, the wild poppy (Argemone mexicana), and other plants growing on these slopes.
West of the Frio, in the breaks of the south end of the Edwards Plateau, where the rocks are hotter and more arid than to the eastward, the remarkable and unique resurrection flora of the limestone mountains of Mexico is found. This is characterized by plants growing upon the hot and sterile rocks, and adapted to irregular rainfall, long drought, and scarcity of soil by their thick, coriaceous parts, which ordinarily look dry and dead, but which rapidly unfold and revive after a rain, taking advantage of every drop of rainfall in order to store sufficient moisture to enable them to survive the long periods of intervening drought. Even the ferns, mosses, selaginellas, and kindred plants, which ordinarily constitute our ideals of delicate and tender herbage, have in this region a thick, leathery texture. On the almost barren surface of these rocks grows the melon shaped, edible cactus the devil's pincushion.
Here also, for the first time in proceeding westward across Texas or northward across the Rio Grande Plain from Mexico, one meets the peculiar plants (agaves and yuccas) which become so marked a feature farther westward, the serrated sotol, with its flower stalk rising to a height of 15 feet; the dagger like lechuguilla or ixtle plant, and several species of yucca not seen farther eastward.
The flora of the summit of the plateau is radically different from that of the breaks or of the valleys. All trace of shrubs or trees disappears, save here and there a patch of shin oaks and dwarf evergreens, and in time of verdure the eye beholds apparently a never ending sea of grass, "
In the lower slopes, around the headwaters of the Frio (Frio water hole), the writers last year discovered a large area of the edible Berberis swaseyi Buckley, a species which Coulter says in his Flora of West Texas is known only from the canyons of the Pedernalis. The fruit of this plant is a large edible berry well worthy of cultivation. We have never seen it elsewhere.
212
through which appear many bright colored flowers. This is the southern end of the flora of the Great Plains region, which continues far northward.
Although there is much agriculture in the wide, fertile, plaza canyon valleys indenting the plateau, especially in Blanco, Gillespie, Comal, and Kendall counties, the slopes and summits constituting the larger part of the area are not adapted to agriculture, owing to the rocky character of the soil, the semiaridity of the country, and the impossibility of irrigation. So little are they fitted for agriculture that the extent of the summit of the Edwards Plateau can almost be traced upon the map by the scarcity of post offices and other evidences of population. They constitute, however, good grazing country and support many large sheep and cattle ranches.
GENERAL PRINCIPLES OF ARTESIAN WATERS.
The rocks of the earth forma system of natural works by which water is collected, stored, and distributed. They constitute the basins, reservoirs, conduits, and other portions of the plant for retaining and distributing underground water.
The details of natural waterworks differ in different places in the same way that the details of artificial systems differ in different plants, but the distribution in both is governed by the common principles of hydrostatics. The efficiency of a natural system is determined by the texture of the rocks and the geologic structure of the region, so that an understanding of the availability of underground water in any region necessitates a knowledge of the elementary geology of that region.
It is neither convenient nor advisable here to discuss minutely the source, storage, and distribution of underground water, but in order that the subject may be understood, we shall. give a brief explanation of the elementary principles governing its occurrence.
213
escapes at the bottom of a hydrostatic column. If sheets, beds, or masses of rock containing water be cut into by a well hole, a ditch, erosion, or other means, to a lower level than that at which the water entered, all the water contained in the rocks above the level of the incision will be drained from the rocks by the simple process of seepage, just as water escapes from the bottom of a moist sponge or from the faucet at the bottom of a barrel. This is the method by which water occurs in ordinary nonartesian or dug wells, and in seepage springs so frequently found along the low banks of rivers, valleys, or other natural incisions into the strata. Modern agricultural field subdrainage depends upon the same principle.Water which falls upon the surface of the earth as rain is disposed of by surface run off, by evaporation, and by absorption into the underlying rocks. The water forming the run off passes over the surface to form streams and lakes; that which is evaporated passes again into the atmosphere; the remainder sinks below the line of evaporation into the rock mass beneath, supplying wells and springs. The water of the last class forms the subject of this paper.
CAPACITY OF ROCKS FOR ABSORBING MOISTURE.
Rocks of open texture, such as loose sands and sandstones, gravels, and chalk, have a sponge like capacity for imbibing water. Water poured upon sand will quickly disappear by imbibition. If we wish to filter water, we run it through beds of sand or gravel. Bricks are sprinkled before they are put into buildings, and they absorb from 20 to 60 per cent of their weight of water. On the other hand, if one wishes to shed water or otherwise prevent its percolation, one constructs roofs of tile, makes tables of marble, and builds tanks and cisterns of cement or clay. Few stop to consider, when thus using rocks, that they are making practical application of the broad principles which control the occurrence of underground water. By careful and accurate experiments, such as anyone can make, the capacities of all the known rocks for the imbibition and transmission of water have been determined, and it is shown that sandstone or chalk will absorb many times as much water as slate, marble, or granite. These facts can be observed after every rain. If the rain falls upon loose, sandy soils, like those of the two Cross Timber regions of Texas, it quickly disappears by absorption; if, on the other hand, it falls on the clay
214
soils of the prairies, it stands for some time in pools. such as those called "hog wallows" in Texas.
The capacity of rocks for the transmission of water is entirely different from their capacity for imbibition. If one could construct of sand, clay, slate, granite, chalk, and close textured limestone filtering vessels of equal capacity, and then fill them with water, one would find diverse results, illustrating the capacity of these rocks for transmission of water. Water would pass so slowly through the close textured limestone, slate, and granite that the quantity filtered would be practically imperceptible. At first the sand and chalk would drink in the water equally fast, but after complete saturation it would require longer time for the water to percolate through the chalk than through the sand.
The distribution of underground water is dependent upon the arrangement of rocks in sheets or strata. By a simple arrangement of the porous sands between impervious materials, nature has constructed reservoirs and conduits for the retention and distribution of water, and it is by the character of the arrangement of the rock sheets that the negative or positive conditions for the procurement of artesian waters are determined.
A stratum usually consists of two related parts the outcrop and the embed. That portion exposed at the surface of the earth is the outcrop, and that portion which is concealed underground beneath and between the other rocks may be termed the embed. The outcrop of a water bearing stratum constitutes its main catchment or receiving area, and the embed constitutes the storage reservoir.
If the water bearing rock sheet is inclosed between impervious beds, and inclines beneath the surface, the water will be conducted to a lower level than the outcrop and will remain stored in the earth under hydrostatic pressure until an outlet is provided for it. A water impregnated stratum embedded in this manner is an artesian reservoir. The water bearing strata, together with the impervious strata beneath and above them, constitute an artesian system.
When water is conducted downward to a lower level by an embedded stratum, it acquires a tendency, due to hydrostatic pressure, to rise higher than the overlying retaining bed. When the beds overlying such a water bearing stratum are penetrated by an opening, artificial or natural, the contained water will rise through the overlying bed. Such earth waters which rise under the influence of hydrostatic pressure through an opening to a higher level are known as artesian waters. Hence an artesian well may be defined as one in which the water rises by means of hydrostatic pressure above the top of the embed. If artesian waters rise to the surface, they are known as flowing artesian wells; if the water fails to rise to the surface in a well, such a well is known as a nonflowing artesian well. The height to which the water will rise depends upon several conditions. If the water bearing stratum is embedded and incised at only one place down the dip from its surface outcrop, the water at the place of incision will rise to the same level as
215
the line of complete saturation of the bed. This level is somewhat lower than that of the lowest surface outcrop. If there are several incisions, the question is not entirely one of static equilibrium. For instance, if water is naturally received by the stratum in one part of its outcrop and discharged from another lower part, the conditions in the intervening area are those of dynamic rather than static equilibrium, and the water will rise in a well to a level intermediate between the receiving and discharging levels.
The artesian water bearing strata of the State east of the Pecos River are composed mostly of extensive sheets of sands, clays, and limestones, succeeding one another in orderly arrangement, except along the Balcones zone of faulting, and in general having a gentle inclination toward the sea, so that intraveling northwestward, although constantly ascending in altitude, one encounters the outcropping edges of rock sheets of lower and lower stratigraphic position. This produces the simple arrangement of a tilted plain built up of a series of alternately impervious and pervious layers. The rain falling upon the outcropping edges of the latter, sinks into the embed, and by gravity is conducted seaward down the plane of its inclination to lower levels beneath the surface. Each different stratum, including any particular water-bearing stratum, becomes embedded deeper and deeper to the southeastward of the point where it outcrops at the surface. This structure is very simple and its detail can be traced out, measured, and mapped as accurately as that of the successive layers of stone in a building. To do this is the work of geologists, and it is not a matter of speculation and hypothesis, but simply the application of specially acquired knowledge, similar to that which one must possess to be proficient in any profession.
The foregoing principles are all applicable in explaining the numerous artesian wells and springs of the Rio Grande Plain and the headwater springs of the Edwards Plateau, but the explanation requires also an understanding of the order and arrangement of the strata, and we therefore proceed to consider the geology of the region.
GEOLOGY OF THE REGION.
The rocks of this region consist of various beds of marl, clay, limestone, sandstone, etc., a few igneous rocks (proportionally a very small part of the whole), and some alluvial or surface deposits derived by erosion from the older formations. With the exception of the igneous and alluvial rocks, the strata are all composed of material which was laid down beneath the surface of the ocean, and embedded in them are found many marine fossils, the remains of animals which inhabited the waters of the old ocean. These fossils are of the greatest value in determining the geologic position of the beds containing them, and therefore we give from each of the principal beds a few illustrations of the most important and most abundant forms, by the aid of which the layman can readily recognize the geologic horizons. (Pls. LI-LXIV.)
216
The nomenclature used in describing these rock sheets is as follows: The geologic unit is the bed or stratum. Series of lithologically and paleontologically related strata are grouped into formations, as the "Glen Rose formation." When the strata are all of one kind of rock the name of the rock is sometimes substituted for the word formation, thus: "Edwards limestone" or "Edwards formation." Groups of this class may be embraced in still larger groups called divisions, as the "Fredericksburg division," and these may be grouped into series, as the "Comanche series," and these compose the still larger geologic groups, such as the Cretaceous or the Eocene.
As the work of geologic investigation. has progressed new facts have made necessary the revision of classification, and new names have from time to time been substituted for those first employed. In the classification herein presented the previous nomenclature has been refined by substituting appropriate geographic names (mononyms where possible) for all formations and abandoning the use of paleontologic and mineralogic names. Thus the terms "Dinosaur sands","Caprina limestone," "Exogyra texana beds","Exogyra arietina clays","Fish beds","Exogyra ponderosa marls","Hippurites limestone," etc., have all been replaced by appropriate geographic names. Even some geographic names originated in earlier writings have been abandoned because found to duplicate names previously used for different formations elsewhere.
The sequence and systematic classification of the formations of the region tinder discussion are presented in the table below, and in the following pages the several formations will be individually described. They will be taken up in the order in which they were deposited, the oldest and lowest first.
 |
217
PALEOZOIC FORMATIONS.
Below the Cretaceous rocks which constitute the surface of the Edwards Plateau and Rio Grande Plain lies a foundation of the older and different rocks of the Paleozoic system. These are exposed by erosion only along the foot of the northern scarp of the plateau in Burnet, Blanco, Gillespie, and Mason counties. So far as known, no drill has as yet penetrated them beneath the Rio Grande Plain, and as they are so deeply buried they need little consideration in the presemt discussiom. In the Edwards Plateau, however, the well drill has reached the Paleozoic rocks beneath the Cretaceous in several places, notably at Kerrville and at Morris ranch, in Gillespie County. From data derived from these drillings we are led to believe that the horizontal Cretaceous beds of the Edwards Plateau lie upon an uneven floor of the older rocks. The probable relation of these Paleozoic rocks to the overlying Cretaceous is shown in the figure of the wells at Kerrville (fig. 68, p. 270). One hundred feet of Carboniferous rocks form the base of the section exposed at the month of Hickory Creek on the Colorado River (see p. 220).
THE CRETACEOUS.
The Cretaceous formations of Texas are by far the most important in the State, in both areal extent and economic value. They are seamade rocks, and rest unconformably upon the older Paleozoic rocks, which formed the ocean floor when their deposition began. They constitute almost the entire surface of the Edwards Plateau and much of that of the Rio Grande Plain. Their southern margin is covered by the Eocene overlap, and they are overlain in places by superficial deposits of gravel of Neocene and Pleistocene age. They contain all the artesian water described in this paper, and for this reason it is important that their sequence and occurrence be well understood. They likewise supply the most valuable building material stone, lime, and cement-and some of them. contain oil and gas.
They are mostly limestones and clays, sometimes containing slight admixtures of very fine silica, but there are also great beds of sand and sandy mixtures, occurring principally at the base, middle, and top of the group. The limestones are of many kinds, predominantly light colored, more or less chalky in texture and composition, and rarely of the hard, ringing, close textured character, such as is usually met in American Paleozoic limestones. Some of them are very porous, even cavernous, in texture; others are more massive; some have slight admixtures of magnesia, clay, or silica; others are almost pure carbonate of lime. In some instances they are soft, often marly; in others, quite indurated. The clays are likewise very chalky, so that on exposure to air they readily crumble and weather into soils. The many
218
beds of clay and limestone resemble one another so much that it is difficult for one not accustomed to making geologic observations to discriminate them. The geologist who has studied them carefully can readily recognize them, not only by slight lithologic differences, but by the peculiar fossils which characterize them.
The many formations which make up the Cretaceous system are classified into two great series, the lower of which has been termed the Comanche and the upper the Gulf series. They have also in various parts of Texas been classified into smaller groups called divisions. As most of the formations are of limited extent as compared to the whole system, these divisions are of importance for the indication of the chronologic relations of formations occurring in different regions, and they have therefore been indicated in the table on page 216, but they are not essential to the discussion of the economic questions of this paper.
COMANCHE SERIES (LOWER CRETACEOUS).
In southern Texas this series includes the greater part of the Cretaceous system, and its total thickness is somewhat more than 1,500 feet. At the base the beds are of feebly coherent sand, then come marls and limestones in alternation, and then a heavy body of limestone, followed by other marls and shales.
THE BASEMENT BEDS.
The basement beds of the Comanche differ from the overlying formations of the series by the fact that they are usually composed of sands instead of calcareous marls and limestones. These are more or less fine grained and slightly compact, accompanied in places by small pebble conglomerate, and locally varying in composition according to the material of the adjacent rocks from which they were derived. Thin beds and lamina, of clay occur in the sands, and the residual surface usually has a reddish color. Furthermore, the outcrop of these beds usually bears a growth of timber such as is found in the region known as the Upper Cross Timbers. While the basal beds of the local Cretaceous section, wherever exposed, are usually of this sandy nature, these beds are not everywhere of exactly synchronous deposition, for they represent the littoral of a sea which was progressively transgressing northwestward across an uneven land during the whole Comanche epoch.
So far as the region considered in this paper is concerned, the lower beds and their relations to the overlying and underlying rocks are exposed chiefly in the slopes of the Colorado drainage in western Travis, eastern Burnet,northern Hays,Gillespie, and other counties along the northern edge of the Edwards Plateau, and especially along a line between the town of Burnet and Travis Peak post office, via Smithwick
219
Mill. There is no doubt, however, that these formations occur embedded beneath most of the Edwards Plateau and Rio Grande Plain, and that if a drill could penetrate far enough it would reach them anywhere beneath these regions. They exhibit such variation from place to place that the precise correlation of different occurrences is difficult, and it has seemed best to give separate names to the rocks at the two most important localities.
TRAVIS PEAK FORMATION.
These beds are especially well displayed. on both slopes of the valley of the Colorado River, between the mouths of Sycamore and Cypress creeks, in Burnet and Travis counties. The name they bear was given them because they are well exposed in the vicinity of Travis Peak post office.
While they are arenaceous in composition and porous in texture, like the basement beds of the Comanche in general, they differ considerably from the allied beds to the northward. They consist of conglomerate, composed of coarse rounded pebbles of Silurian and Carboniferous limestones, granite, Llano schists, quartz derived from the adjacent Paleozoic rocks, beds of finely cross bedded pack sand, white siliceous shell breccia resembling the Florida coquina, and some clay.
At the base is usually conglomerate. Succeeding this is coarse, angular, cross bedded sand, which becomes more finely triturated until it reaches the condition known in Texas as "pack sand" i.e., a very fine grained, loosely consolidated sand, cemented by carbonate of lime. In the sands are occasional patches of red and greenish white clays, resembling very much the characteristic colors of the Potomac beds of the Atlantic coast, and they are sometimes accompanied by lignite and fossil bones.
The following section by Mr. J. A. Taff will give an idea of the sequence and composition of the formation as exposed in the valley of the Colorado, in the locality between Travis Peak post office and Smithwick Mill, Burnet County:
 |
See Annual Report Geol. Survey Texas, Austin, 1890, p. 118.
All sections given in this paper are described from the top down and are numbered from the hottom up.
Third Annual Report Geol. Survey Texas, Austin, 1892, p. 295.
220
 |
The sandstone contains grains of silica from the size of a pea to the most minute particles, and small subangular fragments of clay in the cement of lime.
Fossils occur in these beds as low down as the contact conglomerates, but they are neither plentiful nor distinct. The upper or coquinalike beds are full of casts and molds, among which are undetermined species of Trigonia, Pholadomya, and Cyrena, and Ammonites justinae.
In these beds also appears the first of the several oyster agglomerates of the Comanche series. This is composed of a solidified mass of large oyster shells, forming a stratum 7 or 8 feet in thickness, just below the junction of Post Oak and Cow creeks.
Accompanying the oyster breccia another noteworthy feature of the Trinity division appears i. e., an excess of epsom salts, or magnesium sulphate. The oyster shell bed effloresces into a powdered earthy substance accompanied by the epsom salts. Magnesian and pyritiferous layers occur in other horizons higher in the division, and their presence is no doubt in part the cause of the mineral character of some of the artesian waters, especially those wells which are not drilled into the basement sands below these layers.
At the top of the sandy beds in the Colorado section a yellow, arenaceous, fossiliferous limestone appears. This marks the first or lowest appearance of the peculiar fossils Monopleura (Caprotina) and Requienia, "
See artesian water discussion, p. 300.
221
and indicates the beginning of the conditions which finally produced the Glen Rose formation.
The Travis Peak formation records a subsidence of the land which was taking place during its deposition. As the waters deepened the deposits changed from coarser to finer material, becoming more comminuted and calcareous at the top of the beds, until the sand grains are so fine as to be almost imperceptible to the eye, the whole mass becoming quite chalky and "magnesian" in appearance.
GILLESPIE FORMATION.
In Gillespie County are other exposures of the basement beds of the Cretaceous, resting upon the Paleozoic. These appear in the valley of Barron Creek at Fredericksburg and along the Pedernalis below that town. They are composed of sandy grits and clays of a more brilliant vermilion hue than is ordinarily met with in similar deposits elsewhere. As shown in fig. 55, amid in fig. 74, p. 314, they rest on the Paleozoic and grade up into the upper beds of the Glen Rose formation. These might possibly be correlated with the basal sands of the Travis Peak formation, but they are more probably the stratigraphic equivalent of the lower portion of the Glen, Rose formation where it rests on the Travis Peak.
 |
GLEN ROSE FORMATION.
This consists largely of beds of flaggy argillaceous or massive chalky limestones, alternating with thin strata of marly clay, white and yellow in color. From this alternation of limestone and clay the beds were provisionally called the "Alternating beds" when first differentiated by the senior author.
The indurated beds consist of white and yellow limestones of either brecciated, crystalline, arenaceous, magnesian, or chalky structure. They are of varying thickness and of great uniformity in extent. They
222
are separated by softer, unconsolidated, slightly argillaceous marls of oolitic structure, sometimes white and sometimes yellow.
The Travis Peak formation grades upward into the Glen Rose without break in the sedimentation, as can be seen in the high bluffs of Cow Creek, immediately below Mr. Heasel's house at Travis Peak post office, in the western part of Travis County.
The basement beds of the Comanche series have been described as being predominantly of an arenaceous character. The Glen Rose formation may be distinguished as being essentially calcareous. Each of the subdivisions, however, is accompanied by the material of the other as accessory. Thus the calcareous Glen Rose formation is slightly arenaceous at its base, the arenaceous material being siliceous grains so triturated that they are reduced to an almost impalpable powder. This gradually diminishes as we ascend in the beds, and the lime and clay proportionately increase.
The lowest Glen Rose beds are marked by the appearance of strata of homogeneous texture, such as "magaesian" marls and hard layers in which the fossil Requienia² occurs. The name "Caprotina Horizon No. 1" has been applied to these beds, because in the earlier geologic literature the fossils now called Requienia were termed Caprotina. The top of the Glen Rose formation is just below a bed of yellow marl which is persistent over a great area in central Texas, and is the culminating horizon of the oyster Exogyra texana, after which this bed has been called.
In nearly all complete sections the Glen Rose formation shows three marked subdivisions. The lower and upper thirds are composed of thinly bedded and alternating marl and flags, usually weathering into terraced slopes; the middle third is made up of thicker and more massive beds which constitute bluffs. Some of the beds near the base of the thicker layers are quite chalky in texture and carry many peculiar fossils, especially noteworthy being a large foraminifer (Orbitolina texana), besides many large casts of mollusks. The lower portion of the formation carries much fine arenaceous material accessory to the calcareous material. and its indurated and unindurated beds do not occur in such uniform alternations as do those of the upper third. For instance, there will be 10 or 12 feet of soft, friable material and then a thin layer of less than a foot of indurated stone. In weathering this results in wide terraces with steep slopes.
The yellow magnesian strata also increase in thickness in ascending series, and become very conspicuous in the middle portion, often being from 5 to 15 feet in thickness, as seen in the bluffs of Mount BonNel, near Austin. These magnesian limestones are soft and of a cream or brownish yellow color, and alternate with strata of marls similarly constituted, "
The term magnesian has long been applied to certain yellow strata in these beds. Whether they are or are not magnesian in composition we can not state positively.
A species of this genus of shells from the Edwards limestone is figured on P1. LIV, fig. 1 a,1 b.
223
and are sometimes accompanied by pockets or nodules of calcite, aragonite, strontianite, celestite, and epsomite.
The upper third of the formation, as seen at the top of Mount Bonnel, presents alternations of friable marls and hard limestone strata. The limestone strata usually average less than a foot in thickness. These alternations occur with great regularity and persistence. Clay is the chief accessory of the calcareous beds. The marls are soft and laminated and are composed largely of minute shell fragments, giving the beds a distinctly granular, oolitic character. They have little clay and imbibe the moisture very freely.
While possessing no great agricultural possibilities, the basal or alternating beds are capable of producing valuable building material, among which are building stones. Some of these have rich "magnesian" buff yellow colors, while the limestones often resemble the stones of Caen, France, which are imported into this country. Some of the beds are also valuable for the manufacture of hydraulic cements, although at present they are not utilized. These rocks also contain undeveloped beds of epsom salts, strontianite, and other materials.
The alternations of horizontal beds of soft marls and hard limestones above described produce the bench and terrace topography of the slopes of many of the canyons and along the margin of the Edwards Plateau from the East Fork of the Nueces to the Colorado, where the streams have cut downward through
224
the Edwards limestone. Of this character are the beautiful terraces and bluffs of the Colorado, as seen from Mount Bonnel westward to the Burnet County line, as well as those of the Pedernales, Guadalupe, Comal, Medina, Hondo, Frio, and all the numerous streams indenting the southern margin of the plateau. In addition to the localities already mentioned the beds are exposed west of San Antonio along the line of railway between Aue and Boerne and in the valley of the Guadalupe above Kerrville as far as Via. They are also exposed in the lower slopes of the valley of the East Nueces and its tributaries below Vance to a point a few miles south of Montell. The channel of the West Nueces has barely cut down to their top and exposes them at only two places along the stream bed at Kickapoo springs and in the north bend of the river in northern Kinney County.
The accompanying detailed section of the entire thickness of the beds of the bluffs of the south side of the Colorado in the vicinity of Round Mountain, Travis County, is typical of these beds. It coincides almost exactly with Mr. Taff's Sandy Creek section, previously measured, on the opposite side of the river.
 |
Mr. Taff's section measured 447 feet, or 8 feet less than ours. Geol. Survey Texas, Third Annual Report, 1891, Austin, 1892, pp.298-299.
225
 |
The rocks of the middle of the Glen Rose formation are the oldest exposed in the Nueces Valley. The thickness of the formation in this region, estimated from studies in the vicinity of Kerrville, is approximately 500 feet, which if true would indicate a uniform thickness along the entire line of strike across the region treated in this paper. This uniformity of thickness is not maintained along the line of dip, however, as will presently be shown. Taff's measurement of the Travis Peak formation makes it about 213 feet in thickness at its outcrop in the Colorado Valley.
It is difficult to determine the thickness of the embedded portions of these strata, especially the Travis Peak. The careful measurements
226
of the surface outcrop do not coincide with the artesian well borings made to the east, the latter showing much greater thickness. For instance, the borings of the San Marcos artesian well (see p. 287) show a thickness of 904 feet of the Glen Rose and Travis Peak penetrated by the drill at that place. At Austin also the well records indicate that at least 1,215 feet of the two formations are penetrated. Neither of these wells has as yet reached the bottom of the Travis Peak. These data indicate that the aggregate thickness of the formations increases rapidly coastward away from the line of outcrop. The beds were deposited on the eastern slope of an old, subsiding, preexisting Paleozoic upland, against which the formations of the Cretaceous in general and of the lower beds in particular were cumulatively deposited until it was buried, and toward which the calcareous beds changed into an arenaceous character.
WALNUT FORMATION.
If the classification of the Cretaceous formations had originally been made in the region here discussed, the Walnut formation would probably not have been given separate status, for, though well developed farther north, it is here thin and unimportant. In its typical development along the Brazos River it consists of laminated clays alternating with limestone flags, and both clays and flags are accompanied by great quantities of the two peculiar species of oyster, Exogyra texana and Gryplaea marcoui sp. nov. H. & V., Pl. LIII. Along the Colorado River at the northern limit of the Edwards Plateau the formation is only 10 or 12 feet thick, consisting of thin, friable, yellow, arenaceous marls, in which are great numbers of Exogyra texana. It determines a distinct bench near the summit of the high mesas west of Austin. Southwestward, toward the Nueces, it becomes less and less distinguishable, until at that stream there is only a foot or two of yellow clay, accompanied by the characteristic fossils.
COMANCHE PEAK LIMESTONE.
This is a persistent bed of white chalky limestone, presenting a shattered reticulated appearance on weathering. It is partly characterized by an abundant fossil fauna containing a large number of Exogyra texana, which is especially abundant in its basal portion. It is from 40 to 50 feet thick, thinning toward the Rio Grande. Although it is insignificant as regards thickness, and lithologically might be considered the base of the Edwards limestone, it is one of the most persistent paleontologic horizons of the Texas Cretaceous section.
"Gryphaea pitcheri(in part) of previous writings; described in Bull. U. S. Geol. Survey No. 151.
227
EDWARDS LIMESTONE.
This formation is the most conspicuous and extensive in the Texas Mexican region. It is composed mostly of limestone, but there are some marly layers. It shows slight variation in color, composition, texture, and mode of weathering. In general the beds are whitish, although layers of buff, cream, yellow, or dull gray are frequent. These colors depend much upon weathering. In composition most of the beds are as nearly pure carbonate of lime as can be found in nature, but some have small admixtures of silica, epsomite, chloride of sodium, and perhaps other salts as yet undetermined. Clay is absent except as a minor constituent in the few marly layers. Iron is sparingly present as pyrites, and is revealed by the red color of the clay that weathers out of a few beds. Exceedingly fine siliceous particles occur in the so called "magnesian beds " light brown porous beds which appear southward from Comanche County but no pebble, bowlder, lignite, or other undoubted piece of land derived debris has ever been found.
The limestones vary in degree of induration from hard, ringing, durable strata to soft pulverulent chalk that crumbles in the fingers and resembles very much the prepared article of commerce. Some of the beds are coarsely crystalline, with calcitized fossils, and are susceptible of high polish. The beds also vary in texture. Some of them are quite porous and pervious, while others are close grained and impervious. Some are homogeneous throughout; others have bard and soft spots, the latter dissolving by the percolation of underground water and constituting what is popularly termed "honeycombed" rocks. The harder spots in some cases seem to be in process of induration, suggesting a step in the formation of flints. The holes in the honeycombed layers often represent what were once spots containing soluble salts of iron and other accessory minerals.
South of the Paluxy River the formation can always be distinguished by the immense quantity of flint nodules which are embedded in and between the limestones and which lie scattered over the surface everywhere. These are of many shapes (Pl. XXX); some are fusiform, like elongated roots; others are knotty, like warty potatoes; others are parts of extensive sheets or very flat lenses. They vary in size from that of a lien's egg to a foot or more in diameter. They also vary greatly in color. Upon fresh fracture some are almost jet black; others light blue, gray, or opalescent; still others are delicate pink in color. There is some evidence that each particular kind occupies a definite horizon, but we are not prepared to state this as a positive fact.
"The geographic name Edwards is here substituted for the "Caprina limestone" of Shumard, the latter being abandoned because it is a paleontologic term; also for the term "Barton Creek limestone" of Hill, abandoned by him and revived by Cragin. The term "Barton Creek" is objectionable, and was abandoned by its author because it was not a good locality name and because it is a two word name. The name Barton has been applied to a division of the English Eocene for many years, but we doubt if, in the absence of any defined code of geologic nomenclature, such an objection may be considered valid.
228
In most cases the Edwards limestone may also readily be distinguished by the peculiar aberrant mollusks of the genera Monopleura, Requienia, and Radiolites bivalve fossils which have cornucopiate form, suggesting a resemblance in shape to the horns of cows, goats, and sheep.
The formation is stratified into a succession of massive beds accompanied by very few flaggy and marly layers. Some of the strata are harder than others and project beyond the softer layers in the profile of the hills as overhanging shelves; others are soft and erode very rapidly. South of the Colorado, where the Walnut formation becomes insignificant, the Edwards limestone is almost inseparable from the underlying Comanche Peak, since both are composed chiefly of carbonate of lime. The Comanche Peak strata are less consolidated, and, as they are somewhat argillaceous, possess a more marly texture than the Edwards limestone, which is usually a firm, white, ringing limestone of great hardness and durability; so that the Edwards weathers into cliffs, while the Comanche Peak is wrought into lower lying slopes; but in most cases reliance must be placed upon paleontologic determinations to distinguish the two formations.
Neither is the Edwards limestone always sharply defined from the overlying Fort Worth, except by paleontologic criteria. It is true that the Fort Worth limestone is slightly more arenaceous, but the differences are so slight that their detection requires the trained eye of the geologist. As the upper limestone is less than 75 feet in. thickness, the layman or well driller unversed in paleontology can nearly always be sure that any rock occurring 75 feet below the Del Rio clays belongs to the Edwards formation.
The Edwards limestone, being more purely calcareous than any other of the Comanche series, probably corresponds to the deepest and most extensive submergence of the Comanche epoch. It is true that in the Glen Rose formation occasional thin beds of chalk are met with. and that some of these are composed almost entirely of foraminifera, but such chalks usually contain a considerable percentage of clay, recognized as an offshore deposit.
Occasional bands of soft brownish yellow stone are intercalated with the limestone. These bands are popularly called "magnesian," and are composed largely of all exceedingly fine grained siliceous element like tripoli. As these beds often contain flints, the silex may be of organic origin.
Topographically, the Edwards limestone is one of the most important formations in Texas. In fact, it is the determining factor in the topography of the whole of the Edwards Plateau and Grand Prairie regions. "
See Plates LIV and LV, of typical fossils.
It is doubtful if the interior of the Edwards limestone is always as hard as the surface outcrop, for many of the chalky rocks of Texas harden or set on surface exposure.
229
Its hardness being superior to that of the overlying and underlying beds, its consequent resistance to erosion has preserved it as the capstone of the innumerable round II mountains" (buttes) and mesas of the State and of the extensive Edwards Plateau and Grand Prairie regions. Not only are most of the buttes and mesas capped by it, but these are accompanied by scarps overlooking the lower lying valley prairies which follow the stream. The walls of the canyons which many of the streams have cut are also composed largely of the Edwards limestone, especially the higher and headwater portions of those rising in the Edwards Plateau. To its hardness is also largely due the topography of the limestone mountains of Mexico.
It shows many types of weathering. Some of the strata make bold cliffs nearly 50 feet in height, the faces of which, although apparently of homogeneous texture, weather into small open caverns (Pls. XXXI and XXXII). This weathering sometimes brings out a thinly laminated structure associated with white efflorescence. The bottoms of caverns of this character are filled with a layer of white, pulverulent earth. The residual products of other massive ledges weathering into caverns are vermilion colored clays, in which are beautiful fossils composed entirely of crystallized calcite.
The hard limestones weather into vertical, square cut bluffs, while the soft and more homogeneous beds of marly or chalky texture form slopes. Where these hard and soft beds occur in alternation there is a corresponding alternation of scarps and slopes in the topographic profile. Some of the beds of homogeneous texture having great thickness weather into pyramidal hills, as seen along the monoclinal fold at the south edge in the northern part of Kinney County (Pl. XXXIII).
Near the summit of the Edwards Plateau, where the flaggy layers prevail, the slopes of the stream ways are gentle and are characterized by low, vertical steps, from 2 inches to 2 feet high, over which the stream descends from one rock layer to another. This slope is usually interrupted by vertical bluffs, composed of thick strata, which constitute a cornice in the profile of the canyon of the plateau. These slopes and scarps alternate until the base of the Edwards limestone is reached, beneath which the Comanche Peak bed weathers out in concave profile.
Where the Edwards formation forms extensive stretches of level country, such as that between Manchaca and Oak Hill in Travis County, in the western part of Williamson County, and the summit of the plateau, and the surface stratum is of homogeneous texture, it weathers into millions of miniature ridges, crests, and drainage lilies, illustrating the whole process of erosion and mountain carving. These minutely eroded limestone surfaces are technically known as "karrenfelder" (see Pl. L, p. 318), and they are formed by the solvent effect of the rainfall upon the sun heated limestone surfaces. The crevices in these level areas of Edwards limestone country are usually grasscovered,
230
with occasional patches of scrub oak. The surface is very rocky, the karrenfelder protruding in jagged points through the rich but, scanty soils. Sometimes residual flints occur in such immense quantities over these surfaces that one is apt to mistake them for a water rolled gravel formation.
In Kinney County the high hills north of Fort Clark, with the exception of Las Moras Mountain, are made up largely of the Edwards limestone. The beautiful canyon of the Pecos, near its mouth, as shown in Pl. XXVIII is one of the finest 39 examples of its outcrop.
A small, narrow belt also outcrops on the downthrown side of the Balcones fault. In Travis County an area of it occurs, as shown on the map (see Pl. XLVI, p. 282), along a belt lying between the International and Great Northern Railway and a line drawn through Oatmanville (Oak Hill) and Mount Bonnel, and many other places as far west as the Frio. Aransas Pass Railway crosses it just south of Leon Springs.
It is well displayed in the banks on the south side of the Colorado in the western part of the city of Austin between McDonald's brick yard and the city dam. Owing to faulting, this section is somewhat complicated and not continuouly exposed at any single locality. The details as made out at three localities at Austin are shown in the accompanying sections and figures (fig. 57). It will be noticed that in the lower portion of the Bee Creek section, which is still above the base of the whole of the formation, arenaceous marls and limestones are quite numerous. These play an important part in the artesian conditions from the Colorado southwestward.
The accompanying sections, Nos. 4, 5, and 6( see figures 57, A, B, C), represent the entire thickness of the Edwards limestone exposed on the down thrown side of the fault in the bluff's of the Colorado between Austin and the river level at the mouth of between Bee Creek.
The base of the beds is concealed, lying probably less than 100 feet below No. 1 of section C, but can be seen on the upthrown side of the fault, capping the remnants of the plateau.
231
 |
 |
The Fort Worth limestone is assumed to be 70 feet thick at Austin. Future study may modify this estimate. The uppermost layers, characterized by Kingena wacoensis, are missing in the section.
The beds in the Barton Creek section below 43 can not be correlated layer for layer with the Deep Eddy Bluff section; therefore numbers are not used in the description of the former section for beds below the one numbered 43 (except in one case, 29). The numbers in the different sections indicate equivalence.
232
 |
 |
233
 |
The total thickness of the Edwards limestone exposed in the vicinity of Austin, as determined from the foregoing sections, is as follows:
 |
In the Nueces section the upper 100 feet consist of flaggy layers of bard white limestone devoid of flints. Below this are ledges of yellowish limestone of considerable thickness, marked by numerous black flints. The central portion is of white limestone of homogeneous texture, in which large caverns occur. The lower portions consist of thick and thin ledges and flags containing considerable numbers of flint nodules or strata of flint and many honeycombed layers.
234
Overlay. On the summit of the plateau in Edwards County (altitude 2,375 feet) are occasional low, monad shaped hills of clay marl containing thin bands of limestones with Terebratula (Kingena) wacoensis and Exogyra arietina, estimated not to exceed 50 feet in thickness. These beds belong to the lower formations of the Washita division.
 |
235
 |
The foregoing section is a composite of several small sections and is only approximate, having been measured by aneroid barometer, but it will suffice to give a general idea of the Edwards formation in the Nueces canyons it will be noticed that the total thickness is more than double that of the Colorado section.
FORT WORTH LIMESTONE.
This formation consists of a group of impure white limestones, regularly banded, and alternating with layers of marly clay. (See upper part of fig. 57, p. 230, and lower part of fig. 58, p. 236.) Before exposure they are dull blue in color, but when weathered they are white or yellowish. The lower portion of the section as exposed at Austin contains thicker and more massive beds than the upper. They are paleontologically characterized by Epiaster elegans,Ammonites (Schlcenbachia) leonensis,Gryphaea washitaensis,Exogyra americana,Kingena wacoensis, etc. These fossils occur throughout in definite zones and associations, and some of the strata are composed almost entirely of them.
Above the more massive lower layers is an agglomerate of Gryphaea washitaensism. Associated with this is found an oyster, Alectryonia carinata, a familiar European form, occurring only at this horizon in the Austin section.
At the top is a stratum of massive limestone less than 3 feet thick, consisting of a homogeneous calcareous matrix thickly studded with Kingena wacoensis. (See Pls. LVI and LVII for figures of most of these fossils.)
The formation does not exceed 75 feet in thickness in the Colorado River section. Southwestward, toward Brackett, it becomes less and less distinguishable from the underlying Edwards limestone and is recognizable at only a few places. So far as the question of artesian waters is concerned, it would be better to consider it as the upper 70 feet of the Edwards limestone rather than as an independent formation. As a rule these beds outcrop only immediately along the western margin of the Rio Grande Plain, at the foot of the Balcones escarpment, and occupy an exceedingly narrow belt from Austin to Del Rio.
"Paleontologically, the term Fort worth limestone should be limited to the lowest member of this Austin section or to the thicker beds carrying Epiaster elegans and Schlcenbachia leonensis, for the few feet of upper marls are probably the southern attenuation of the Denison formation. The lowest formation of the Washita division (Preston) has not been found at Austin, or to the southward, although its equivalent has been reported by Taff at Georgetown. Williamson County, and is probably represented by a few feet of limestone below the Fort )Forth just south of Round Rock in the same county. For definitions of Denison and Preston formations consult Bull. Geol. Soc. America, March 1894, Vol. V, pp. 303, 324-332.
236
Hitherto it has been supposed that these beds did not occur on the summit of the Edwards Plateau, but our observations in 1895 showed that oil the highest summit of the plateau in Edwards County small areas are preserved, as can be seen at Anderson's ranch, between the headwaters of the Guadalupe and the Frio rivers, on the Kerrville and Rock Springs road.
From the paleontologic notes of Dr. G. G. Shumard, who made an expedition across the region before the present geologic classification was made, we are led to believe that fragments of the beds are quite extensively preserved on the western side of the plateau, between Fort Clark and the Pecos. There can be little doubt that the whole plateau was once capped by these beds, and probably higher beds that have since been almost entirely removed by erosion.
DEL RIO CLAYS.
These are peculiar greenish blue laminated clays which weather (lull brown or yellow and form a very black soil. They are some 80 feet thick at Austin, where they have their typical occurrence in Shoal Creek and at Fish Pond Bluff, at the mouth of Barton Creek. They outcrop immediately beneath the Shoal Creek limestone, and rest upon the Fort Worth limestone, the uppermost band of which is characterized by the occurrence of Kingena wacoensis. Theyare an especially important landmark in the geologic column, marking a break in a monotonous sequence of limestone beds, and possessing lithologic and paleontologic characters which render them easily recognizable. They can always be identifiedby means of a peculiar fossil, Exogyra arietina, a little oyster shown on
A Partial Report on the Geology of Western Texas, etc., during the Years 1855-1856, by Prof. G. G. Shumard, Austin, 1886, pp. 69-77.
The geographic name Del Rio clays is here substituted for the paleontologic designation Exogyra arietina clays of previous writings.
237
P1. LVIII, figs. 2a-2e. This occurs in the greatest abundance, weathering out by the thousand in a state of perfect preservation. Attached to these shells, especially the umbonal region, are small cubes of iron pyrites. Upon decomposition this coats the shells with thin layers of brown hematite, and converts the lime into numerous crystals of fibrous selenite, which are intercalated in the seams adjacent to the shell horizons. In places the shells are cemented into thin layers of indurated argillaceous limestone, making persistent bands in the middle of the clay bed.
Above the zone of Exogyra arietina the clays are somewhat barren of fossils until near their summit, where they become slightly arenaceous and contain impure limestone slabs bearing other fossils, some of which also occur in the upper layers of the Fort Worth limestone. Among these fossils is a gryphaeate oyster, G, mucronata of Gabb.
The Del Rio clays occur as occasional patches on the Edwards Plateau, in central Edwards County, between the headwaters of the Frio, Nueces, and Llano rivers. They were noted on the road from Dieter's ranch to Rock Springs. According to the paleontologic notes of Dr. G. G. Shumard previously quoted, there are also areas on the plateau between Fort Clark and the Pecos River.
South of the Colorado River these clays appear at various places along the interior margin of the plain adjacent to the Balcones fault, in Hays, Comal, Bexar, Uvalde, Kinney, and Valverde counties, at least as far west as Del Rio, 200 miles southwest of Austin,
SHOAL CREEK LIMESTONE.
This has its characteristic exposure along the Colorado, in the steep scarps of Shoal Creek in the city of Austin, and in the bluffs on the south side of the river, where Bouldin Creek enters the valley, at the crossing of the International Railroad and the Oatmanville road. It forms precipitous cliffs, with toppling projections, owing to its jointed structure.
Its outcrop oxidizes to a slightly darker color than the limestones "
The senior author in a previous paper has said that "this fauna is the upward limit of the grand fauna of the washita Division, the subfaunas of which show connection by a few common binding species," and that, on the other band, not a single species passes upward from the Exogyra arietina beds into the Shoal Creek (Vola) limestone, thus showing between these beds a life break as marked as is the lithologic change. Later studies have shown a few connecting species between the Shoal Creek and Del Rio formations.
This has been called "Gryphaea pitcheri" by Roemer, and is the "G. navia," or "G. pitcheri var. navia," in part, of the writings of Shumard, white, and of all the reports of the Texas survey.
238
already described. On fracture it is light yellow, with blotches or spots of pale pink, as if it had been subjected to fire. lit places it is very hard, but in general is of varying texture, usually lumpy; in some spots it is efflorescent and decays into a soft, pulverulent material with slightly saline taste.
The minute red and pink blotches are peculiar to this limestone and have given to it the local name of "Burnt limestone." Microscopic study has revealed the fact that the rock is made up largely of foraminifera, filled and coated with a mineral, which in all probability is glauconite. Exteriorly the limestone presents no appearance indicating that it contains foraminiferal remains, but, so far as examined, it is more largely composed of them than any rock of the whole series. In one thin section Rotalia, Textularia, Globigerina, and fragments of three or four other genera of Foralninifera have been recognized.
The outcrop of this formation is proportionately very limited, being better displayed at Austin than at any other locality. It can be found in the bluff's just below the Balcones scarp line near by many of the streams, such as Bear Creek at Manchaca;Onion Creek at Buda;the San Marcos at San Marcos Springs;the Frio, and along the West Fork of the Nueces,between Turkey Mountain and its mouth.
In the Austin section the Shoal Creek limestone rests without apparent gradation upon the Del Rio clays, indicating a rapid physical change in sedimentation, but in the Uvalde country there is intergradation.
GULF SERIES (UPPER CRETACEOUS)
The Upper Cretaceous rocks in the portion of Texas under discussion in this paper are found only in the Rio Grande Plain. They are composed mostly of calcareous clays or other soft and unindurated strata. Owing to the generally soft and friable character of the material, the surface composed of it is naturally rather level or gently undulating.
The rocks of this series contain but little water, and their principal function in relation to the water question is that they serve as a cover for the water bearing rocks proper. It is essential, however, to have some knowledge of them to be able to estimate the depth of the water-bearing beds beneath the surface.
"The relations of the Upper Cretaceous formations of southwestern Texas to those of other regions are discussed in a paper, nearly ready for publication, by T. Wayland vaughan, entitled Reconnaissance in the Rio Grande Coal Fields of Texas." intended for the Bulletin series of the Geological Survey.
239
EAGLE FORD SHALES.
This formation consists of laminated clays, shales, and impure limestones, usually blue or black when unweathered, but becoming light yellow and white on exposure. Both the shales and clays are distinguished from the other Cretaceous rocks, especially those immediately above and below them, by their laminated character. The beds usually contain remains of fishes, such as scales, teeth, and small bones, and a few mollusks, principally inocerami. (Pl. LX.)
At Austin the shales are exposed in the Sixth Ward and along the breaks of Shoal Creek, especially where Pecan street crosses it. They are finely displayed also in Bouldin Creek, on the south side of the river. At San Antonio they outcrop near the cement works and furnish the. material from which Portland cement is made. They are also greatly developed around Fort Clark and at other points in Kinney and Valverde counties, the town of Brackett being situated upon them. In the last mentioned area the formation consists of thick, flaggy, hard, or chalky limestones with interbedded or interlaminated marly layers.
The Eagle Ford shales are rarely over 50 feet in thickness anywhere in the vicinity of Austin. Near Brackett they are fully 250 feet thick.
AUSTIN CHALK.
This formation consists of impure white chalky limestone with a conchoidal fracture, and is usually free from grit. It is so soft on fresh exposure that it is easily cut with edged tools. In places massive beds are interstratified with very chalky marls. Under the microscope the chalk exhibits a few calcite crystals, particles of amorphous calcite, and a great slumber off the shells of foraminifera and other minute organisms. The air dried, indurated surfaces are white, but the saturated and unoxidized rock below the surface has a bluish color. The rock usually weathers in large conchoidal flakes.
In composition it varies from 85 to 94 per cent of calcium carbonate, the residue consisting of magnesia, silica, and a small percentage of ferric oxide.
It is easily distinguishable by its characteristic fossils, but superficially resembles closely some of the beds of the Comanche series. The Comanche limestones are usually harder and more crystalline, but this distinction can not always be made.
The Austin chalk is of great uniformity, presenting few local variations. Its thickness is difficult to determine, but averages about 500 feet in other parts of the State. The Manor well shows the thickness at Austin to be 410 feet, and from San Antonio westward to the Rio Grande it is probably 500 feet . It is a most important bench mark or datum plane in the determination of the depth of underground waters, and will be frequently referred to in the economic discussions in this "
Fifteen hundred feet has been given as the thickness of this formation along the Rio Grande (Bull. Geol. Soc. America, Vol. III, 1892, p. 229), but this is probably excessive.
240
paper. The outcrop follows the northwestern margin of the Rio Grande Plain from Austin, via New Braunfels and San Antonio, to Fort Clark, crossing the Rio Grande between Del Rio and Eagle Pass. The greater portion of the city of Austin east of Shoal Creek is situated upon this rock, and the foundations of the State capitol are built upon it.
TAYLOR FORMATION.
The Austin chalk is overlain by a deposit of calcareous clays locally known as "joint clays," estimated to be about 540 feet in thickness in the Colorado River section. When fresh, these beds are fine grained, tough, unctuous, blue clays. They are apparently unlaminated until exposed to weathering, when their laminated character is developed. Their accessory constituent is lime in a chalky condition. Upon atmospheric exposure their color, owing to oxidation of the contained iron, changes to a dull yellow.
Because of their rapid surface disintegration the character of the unaltered beds is seldom seen, except when fresh material is brought up by the well-digger or exposed in freshly cut ravines or creeks. At the Blue Bluffs of the Colorado River, 6 miles east of Austin, there is a good fresh exposure.
At the top, as seen at various places in Travis County east of Austin, they grade into the marls of the Webberville formation. Their middle portion apparently contains no well-preserved fossils, but impressions are abundant in places. In the base of the beds Exogyra ponderosa, a large, heavy oyster, is abundant. (P1. LX I II, fig. 3a, 3b.)
ANACACHO FORMATION
In Uvalde and Kinney counties, in the stratigraphic position occupied to the eastward by the Taylor marls, is a series of hard yellow and white limestones with interbedded marls and occasional sandstone ledges, for which the local name Anacacho formation is proposed, after the locality of their characteristic occurrence, the Anacacho Mountains of Kinney County, which are capped by this formation.
The following is a section at the east end of the Anacacho Mountains. "
Exogyra ponderosa marls of the earlier literature.
This name is applied to massive clays, because of their tendency to break into blocks.
241
It gives a good idea of the constitution of the beds. Fig. 61 represents in a diagrammatic manlier the general character of the beds, showing those that form slopes and those that because of their greater hardness produce escarpments.
 |
The measurements were made with an aneroid barometer and must be regarded as only approximately correct.
The deposit of asphalt at the Lithocarbon Rubber Company's mine is in the Anacacho limestone. The asphalt-bearing horizon corresponds with the upper part of No. 6, above.
WEBBERVILLE AND EAGLE PASS FORMATIONS.
The highest beds of the Cretaceous outcrop along the eastern margin of the Black Prairie region and of the northern portion of the Rio Grande Plain. In the Texas region they present two distinct lithologic "
These beds contain a peculiar form of Rudistes not hitherto found in the Upper Cretaceous of this country. It will be studied by Mr. Stanton of this Survey.
242
facies, though the beds are probably synchronous. In northern Texas and as far south as the Colorado River the general aspect is that of the Upper Cretaceous formations of the Atlantic and Gulf States from New Jersey into Texas, which consist of sands, marls, dark clays, or other impure sediments, usually characterized by the appearance in some form or other of particles of the mineral glauconite (greensand) in the various beds. From San Antonio westward to El Paso the beds of the uppermost Cretaceous consist of yellow clays, impure ferruginous limestones, and beds of lignite, and have a general increase in thickness, showing a transition to the facies of the synchronous beds of the Rocky Mountain region.
Along the Colorado River, below the mouth of Onion Creek, the Taylor formation grades upward into glauconitic marls with beds of impure limestone and black clays. The local term Webberville has been used for these. They are exposed at only a few localities in eastern Travis County.
In the First Annual Report of the Texas Geological Survey, page 20, the characteristic outcrops of Webberville beds are well described, but the formation is erroneously referred by Penrose to the Wills Point beds of the Basal Tertiary, as follows:
On the Colorado River it is seen outcropping at a point 16 miles by river below Austin, and 1 mile below the mouth of Onion Creek, in a bluff some 40 feet high and a mile long. Also at Webberville, on the line between Travis and Bastrop counties, where it is seen in a low bluff just above the water's edge. This is a much darker and more massive clay than that seen in most other outcrops. In the bluff 16 miles below Austin are found a few fragments of fossils, but they are all so broken as to make their determination very doubtful.
The investigations of this Survey in 1894 have shown them to contain fossils characteristic of the upper division of the Upper Cretaceous, identified by -Mr. T. W. Stanton, as follows : Anomia conradi, Leda protexta, (Corbula crassiplica, Drillia ? distans, Sphenodiscus lenticularis.
Although probably not the highest beds of the Cretaceous system, the Webberville beds are the highest Cretaceous exposures seen along the Colorado River, for below Webberville they are overlain by the basal division of the Eocene Tertiary.
"In order to distinguish the allied facies, the senior author has in previous papers spoken of the northernmost of these areas, presenting the Atlantic States facies as the glauconitic division and those of the Rio Grande and trans-Pecos region as the Montana division, the latter name having been previously used for the allied formations in the Rocky Mountain region. The term "Glauconitic" has always been considered unsatisfactory, inasmuch as it is not a geographic word. The word Montana can not be used with certitude for the beds of the northern area until they are proved to be identical on paleontologic grounds. The beds of the two regions are quite different in lithologic aspect, and they must be distinguished until they are proved to be the same. The term "Ripley " has been used as a generic one for beds which we know belong to this division in Texas, but without specific definition. Since the original use of the word Ripley in Mississippi was restricted to some of the beds of the many in that State composing the equivalent of this division as a whole, it is hardly appropriate to apply that name to the entire uppermost division in Texas.
Preliminary cheek list of Cretaceous fossils of Texas, by Robert T. Hill: Bull. 4, Geol. Survey Texas, Austin, 1889, p. xxx. Also, First Ann. Rept. Geol. Survey Texas, p. 115.
243
From the Colorado River southwestward through San Antonio and onward to the Nueces the beds of this upper division appear only occasionally along the watercourses, where erosion has cut through the overlying Neocene and Pleistocene deposits.
Near San Antonio, as shown by the Terrell artesian well borings, they are represented by a great thickness of lignitic joint clays, estimated at 600 feet.
In the Rio Grande Plain, southwest of San Antonio, and in the foothills of the Santa Rosa Mountains of Mexico, they attain great thickness, but differ entirely in detail from the north Texas extension, lithologically resembling very much the Fox Hills and Laramie beds of the Rocky Mountain region. They are well displayed from 12 miles above Eagle Pass to the Webb. County line in Texas, and in Mexico south of the Sabinas River, north of the Santa Rosa Mountains, where they consist mostly of glauconitic sands, alternating with limestones, clays, and beds of lignitic coal. Near Eagle Pass they are probably 2,000 feet thick, and the whole formation may be even greater. They abound in fossil wood and bones, and are of great economic value as coal producers, the mines at San Felipe, Eagle Pass, and Sabinas being located in them. Fig. 62 illustrates the general character of the beds at Eagle Pass.
THE EOCENE.
These beds overlie the highest Cretaceous, and consist of ferruginous sand, clays, and some impure marls. Lignite beds are very abundant. They mark the eastern and southern border of the portion of the Rio Grande Plain described in this paper, and the discussion of them does not properly fall within its province. A line from Littig through Lytton Springs, Lytle, and the southeast corner of Maverick County approximately marks their interior border. The artesian wells deriving water from these formations, such as those at Carizzo Springs, will be treated in a separate paper.
ALLUVIAL DEPOSITS OF THE NEOCENE, PLEISTO- CENE, AND RECENT EPOCHS.
These formations comprise the products of upland degradation, and have been laid down by streams at local base levels. They are composed entirely of the debris of the Cretaceous uplands, except immediately along the Rio Grande and Colorado, where other material is mixed with such debris, the lithologic
244
aspect of the beds remaining, however, practically unchanged. As they are the chief source of the surface wells of the region, as well as of many important springs, they will be briefly described. The final classification of these deposits is not complete, but for convenience they will be temporarily grouped as follows:
The Uvalde formation ("Upland gravel"), supposedly of Pliocene age.
UVALDE FORMATION.
In the Rio Grande Plain lying off the foot of the Balcones escarpment, from San Gabriel to Devils River, and extending coastward many miles, there is a remarkable geologic formation or series of formations to which the name Uvalde has been given. It consists of a vast deposit of gravel, composed almost entirely of rolled flint pebbles, with occasional pieces of limestone, partially embedded in a matrix of chalky marl and clays. Most of these materials have been derived from the decay of the Edwards limestone of the plateau, and spread like a mantle over the lower plain. In places where the marly material predominates over the pebbles, as on the high divide between the Colorado River and Onion Creek, southeast of Austin, east of San Antonio, and elsewhere, the formation weathers into dense black soils bearing great resemblance to the residual soils of the Upper Cretaceous beds, upon which the Uvalde formation rests. No fossils have as yet been found in it. It caps the higher divides in the Rio Grande Plain, and constitutes the highest terrace level in the canyon valleys of the, plateau. North of the Colorado River it caps the hills west of Manor, having an altitude of 750 feet. South of the river it covers much of the high divide, the St. Elmo Plateau, between it and Onion Creek. The divide of the Blanco, Onion Creek, and Colorado drainages in northern Hays and Southern Travis counties is also made up largely of it. Similar remnants are found in many places southward toward "
None of the superficial and gravel deposits of the Texas region are of glacial origin, as people frequently suppose them to be. There is no reason to modify the opinion expressed by Dr. Roemer many years ago: "At the close of the remarks on the formations of the diluvium in Texas, it may be proper to point out that no trace is found in Texas of any drift blocks or gravel deposits of northern origin, which, of course, is quite in harmony with the well-known distribution of northern drift in the basin of the Mississippi, in which the erratic blocks, so common in the vicinity of the Great Lakes, nowhere extend as far as the Ohio, thus proving that the conclusion gained in Europe, that the erratic phenomenon is of northern origin, applies also to the American continent." (Kreidebildungen von Texas, 1852, p. 4.)
Am. Geologist, June, 1891, p. 368.
245
San Antonio. The heights surrounding the latter city are largely capped by it. An instance is the Fort Sam Houston hill.
This flint gravel formation is found along the lines of the Southern Pacific from Seguin to Del Rio, constituting, with few exceptions, the divides between the streams, and attaining a considerable thickness. Usually it extends up to the very foot of the Balcones hills, but occasionally, as in Uvalde and Kinney counties, its border lies farther south. A similar gravel constitutes the level prairie upon which the town of Spofford, in Kinney County, is situated, and extends northward to the foot of the highlands. This is 40 feet or more in thickness, and contains locally derived. igneous as well as Cretaceous material.
Southwest of Uvalde, on the divide between the Leona and Nueces rivers, along the Eagle Pass and Carrizo Springs roads, the matrix of the formation is a fine silt that makes a stiff, black soil when wet. In the higher places and around the edges of the waterways in the wide, flat caletas the gravel is exposed. The gravel was at first embedded in the fine silt and covered by it. In high places the silt has been washed off the gravel and has accumulated in the caletas, the gravels having been too heavy to be moved by the sluggish waters. Along the stream ways in the draws the water has swept the silt away, leaving the gravel exposed in the lower part of the banks, while the silt forms the upper part.
Along the borders of the Rio Grande the surface of the formation, in spite of the incision of the drainage, still preserves well its original plain character. The altitude of the formation is higher in its western than in its eastern portion, its surface sloping both east and south. Its height above sea level between Spofford and Del Rio is from 1,000 to 1,100 feet; in the vicinity of Austin its level is between 650 and 750 feet; on the Rio Grande, near Santo Tomas, in Webb County, it occupies an elevation of between 600 and 650 feet. It does not cease at the Rio Grande, but extends far into Mexico.
The distinction to be drawn between the Uvalde formation of Rio Grande deposition and that of the rivers of the region farther east lies in the difference in the character of the material composing the gravels. As has already been noted, the gravel in the latter area is composed almost entirely of flints derived from the disintegration of the limestone of the Edwards Plateau. In the former region the material has been derived to a large extent from sedimentary rocks older than the Cretaceous and from eruptive masses occurring along the course of the Rio Grande above the mouth of the Pecos River. However, mixed with the gravels of igneous material and pre Cretaceous sediments is a large percentage of pebbles of Cretaceous rocks, comprising Edwards limestone, other limestone, and flint pebbles, etc.
Another interesting peculiarity in this region is, the pebbles are frequently cemented into a firm conglomerate by a chalky matrix. Often the pebbles are very rare and the formation is represented by the lime.
246
stone alone. The two hills in Webb County north of Santo Tomas, known as the Dos Hermanos (Two Brothers), are buttes capped by a white chalky indurated limestone containing a very small number of pebbles. The cap rock is almost a pure limestone of "tierra blanca" (white earth), as described on page 256.
Besides the distribution of the formation in the Rio Grande Plain, it extends up the canyons of the larger streams flowing from the Edwards Plateau, making the highest terrace, standing about 50 feet above the stream bed, and is composed mostly of flint gravel. As the streams descend from the plateau and approach the plain country, their canyoned valleys become wider and the Uvalde formation becomes more and more extensive, spreading out and covering all of the divides after the belt of the Balcones fault disturbance has been traversed.
It is evident that there was a long period of canyon cutting preceding the deposition of the Uvalde, and this period was probably during Miocene and early Pliocene time.
The coastward extent and relations of the Uvalde formation have not been investigated. It extends southward beyond Santo Tomas, in Webb County, and eastward of the International and Great Northern Railroad from Seguin to Laredo.
There can be but little doubt that the Uvalde formation is of the same age as the plateau gravel of Arkansas and northeast Texas and the Lafayette formation of the Gulf and Atlantic region east of the Mississippi. These have been correlated by Hill, by Penrose, and by McGee.
Dumble has considered the Reynosa limestone of Penrose as only a phase of the extensive upland gravel deposit described above, and extends the term Reynosa so as to include the whole of the gravels. According to Penrose, this limestone is hard and whitish, and occurs about 50 feet above the Rio Grande in the town of Reynosa, State of Tamaulipas, Mexico. As we have not been able to make studies in the vicinity of Reynosa, we have not sufficient data to pass judgment on Dumble's treatment of Penrose's term. bumble states that the seaward extension of the Uvalde (Reynosa) passes beneath the Gulf coast clays (Port Hudson clays of Hilgard).³
As nearly as can be determined, the deposition of the Uvalde formation took place in late Pliocene time.
It does not appear to the writers that it is necessary to postulate a marine submergence or an absolutely horizontal deposition level to explain the Uvalde formation within the area considered in this paper. At most a large portion of the deposit was laid down by the process later described under the heading "The wash" (p. 254), when the country stood lower than now, but was undergoing gradual uplift, the streams "
The Lafayette formation : Twelfth Ann. Wept. U. S. Geol. Survey, Part I, 1891, pp. 347-521.
First Ann. Dept. Geol. Survey Texas, 1890,1).63.
Bull. Geol. Soc. America, vol. 3, 1892, p. 230.
247
debouching upon the plain seaward of the Balcones scarp, and probably inundating its expanse at various flood periods. The shifting of the channels of the streams was also probably instrumental in distributing the debris.
PLEISTOCENE TERRACES.
After the deposition of the Uvalde gravel sheet or sheets the rate of elevation of the land increased somewhat, and drainage valleys were successively cut through these older gravel formations. In the valley, not only in the general region of the Coastal Plain, but over the entire Central Denuded region, extending up to the breaks of the Plains and into the canyons, are many alluvial talus fans and terrace deposits, varying in number and character with the age, position, and character of the streams. Streams of larger magnitude, like the Rio Grande, Pecos, Colorado, Nolan, Brazos, and Red rivers, show more of these terraces than those of more recent origin. ; Theoretically the different deposits must merge in their coastward extension into delta or estuarine beds, and are no doubt represented in the coast sediments.
TERRACES OF THE COLORADO RIVER.
No better expositions of terrace phenomena can be found than those seen adjacent to the Colorado River as it crosses Travis County, and especially in that portion of its course lying to the east of the main fault line. These phenomena will be illustrated and described in detail in the forthcoming Austin folio of the Geologic Atlas of the United States. Their general relations are illustrated in the sections of Pl. XXX V, A-C. In this vicinity several categories of terraces can be distinguished, as shown in the accompanying table (p. 248), including various stages in the history of the Colorado River, from the oldest (Uvalde) plains on the high divides to the present river bottoms. The light colored, sometimes chalky, matrix of the fundamental formations out of which the Colorado has eroded its valley affords a favorable foundation for the clear demarcation of the various terraces. Their elucidation is further facilitated by the material of the terrace deposits themselves, consisting as they do of three entirely distinct and contrasting materials, viz,
The terrace phenomena associated with the major streams, such as the Red, Brazos, Colorado, and Rio Grande, as well as those of the Trinity, Guadalupe, and Nueces, have been the subject of detailed investigation by the authors.
248
The Uvalde beds previously described may be considered the most ancient or earliest of this series of formations, while the present high water flood deposits of the river are the most recent. Between these there are many steps or stages.
The following table shows the locality, height, and composition of sonic of the terraces recognized on both sides of the river. The altitudes given represent as nearly as possible the original deposition surface of the formation.
 |
On the north side of the river these terraces call be divided into two categories:
249
This terrace lies some 215 feet above the Colorado River, and extends nearly 4 miles back from the present river bed, forming the summit of a high bench that usually marks the exterior margin of the so called Colorado bottoms. In the city of Austin it commences north of the university and constitutes the sandy post oak flat in the northern part of that city upon which the lunatic asylum stands. This terrace extends down the Colorado River, preserving its position at the summit of the bluff along the river flat, into the Bastrop quadrangle, where it may be related to the gravel on the divides on the south side of the river in the vicinity of Lytton Springs. West of Shoal Creek, as the line of the Balcones fault is approached, occasional remnantal patches have survived the vigorous erosion to which the formation has been subjected, but the terrace once extended to the foot of the escarpment.
We are not prepared to determine the stratigraphic position of the Asylum terrace south of the Colorado in the vicinity of Austin, although patches of it may still be preserved south of Barton Creek, on the road to Oatmanville, and in occasional places along the river bluff north of Delvalle. South of Onion Creek and extending far down the river this terrace has a great development, and probably spreads out over a large area. Its material consists largely of granite debris, granite pebbles, quartz, and red colored plastic material derived from the decomposition of feldspar, brought by the river from the Burnet granite region to the northwest. There are also pebbles of schist and Paleozoic limestone from the Burnet Llano region, and a little flint from the Edwards limestone.
The character of the material of the Uvalde formation shows that during the deposition of the oldest of these beds the Colorado River had probably not cut through the Cretaceous covering that overlay the Paleozoic area of central Texas. The Asylum terrace material, however, tells definitely that at the time of its deposition the river had cut into the older formations of the Burnet Llano region and was vigorously eroding them.
There are several minor terraces below the Asylum terrace, the debris of which occurs in the university grounds. Some of these are as follows:
The Capitol terrace.-The next lower conspicuous terrace is that which
250
forms the surface of the capitol grounds and the residence portion of Austin. The best preserved example of this is in the city block northwest of the old Catholic Church. It can be traced eastward down the river, Tillotson University being located upon it. East of that institution it gradually merges into the wide flats of the Colorado. South of the river this terrace forms the top of the high bluff or valley wall of the Colorado bottom at the State Asylum for the Deaf and Dumb. Fragments of it can be traced far down the river. The material of this terrace is the same kind as that of the Asylum terrace; i. e., it consists mostly of granitic debris brought from the Burnett Llano region. It is presumed that the finer silts which originally constituted a part of the Asylum and Capitol terraces have been removed by erosion. This terrace is also well shown west of Austin, between Shoal Creek and the river.
Some 40 to 60 feet above the river at low water and above the present limits of the highest water in times of overflow is a vast flat, or "second bottom," often several miles in width. The bottom in places consists of five or more terraces, forming steplike descents to the river. These are specially well shown near Montopolis Bridge. Their elevation one above another is only 5 to 10 feet. It is very evident that this whole area was once the flood plain of the Colorado, and, although not now subject to overflow, it is locally termed the Colorado bottom. The highest of the second group of terraces is that upon which the main business portion of the city of Austin is built. It is well shown along Fourth street, particularly just back of the Board of Trade building. About 10 feet below this is the top of the terrace exposed in the bluffs of the Colorado at the city bridge. The summit of this is about 60 feet above the river, and is composed largely of red loam. Four distinct later terraces, in all, can be seen above the present stream way in the cross section of the Colorado at Austin from Fourth street to the bluffs just north of the Deaf and Dumb Asylum.
The material of the lower group of terraces is usually fine sandy loam, with occasional beds of gravel or clay. At Austin excellent bricks are manufactured from the clay beds. During the time of the formation of these terraces the headwater drainage of the river had worked its way back and down into the Red Beds country, and was bringing down material similar to that which now gives the Colorado water its characteristic color, and from which the river derives its name.
The present flood plainThe bed of the river is usually shallow and sandy, the stream occasionally cutting into the underlying bed rock. The higher flats are covered, by water in time of overflow. The alluvium is reddish, similar to that of the flats above described, having been deposited largely by the "red rises," which are originated by the heavy rains that fall in the Red Beds region near the principal head waters of the river. There are also occasional deposits of debris of
251
the Paleozoic and Cretaceous rocks, mostly derived from the sediment of the "white rises," which usually originate in the drainage basin of the Llano River.
Terraces on the south side of the Colorado RiverThese differ from those on the north side in two particulars: First, older and higher deposits are preserved on the south side of the river, such as those seen capping the hills 3 miles southeast of Manchaca and Creedmoor, at elevations as high as 750 feet. No stream deposits of equivalent height are found on the north side of the river.Second, below this level, occupying the same level as the Bald Knob Plateau near Manor, are several gravel capped outliers. Some of these, such as the one at the fork of the Oak Hill and Manchaca roads, are composed of the old granitic debris, while the St. Elmo divide, occupying the same level, is made up entirely of flint material.This would indicate that at the time of their deposition a stream of the former material was coming down the Colorado, while the flint was being deposited by its laterals. This is the highest elevation at which the granitic material has been found. There are other instances of terraces on the south side of the river occupying the same level as those on the north side but composed of radically different material. For instance, the Onion Creek marls, as seen at Bluff' Springs bluff, occupy an elevation almost identical with that of the Capitol terrace, while those of Delvalle are analogous to those of the bluff at the city bridge in Austin. The Onion Creek and Delvalle terraces are composed of calcareous marl derived from the Cretaceous formations, and the others of granite debris and of the fine brick-clay silt derived from Permian and granitic materials. These differences in the material at the same level on opposite sides of the river are readily explained : Barton Creek and Onion Creek, two of the principal laterals of the Colorado, drain only Cretaceous formations, and have brought down only material derived from them,while the granitic, Paleozoic limestone, and Permian materials were brought down by the Colorado itself.
TERRACES OF THE RIO GRANDE.
The Rio Grande, although a stream of major magnitude, presents, between Del Rio and Laredo, terraces considerably different in detail from. those of the Colorado River near Austin. The Uvalde formation extends to the edge of a high erosion bluff that forms the outer border of the stream valley of the Rio Grande from Del Rio an indefinite distance southward. The bluff is steep, and its summit is 150 or 200 feet above the river. The foot of the bluff marks the outer margin of a terrace usually several miles wide and about 120 feet lower than the top. The descent from this wide terrace to the present flood plain of the river is by a succession of minor terraces, usually three or four
252
in number. The usual relation and succession is indicated by fig. 63. Plate XXXVI, reproduced from a photograph, shows the topographic expression.
At Palafox, Webb County, there are remnants of a greater number of terraces than were seen elsewhere. Fig. 64 illustrates their occurrence.
ONION CREEK HAUL AND ALLIED DEPOSITS.
Occupying an intermediate altitude below the level of the Uvalde formation and above the present flood plains of the numerous secondary streams of the Edwards Plateau and Rio Grande Plain, there is a formation which consists of a faint yellow or salmon yellow calcareous marl, sometimes accompanied by fine pebble conglomerate, all of which is derived from the Cretaceous limestone material. It is usually less than 50 feet in thickness.
A type locality for this kind of material is the valley of Onion Creek from near its mouth to the eastern end of Pilot Knob Canyon, and from the western edge of Pilot Knob Canyon up that stream west beyond
253
Cope, a species hitherto found only in Nicaragua, where it is associated with an abundant fauna regarded as the equivalent of the Equus fauna of early Pleistocene age.
Marl of this character has been observed south of the Colorado in nearly all the streams of the Black and Grand prairies and the Edwards Plateau, notably along the Conchos,Llano,Guadalupe above Kerrville, the Pedernales above Fredericksburg, at many places along the East and West forks of the Nueces, and on the Frio,Medina,Devils, and Pecos rivers.
North of the Colorado similar deposits occur in all the larger secondary streams of the Central Denuded and the Black Prairie and Grand Prairie regions as far as the Ouachita Mountains, especially in the valleys of the Nolan, San Gabriel, Cow House, Leon, Bosque, Paluxy, and Trinity. Like the Leona formation, next to be described, and some of the later terraces of the major stream ways, the Onion Creek formation is usually deposited in furrows which have been eroded in the older Uvalde formation.
TERRACES OF THE NUECES AND LEONA RIVERS, AND THE LEONA FORMATION.
The highest terrace of the Nueces River, where it flows through the canyon of the plateau, is the equivalent of the Uvalde formation, as has been previously stated. In the same canyon, below the Uvalde formation terrace, is a second, narrower terrace. Where the stream passes southward out of its canyon the Uvalde formation, as previously stated, spreads out over the divides as soon as the faulted zone is crossed. There is a bluff or low escarpment along the western side of the Nueces from the boca of its canyon entirely across Uvalde County. The eastern side is bounded by an escarpment almost as far south as the crossing of the Southern Pacific Railroad. Four and a half miles south of the railroad the escarpment or bluff, on the eastern side, begins again and continues southward entirely across the county. There is no escarpment on the east side of the river in the space between the Southern Pacific Railroad and a point 4½ miles below it, so that a wide flat extends from the Nueces eastward toward the Leona, limited on the south by a sinuous escarpment extending between the two rivers from a point 3 miles south of Uvalde, near the Leona, westward to a point 4½, miles below the crossing of the Southern Pacific Railway over the Nueces. Where the river is inclosed by escarpments there is a terrace flanking it on each side, occupying a position in general 25 to 50 feet above the stream bed and 75 to 100 feet below the Uvalde formation level. The material of the terrace is fine calcareous silt at the surface, and grades downward into coarse gravel.
Below a point 3 miles south of Uvalde the terraces of the Leona are exactly similar to those of the Nueces. The old (Pleistocene) floodplain "
Jour. Acad. Nat. Sci. Phila., Vol. IX, 1895, pp. 457-458.
254
deposits of the two streams fused through the low saddle or plain above described. The town of Uvalde is situated on the eastern side of the saddle.
The name Leona formation is proposed for the deposit making the first wide terrace of the Nueces and Leona rivers, below the level of the Uvalde formation, and for the flood plain deposit extending westward from Uvalde on the Leona to the Nueces River. The Leona formation is a Pleistocene flood-plain deposit bearing certain definite relations to the older Uvalde formation and the streams that have laid down its component materials. Where the streams are flanked by bluffs or escarpments it occurs as a terrace; in one instance it extends across a saddle and constitutes the junction of the flood plain deposits of two neighboring streams. The width of the formation in any particular portion of the course of a stream depends upon the topography of the country at the time of its deposition. Where the stream flowed through a narrow canyon the Leona formation is a narrow terrace; if the surrounding country was flat the flood that brought the material down may have covered many square miles, spreading the silt and gravel over vast expanses of country. Such is the case along the west side of the Frio.
We designate as type localities for this formation the courses of the Nueces and Leona rivers in the Uvalde district, as has been indicated in the preceding discussion (see map, fig. 70, p. 275). The formation presents the same general characters along the Frio River, and the wide silt terrace that occurs about 150 feet below the Uvalde formation level along the Rio Grande is undoubtedly of the same age. The Leona may ultimately be correlated with the Onion Creek formation. Along both the Leona and the Nueces rivers there is usually a smaller terrace, some 10 to 20 feet below the Leona terrace level. It is rather insignificant.
The present stream deposit consists mostly of limestone and flint pebbles derived from the Edwards Plateau.
THE WASH.
The edges of the outcropping ledges of hard limestone forming the scarps and crests of the hills in the Plateau country are shattered into fragments by alternate expansion and contraction, due to the diurnal variation of temperature. The loosened pieces may remain temporarily in situ or may roll down the steep slopes. When a sudden rainfall occurs they are washed down the slopes by the torrents, and scattered in great sheets over local lower levels.
A peculiarity of the cloud-burst type of rainfall of the semiarid region is that, although the water forms great torrents and spreads out into sheets, it usually disappears entirely upon reaching lower, more level areas, where it is either imbibed by the underlying rocks or evaporated without being conducted into the larger streams. Sedimentary
255
material deposited by these rain storms constitutes formations of vast areal extent in the arid and semiarid region, and may be appropriately designated "the wash."
The character and appearance of the wash vary with the geology and topography of the country in which it occurs. In southern Texas, along the Balcones escarpment and up the valleys and canyons of the Edwards Plateau, where the country rock is mostly the Edwards limestone, it consists largely of flint nodules, which are brought down from the decaying plateau and scattered over the Rio Grande Plain at its base. In the traps Pecos mountain region the wash is of the type of talus fans, spreading out at the mouths of the mountain canyons over the margins of the bolson plains. The surface of the Llano Estacado is all wash.
This material is found extensively in both the plain and the plateau, especially in the wide hemispherical headwater valleys of the minor stream ways of southwestern Edwards and northern Kinney counties, such as that of Griffin, Dry Sycamore, and Hackberry creeks. The wash is of especial interest because it enables us to a certain extent to interpret the origin of some of the older deposits of the Uvalde and Leona type. Each of these formations begins as a ribbon of aggradational material in the narrow canyons of the plateau, and flares out into a broad sheet in the coastward regions. The Uvalde gravel, for instance, widens out immediately below the Balcones scarp line; the Leona widens out a little farther on. These sheets may ultimately expand into the broad alluvial tracts of the Coastal Plain.
FRY PAN DEPOSITS.
The peculiar basin valleys to which the cowboys have given the appropriate name "fry pan" are pouch like indentations into monoclinal escarpments opposing the course of a stream. The wider or flaring portion of the valley, constituting the body or pan portion of the fry pan, lies upstream, and the valley constricts downstream, forming the handle of the pan, until the stream cuts its way out through a narrow gorge. In the arid and semiarid regions at times of flood the water is checked in these gorges, causing a precipitation of the material held in suspension. Typical deposits of this character are found in western Uvalde County, above the point where Turkey Creek crosses the monoclinal scarp line of the Anacacho Mountains, and above that where Elm Creek cuts across the monoclinal scarps of the Shoal Creek limestone. The southern end of the great Pecos Valley between the Rocky Mountains and the thirty first parallel, where the plaza country ends against the northern edges of the Edwards and Stockton plateaus, is a gigantic fry pan of this character. The fry pan alluvium by marginal gradation merges into the alluvial deposits described above as "the wash."
"Bolsons are basin alleys which have not, or had not originally, any outflowing drainage, and are lined with sedimentary debris derived from the surrounding country. Hill, Descriptive topographic terms of Spanish America: Nat. Geog. Mag., Sept., 1896, Vol. VII, No. 9, p. 295.
256
TEPETATE AND "TIERRA BLANCA," CHEMICAL LIME DEPOSITS.
Throughout the limestone regions of the hot climates of America a superficial crust of white lime material is found, called tepetate. Sometimes it is comparatively free from foreign material, or occurs as the matrix or cement of conglomerates. This is a concentrate of the lime which has been dissolved from the surface, transported in solution by the torrential streams, and redeposited through evaporation. In some cases it infiltrates downward into the embedded matrices of permeable formations and is there consolidated, in which case it is called "tierra blanca." If the country rock is a chalky limestone, the slightest rainfall is sufficient to take some of the lime in solution and redistribute it over the surface as "tepetate." For instance, the outcrop of the marly Eagle Ford beds in Kinney and adjacent counties of Texas is everywhere coated by this secondary deposit. The tepetate is forming great incrustations around the margins of the bolson plains of northern Mexico. The material often constitutes the matrix of the Uvalde formation gravel along the interior margin of the Rio Grande Plain and the Black Prairie region south of Austin.
IGNEOUS ROCKS.
KINDS OF IGNEOUS ROCKS.
All of the igneous rocks of the Rio Grande Plain are very basic, belonging to various types of basalt, viz,
MODE OF OCCURRENCE OF THE IGNEOUS ROCKS.
The igneous rocks all occur along the interior margin of the Rio Grande Plain, usually oil the downthrown side of the Balcones fault. The geologic relations of the Pilot Knob area have been described in some detail ill au article entitled "Pilot Knob, a marine Cretaceous volcano." The central mass is a volcanic neck or stock of nepheline basalt that has been pushed up through the Austin chalk, which is "
These rocks were identified by Mr. Whitman Cross.
Hill and hemp, Am. Geologist, Nov., 1890, pp. 292-294.
Hill, Am. Geologist, Vol. VI, 1890, pp. 286-292.
257
often marmorized along the contact and lies against the flanks of the igneous mass, dipping away from it on all sides.
The basalt has an imperfect columnar structure, nearly vertical at the south extremity of the hill and nearly horizontal at the north side. The flat region between the basaltic hills and the chalky perimeter of the igneous area is filled with a soft, yellow, amygdaloidal, exfoliating material, some of which is undoubtedly the product of basaltic decomposition, while in other places it resembles volcanic ash. . . . In any direction from the basaltic bills which form the center of the whole outcrop, the average distance to its edge is about one-half mile, where excellent contacts with the chalk are found. The chalky stratum forming the margin of this area throughout its whole extent is crenulated into gently waving undulations and presents different degrees of hardness. In places of direct contact with basaltic material the chalk is converted into hard marble; when the ash like material intervenes between the basalt and chalk the latter retains its soft, unaltered, pulverulent nature.
The occurrence of volcanic material interbedded with chalk is adduced in the same paper as evidence to prove that a part of the volcanic disturbance took place as early as the latter portion of the time in which the Austin chalk was deposited.
Intrusive sheets and dikes of soft, much decomposed material of a basaltic nature occur at numerous other places within a radius of 10 miles.
The only other areas that have been studied in detail are the Uvalde and Brackett quadrangles. The basalts and phonolite probably are all intrusive. No undoubted lava flows and no pyroclastic material of any kind have been found. The rocks present four modes of occurrence:
ARRANGEMENT OF THE STRATA.
THE MAIN SYSTEM OF DIPS.
The apparently horizontal Cretaceous rocks are laid upon each other in sheets of great persistence and regularity, as is shown in many of the illustrations in this paper. Each of these individual sheets ultimately thins out and disappears beyond the area considered in this "
Hill, loc. cit.
The mode of occurrence and petrographic characters of the rocks in the region of Uvalde will be discussed in detail in a paper soon to be published by Mr. Vaughan and Mr. Cross.
258
paper, but the change is so slow and gradual that the beds are practically uniform throughout the region under discussion. Their arrangement usually appears horizontal to the eye, but in fact the strata are all slightly tilted or inclined toward the coast, so that a rock sheet outcropping at the surface in one locality may be carried down by this dip from 10 to 100 feet beneath the surface a mile or two seaward.
North of the Colorado River the beds, as a whole, have the uniform gently tilted monoclinal arrangement characteristic of the general structure of the Coastal Plain, whereby the various rock sheets dip beneath the surface and each other, toward the coast, at an angle greater than the surface slope in the same direction. The dip of the Cretaceous rocks on a life drawn eastward from Dublin through Waco is estimated to be about five times as great as the average continental slope of the surface in the same region.
South of the Colorado the direction of the dip of the rocks is likewise coastward, but the angles of dip are more varied. The rocks of
This change in the dip coincides approximately with the Balcones scarp line, of which an account has already been given (p. 203), and more precisely with the Balcones fault line.
BALCONES FAULT ZONE.
The abrupt southern termination of the Edwards Plateau between the Colorado River and the Rio Grande and the sudden fall in altitude from the summit of the plateau to the lower level of the Rio Grande Plain has been produced by faulting. The line of faulting follows closely the foot of the Balcones escarpment, and for that reason has been named the Balcones fault zone. This fault zone is one of the most important features in the geologic structure of Texas, because it is the only structural break in the continuity of the post Paleozoic strata in
259
the vast stretch of country between the Gulf of Mexico and the Rocky Mountains. It makes the southern boundary of the Great Plains region, because to it the Balcones escarpment, which is the southern boundary of the Plateau of the Plains, owes its existence.
The strata on the seaward side of the faults have been dropped down, so that any particular stratum, the top of the Edwards limestone for instance, lies 500 to 1,000 feet lower oil the coastward or downthrown side of the fracture than can the interior or upthrown side. Another statement of the same idea is: A stratum on the upthrown side of the fault zone is opposed on the downthrown side by a stratum that belongs 500 to 1,000 feet higher in the geologic section. This explains why the Cretaceous rocks at the surface along the northwestern margin of the Rio Grande Plain belong, geologically speaking, hundreds of feet above the summit rock of the plateau, although the latter are 500 feet higher, topographically, than the former, and it enables us to understand certain laws, not hitherto recognized, that govern the occurrence of much of the underground water in the region. These laws will be elaborated under the heading "Source of the underground waters of Rio Grande Plain" (pp. 313-316).
The fault zone really consists of many faults having subparallel directions, all concentrated within a narrow belt of country, as is shown in the Austin section (P1. XXXV, D; figs. 65, 66). By an examination of this cross section of the faulted belt it will be seen that interiorward there are major faults of large downthrow, attended to the seaward by numerous smaller ones. The effect of the numerous faults is to break the regularity of the dip of the strata and to chop them into numerous blocks tilted at various angles within the faulted zone (fig. 67). To trace and map in detail the many small faults and folds would be an almost impossible task, the sum of them all amounting to a general downthrow of several hundred feet.
The series of subparallel faults has produced a narrow belt of country, occurring in Travis and through other counties southwest, the surface of which is composed of the outcrop of the Edwards limestone, which has fallen below the level of the Edwards Plateau. As this zone is of small area and of exceptional character, we do not devote much
260
space to it. It usually lies between parallel faults. Its type is splendidly developed between Manchaca and Oatmanville, in Travis County. From the rough, broken character of its rocky surface, clothed with a dense growth of junipers and small scrubby oaks, it is locally called "4' hard scrabble." In Uvalde County, west of the Nueces River, the fault is replaced by a simple monoclinal flexure. Instead of being broken across and dropped down bodily along the plane of fracture, the strata have a rapid increase of dip at the edge of the plateau, so as to descend quickly to a lower level, and then, by another curve, become again nearly horizontal. The two bodies of nearly horizontal strata on the opposite sides of the flexure thus stand at very different levels, and so far as dislocation is concerned the result is practically the same as in the region of faulting. The effect on the topography is also quite similar, so that the line of the Balcones is not interrupted. The northern part of this flexure, connecting the gently dipping rocks of the plateau with the steeply dipping rocks of the scarp line, is shown in the section. Accompanying the fold are numerous small faults that usually are not parallel to the axis of the folding, but cut it at more or less acute angles.
The general character of the fault line is given in the accompanying illustrations and maps. Its details will be shown more thoroughly upon the Austin, Brackett, and Uvalde folios of the Geologic Atlas of the United States, now in preparation. Near Austin, just north of Mount Bonnel, the lowest strata of the Upper Cretaceous, the Eagle Ford, have dropped down so that they now occur against the edges of the upper 100 feet of the Glen Rose beds, as shown in fig. 76 (p. 315). It is quite difficult to determine with accuracy the time during which this faulting was developed. We know that the Uvalde formation, which has been deposited since this faulting began, is probably of Pliocene age. Its deposition did not take place until after the Edwards Plateau was elevated into its present position relative to the plain and the deep canyons had been cut into it. That period of vigorous erosion approximated the close of Miocene time. As the faulting preceded the period of erosion, it is very probable that it may have been in operation during Eocene time. There is but little doubt, however, that the dislocation continued through later epochs.
The continuity of the stratified rock is likewise, though rarely, broken in places by dikes and sills of igneous rock. These are confined mostly to the Rio Grande Plain, and are quite unimportant except in Uvalde and Kinney counties.
CAPACITY OF THE VARIOUS ROCK SHEETS FOR WATER.
The composition, texture, sequence, and arrangement of the rock sheets constituting the systems of underground waterworks in the region discussed having been described, the part that each particular rstatum plays in the distribution of water will now be considered.
261
Of the numerous beds mentioned, all those composed of impervious material, such as clays. and close-textured limestones, may be considered as non-water-bearing, and their function in the transmission of water is solely that of control, not of supply, since they retain the water in the water-bearing beds. Nearly all of the Cretaceous rock sheets south of the Colorado lying above the Fredericksburg division belong in this category of impervious beds overlying the water-bearing strata. There may occasionally be a few arenaceous layers, sufficient to furnish a scant supply for a few dug wells, but as a rule, as all those who have endeavored to obtain wells in them can testify, the underground water they contain is neither abundant in quantity nor good
Rocks of open texture, such as sands, conglomerates, porous, chalky limestones, and massive rocks broken by joints, fissures, honeycombs, or other openings, are usually water bearing. These are mostly found below the Del Rio clay.
262
It is from these lower beds that we are to expect artesian supply in the region south of the Colorado River south of the Brazos, in fact. That certain of these beds are completely charged with water is demonstrated by observations upon the source of the springs of the plateau region and by the experiments of well drillers.
 |
The chief water bearing strata of the Edwards, Glen Rose, and Travis Peak or Gillespie formations will now be described more fully.
The barren limestones of the Edwards formation have little in their appearance to suggest the occurrence in them of underground water; yet in the series there are several horizontal layers of water bearing rocks which, when cut by streams or penetrated by the well digger, afford abundant and pure supplies.
The proof that the Edwards limestones are water bearing is three fold, as follows:
Some of the water bearing strata are pervious arenaceous beds intercalated between the limestones. The character of these beds is often difficult to discover, the fact that they are very calcareous causing hem to resemble the limestone ledges, and their surface being often
263
coated by calcareous incrustations which completely obscure their true nature. Where they are exposed, so that they can be studied, they are seen to consist of from 20 to 50 feet of pervious beds, composed of very fine particles of sand embedded in a white or yellowish limy matrix.
The horizontal distribution of water in the Edwards formation is also facilitated by the occurrence in the series of certain limestone strata that have been honeycombed or made cavernous by underground solution. The limestones, as exposed in cliffs, may appear hard, durable, and of homogeneous nature, but the interior when examined may prove to be very heterogeneous. There will be hard and soft spots, the latter being much more soluble than the former. The soft, soluble portions of the limestone may represent the original loci of fossils, small particles of iron pyrites, or tubular molds of fucoidal character. The irregular cavernous decomposition of the Edwards limestone is well shown in the bluffs along the Colorado River west of Austin. There are fucoidal layers, which weather into honeycombed rock, and peculiar red cavernous blotches develop in certain fossiliferous horizons of the massive limestone. (See fig. 75, p. 315.)
Whatever may be the origin of the honeycombed beds, it is a fact that they transmit immense quantities of water, and it is through them and the arenaceous layers of limestone that the headwater springs of the rivers of the Edwards Plateau above mentioned and the artesian wells of the San Antonio system deliver their water at the surface. The impervious roofs and floors of the water bearing limestones can not be so sharply delineated as those of the water bearing strata of other formations, owing to the fact that the various limestone strata resemble one another so much as to be indistinguishable for mapping purposes. Furthermore, the paleontologic and lithologic research thus far conducted has not been. sufficient to secure data to differentiate and define them accurately.
While water is distributed horizontally by thin pervious beds, separation planes, and cavernous strata or honeycombed rock, it is also distributed vertically in the Edwards limestone by joints, fissures, and crevices. Much more concerning the water bearing beds of the Edwards limestone may be ascertained when that formation is made the subject of still more detailed investigation.
In Southwestern Edwards County there are several water bearing layers that occur at various levels in the Edwards limestone from the base upward. These layers, in ascending series, supply the Kickapoo springs, the Black water hole, and the Justice spring. The supply from the uppermost layer is trivial and unreliable in character; the other two are of great economic importance. The ranchmen of the summit region of the plateau bore deep wells down to these strata.
One of the water bearing strata of the Edwards beds lies approximately 300 feet below the summit of the formation in sections measured at the headwaters of the Frio River in Edwards County. Another
264
measurement makes the distance 380 feet below the Del Rio clays. The uppermost of the embedded water bearing strata of the Edwards formation at Austin, San Marcos, and Manor, so far as can be estimated from the meager artesian well records at hand, lie about 50 feet below the summit of the formation. These beds are further discussed under the head of "Availability and limitations of the underground waters" (p.316).
The Glen Rose formation contains considerable quantities of water, as is well shown by springs, surface wells, and artesian wells. Some of the springs can be seen in the stream cuttings near Anderson's mill, in Travis County. There are springs along the courses of many of the streams that have cut down into it.
The Travis Peak and Gillespie formations contain a greater quantity of water than any other beds of the Comanche series. The water contained in the Travis Peak and Gillespie formations has previously been spoken of as Trinity water, because the name Trinity has been applied to the sands at the base of the Comanche series farther north. Since the name Trinity is applied to the division including the basal sands, which in the Austin region have been designated the Travis Peak sands and the Glen Rose formation (called also farther north the Paluxy sands), it seems best to dispense with the name Trinity as applied to the water derived from the lowest sands, and to use it only for the waters of the Trinity division as a whole. Therefore, we shall speak of the Travis Peak or Gillespie water and the Glen Rose water, as well as of the Paluxy water. Numerous springs, such as those in the vicinity of Travis Peak and Fredericksburg, derive their water supply from them. There are also numerous surface wells where they outcrop, and many artesian wells are supplied by them where they are embedded. The water that they contain is also the purest found in the strata of the Comanche series. The quantity and purity of the water increase as one goes downward in the series. The water of certain of the higher beds of the Glen Rose and Edwards formations is strongly impregnated with mineral matter and must be cased off in wells, for it is not potable.
UNDERGROUND WATER OF THE REGION.
WATER SUPPLY OF EDWARDS PLATEAU.
The summit region aid the canyons of the Edwards Plateau present different conditions for supplying underground water. The summits are so high above the surrounding regions' that both the surface and the underground water naturally drains from them toward the lower canyons. Therefore the conditions on the plateau and the canyons will be discussed separately.
The whole plateau is underlain by the water bearing strata of the Edwards, Glen Rose, and Travis Peak formations. The upper strata
265
of the Edwards compose the summits, while the Glen Rose beds are in part exposed in the deeper canyon cuttings, as can be seen in the Guadalupe, Nueces, Frio, and other streams. There are strong probabilities that west of the Frio below the lowest exposures of these beds there are still lower strata containing a great amount of water.
No artesian water has ever been obtained from wells sunk on the summit of the plateau, although several drillings have been made, but in most places nonartesian water is found.
NONFLOWING WELLS.
The horizontal arrangement of the strata of the Edwards Plateau is not favorable for the obtainment of artesian water upon the summit areas. As the uplands are higher than the outcrop of any known water bearing beds occurring beneath them, the hydrostatic pressure necessary for flowing wells can not exist.
The ranchmen of the summit region rely solely upon deep pumping wells for stock water. These wells are from 300 to 500 feet deep and obtain their water from the water bearing strata of the Edwards beds, described in the previous section.
We have seen many records of the deep wells sunk on the summit of the Edwards Plateau in Kerr,Kimble,Menard,Schleicher,Sutton, Edwards,Valverde, and Crockett counties, and in nearly all instances they are drilled to the depths above mentioned through the solid Edwards limestone. Their depths check closely with the measurements of the rock sections in the adjacent canyons.
We present the following records of the deep nonartesian Edwards limestone wells of the plateau region, including in the list some on the monoclinal slope constituting its southern edge in Kinney County.
DEEP WELLS OF THE PLATEAU SUMMIT.
Grass Valley pasture, Fort McKavett. Total depth, 255 feet. Limestone all the way except a few feet of sandstone at the bottom. Good water. (M. C. Ott.) Water probably struck in the Edwards formation.
Rock Springs, Edwards County. In the Edwards formation. Two deep wells, 400 feet; pumped for use of town. Another well, 129 feet. (Ott.)
Hillcoat well, No. 2, Edwards County, 3 miles west of McKenzie trail, Nueces quadrangle, 300 feet deep. Through white limestone (Edwards) all the way. Soft drilling. (Ott. )
Griffin Creek, Nueces quadrangle, Edwards County. Surface gravel a few feet; soft white limestone (Edwards), 275 feet; sandstone (Edwards), with water. (Ott.) Valverde County. B. L. Croucher's well, 30 miles north of Del Rio and 10 miles from Devils River. Depth, 475 feet. An unlimited supply of permanent non-artesian water 300 feet from the surface. Raised by an 18 foot windmill.
Valverde County. B. N. White's well, 20 miles north from Del Rio and 12 miles from Devils River. Depth, 300 feet; water struck 250 feet from surface.
R. W. Prosser's well, 20 miles from Comstock, Valverde County. Total depth, 569 feet; depth to water, 340 feet. Entire distance through solid limestone, probably all Edwards.
"We are indebted to Mr. M. C. Ott, of Brackett, for the records to which his naive is appended.
266
VALLEY OF GRIFFINS CREEK.
Hillcoat's well, northern Kinney County. Total depth, 124 feet. Bowlders of hard limestone flints (Uvalde gravel), 88 feet; hard (Edwards) limestone, 36 feet. Fine water. (Ott.)
DEEP WELLS OF THE PLATEAU MONOCLINE.
North end of Grass Valley pasture, Kinney County. Total depth, 205 feet. Good bard limestone (Edwards) all the way. Few feet of sandstone at the bottom. Pumps 180 feet. (Ott.)
Hitchcock well, Grass Valley, Kinney County. Total depth, 204 feet. Gravel, 9 feet; limestone (Shoal Creek), 40 feet; slate (Del Rio beds); limestone (Edwards); hard sandstone (Edwards). Water rises to Within 60 feet of surface. (Ott.)
Grass Valley pasture, south end. Two holes; no water; gas wells.
Grass Valley, north side of the creek. Total depth, 336 feet. Slate, 186 feet (Del Rio?); drilled 150 feet below slate into limestone; limestone all the way down to sandstone (Edwards); 10 feet of bottom water. Inexhaustible water. (Ott.)
Grass Valley, 1 mile west of Hitchcock ranch. Running stream in cave in mountain rock. (Ott.)
Weymuller well, Kinney County. Surface gravel (thickness not given); mountain limestone (Edwards), 180 feet; whitish sand (Edwards), 20 feet; water in last; non-artesian. Windmill pump. (Ott.)
SURFACE WELLS OF TILE CANYON VALLEYS.
These can usually be obtained in those portions of the canyon valleys of the region where the stream way has cut below the level of the Edwards beds. Water is found in the outcrops of the Glen Rose and lower beds and in the alluvial deposits of the Uvalde, Onion Creek, and fry pan type.
The basement sandy beds of the Cretaceous yield constant supply of wholesome well water. The Glen Rose beds and alluvial deposits are somewhat variable.
WELLS NEAR BEE CAVES, TRAVIS COUNTY, SUNK IN THE GLEN ROSE FORMATION.
Information concerning the following wells was furnished by T. C. Bohls, Bee Caves, Texas:
At Bee Caves. Depth of well, 160 feet. Water supply constant. Constant depth of water in well, 11 feet. Water found in black sand.
Three fourths of a mile west of Bee Caves. Depth, 165 feet. Constant depth of water in well, 25 feet.
One and one half miles west of Bee Caves. Depth, 163 feet. Constant depth of water in well, 40 feet.
On T. C. Bohls's place, 2½ miles east of Bee Caves, there are two wells, as follows: One 221 feet deep; 140 feet of water sometimes; supply not constant; well goes dry. Another 264 feet deep; 80 feet of water; can dip dry.
There is a well at the junction of the two forks of Barton Creek, bored 100 feet above the creek bed; well 54 feet deep. There is 10 feet of water in well, and water can not be lowered.
WELLS OF SOUTHERN PACIFIC COMPANY IN PECOS COUNTY
The following wells drilled by the Southern Pacific Company in Pecos County are west of the Pecos, and therefore beyond the field
267
of this paper, but are allied to the deep wells of the Edwards Plateau system:
Longfellow well; 683 feet deep; unlimited supply of water; capacity, 1,250 gallons per hour.
Sanderson well; 987 feet deep; large supply of good water; capacity, 1,700 gallons per hour.
Dryden well; 1,797 feet deep; capacity, 1,000 gallons per hour; unlimited supply of good water. Well begins at Shoal Creek limestone.
Lozier well; 770 feet deep; limited supply of water; capacity, 1,000 gallons per hour, which exhausts the well in two hours. It then takes two hours to fill up. Well begins in Del Rio beds.
Thurston well; said to have been drilled 1,800 feet through solid limestone.
None of these wells are flowing, nor have their geologic relations been studied.
GRAVITY SPRINGS OF THE CANYONS OF THE PLATEAU.
In the investigation of the springs of the region to which this paper pertains we have found it important to distinguish two classes. In one case water bearing strata lying nearly level outcrop one slopes, gentle or steep, and the water is drained away at the horizontal outcrop; the underground water is merely a continuation of the descending surface water, which may have been diffused through sandstone or concentrated in tunnel like channels dissolved from limestone. In the second case the water bearing strata do not locally outcrop at the surface, but are deeply buried, and communication with the surface exists through natural fissures. The water in the strata, being under hydrostatic pressure, is forced up through the fissure conduit so as to flow out at the surface of the ground. The two classes are here distinguished as gravity springs and fissure springs.
In the description of the rivers of the plateau we have mentioned the springs which were met in ascending the canyons of the streams. These springs break out near the water line in the rivers, the water draining by gravity from the horizontal strata of portions of the Edwards, Glen Rose, and Travis Peak or Gillespie formations.
Such springs usually occur in all. the streams of the plateau front the Llano to the Pecos. Wherever the gradient of the stream, in descending the summit of the plateau through the canyons to the lower Rio Grande Plain, cuts into a water bearing bed the water drains out into the stream way. In many cases this forms large, deep pools of clear running water. Of this character are the so called headwater holes of the various forks of the Llano, Pedernales, Guadalupe, Comal, Medina, Frio, Nueces, and Devils rivers. The constant waters of all these streams are derived from springs of this character. As the rivers have never been measured no idea of the exact quantity of water thus escaping has been obtained, and we can present only general descriptions of it.
About 17 miles north of Leakey is the great headwater hole of the
268
Frio (P1. XXXVIII), which occurs at the very head of the flat bottomed canyon, just below where the steep ascent to the summit region begins. These springs break out from cavernous, slightly arenaceous layers in the Edwards limestone, about 300 feet below its summit. Between this point and Leakey there are many large, deep, and long pools of water in the stream way.
From Leakey the east prong of the Frio was ascended in a northeast direction. Fine springs burst out from the Glen Rose and Edwards beds, and supply the river with much water. About 12 miles northeast of Leakey there are some springs the tufaceous deposits of which make a number of hemispherical concentric pools, arranged in descending series, resembling very much the illustrations of Gardner's springs in the Yellowstone Park. Below Leakey and thence to the mouth of the canyon the Glen Rose beds afford a large number of gravity springs. There are four large ones at Van Pelt's ranch, and at Rio Frio postoffice many acres of land are irrigated with spring water.
The Guadalupe and its several tributaries derive the abundant water which they contain above Kerrville from various strata in the Glen Rose and Edwards beds. At Kerrville the springs can be seen draining out of the Glen Rose beds. The highest springs are about 7 miles above Vix and at Spencers Hole. At the latter place the water breaks out of a horizon high up in the Edwards limestone.
The various forks of the Llano, the East Fork of the Nueces, and its principal tributary, Hackberry Creek, reveal the same wealth of gravity spring water in the upper portion of their flat bottomed canyons, the water breaking out from various horizons of the Edwards and Glen Rose beds.
The waters of the Pedernales are largely derived from the basement beds of the Cretaceous. Many fine springs break out in the course of the stream and its tributaries, as seen around and below Fredericksburg.
In addition to the gravity springs here enumerated there are, no doubt, many others, both in the plateau and in the plain, which the writers have not had opportunity to see. Among these may be the springs of Howards Creek, the Pecos, and Devils River. Concerning the latter, we have taken the following notes from a letter written to this office by Dr. John T. Nagle:
I desire to state that there are two other large springs in Valverde County, Texas, besides the San Felipe at Del Rio. The water is clear, has a very pleasant taste, appears to be pure, and runs over a rocky bed into the Devils River, which empties into the Rio Grande. These springs might be of interest to your Bureau, and I will give their names and location as near as I can.
The first spring is at R. W. Prosser's ranch, which is about 6 miles north and slightly to the west of Camp Hudson, on the Devils River, between Camp Hudson and Pecan spring. The spring is not named, but is known as Prosser's, on account of its being on his land. The second spring is called Pecan spring; it is another sparkling spring of pure limpid water, and its taste is delicious. There has been
269
no chemical test of these waters, but I am inclined to think that there may be a very slight amount of lime in the water. Pecan spring is about 2 miles north and one half mile west of Prosser's ranch house, close to the Devils River, as is also Prosser's spring.
The following details of gravity springs along the forks of the Nueces will illustrate the general character of the plateau springs.
KICKAPOO WATER HOLE.
At Kickapoo springs the West Nueces and Kickapoo Creek are cut down to the level of the Comanche Peak limestone. Here enormous springs break forth, creating a wide, running stream of clear water that continues 4 miles. In places it is 100 yards in width, is bordered by exquisite forests, and teems with aqueous vegetation and game fish. Toward its lower end the streamway narrows; the bold flow which up to this point has rushed through the rocky banks suddenly ceases and is succeeded by a whitened pebble bed. We were informed that the gravel drank in this water; but, suspecting that its disappearance was too rapid for such imbibition, cleared away a thin layer of gravel and discovered that it escaped down a large fissure into the underlying limestone rocks (Pl. XLVIII, p. 314). These springs drain out of the strata near the contact of the Edwards. limestone and the Comanche Peak bed. Observations tend to show that this geologic horizon is elsewhere completely saturated with water. Springs from this horizon are found along the East Fork of the Nueces and Hackberry Creek. Nearly all the abundant living water in the East and West forks of the Nueces, except the Black water hole and the springs at the immediate head of Hackberry Creek, is derived from this water bearing horizon.
Where the rocks of the plateau are horizontal, north of where they bend down to the southward, this water bearing stratum occurs at a level of about 1,750 feet above the sea and about 600 feet below the summit of the plateau, and (except in the lower valley of the East Nueces, which has cut below its level) is available for wells. Along the southern monocline it is reached in the well at Hillcoat's ranch, 124 feet below the surface, at an altitude of 1,447 feet above the sea, and at two wells in the arena of Griffin Creek, west of Hillcoat's ranch, at altitudes of 1,400 and 1,450 feet, respectively.
BLACK WATER HOLE.
This occurs about 150 feet above the base of the Edwards limestone, and occupies a position at Black water hole 1,900 feet above sea level. The water here is not so abundant as at the Kickapoo springs.
Justice spring, Cedar spring, and Cherry spring, on the western border of the Nueces quadrangle, probably derive their waters from a third and still higher water bearing horizon of the Edwards beds. The waters in the vicinity of Seep Springs Mountains may also be derived from this source. These springs are feeble, and the horizon has not
270
been sufficiently studied to justify at present any conclusions tending to show persistent horizontal distribution or availability. There are strong reasons for believing, however, that in the region lying to the east of the Nueces quadrangle this water bearing stratum becomes more productive, especially in the heads of the Guadalupe, Llano, Medina, and Frio. In the plateau region this water lies at an altitude of from 2,000 to 2,1011 feet above sea level, or from 300 to 350 feet above the base of the Edwards limestone. In common with the other water-bearing strata, its plane descends with the dip along the monocline at the south.
Occasional seepage springs also break out in these canyons at the contact of the gravel deposits with the underlying Cretaceous beds, but these are feeble and intermittent in yield, being sympathetic in their flow with the rainfall.
ARTESIAN WELLS OF THE CANYONS OF THE PLATEAU.
Since the drainage ways of the plateau south of the Pedernales do not cut entirely through the Glen Pose formation, it is evident that a large portion of the water bearing rocks of the Cretaceous system, including those which elsewhere yield the largest supplies, must be embedded below the lowest cutting of the stream valleys. At various points in the canyons artesian flows have been obtained from the Glen Rose and Travis Peak sands, as at Kerrville and Center
271
Point, in the Guadalupe Canyon, and 7 miles southeast of Utopia, in the canyon of the Seco. A description of the wells in the valley of the Guadalupe at and below Kerrville will serve to illustrate the conditions controlling their occurrence.
Mr. Charles Shriner has made several interesting drillings at Kerrville. He finally obtained flowing water in the river valley at a place 50 feet above the water in the Guadalupe. The well is a 12 inch hole. The first flow of water was struck at 225 feet and the main flow at 250 feet. The water rises about 8 feet above the surface. This well was commenced in the Glen Rose formation, about 150 feet below its contact with the Edwards limestone, which can be seen in ascending the adjacent divides; hence the water bearing sands struck at 225 and at 250 feet occur in strata about 400 feet below the Edwards, which position corresponds very nearly with the base of the Glen Rose formation (See fig. 68).
Mr. Shriner's first experiment was a well drilled on a hill in the northern suburb of the town, about 50 feet higher than the surface of the present well. At about 750 feet fine water was struck, which rose to within 75 feet of the surface, or not quite to the level of the river. This water must have been very near the bottom of the Travis Peak formation, for below it were salt, clay, and black sand, probably from the underlying Paleozoic rocks. At 1,250 feet it is stated that granite was struck and penetrated to a depth of 75 feet.
A flowing well has been reported at Center Point, Kerr County, in the valley of a small stream flowing into the Guadalupe, but we have had no opportunity to verify this information
A few other wells of a similar character Dave been bored in the Guadalupe Valley below Kerrville. One of these is reported 1½ miles below the town; another is lower down. About fifteen have been reported from within a radius of 10 miles of Welfare postoffice, Kendall County. These have a depth of from 170 to 175 feet and flow about 5 gallons each per minute.
A single canyon well of this type has been reported in the valley of the Blanco, at Wimberley, Hays County. This is said to be 250 feet deep. No information has been obtained as to the amount of flow. The only other flowing wells reported to have actually been obtained in such valleys are two shallow wells in the valley of the Seco, 7 miles southeast of Utopia.
Similar wells should probably be found in the valleys of the shallower streams, such as the Pedernales, Cibolo, etc., but we have no data for making definite predictions concerning them.
The well drilled several years ago at Bulverde, on the Cibolo, north of San Antonio, furnished the only record we have been able to procure of this type of canyon wells. This, together with some experiments near Aue, indicates that the water will rise but not flow. The
272
record of the Bulverde well as reported by Roessler (except the names of the formations, which we have supplied) is as follows:
![Record of R. Mecke's well at Bulverde, Bexar County, on the Cibolo, due north of San Antonio. [Altitude, 1,600 feet; depth, 361 feet.] Thickness Depth in feet in to bottom of feet. strata. Glen Rose formation (?): Soft yellow stone 42 42 Blue stone 20 62 Yellow stone 18 80 Blue stone 2 82 Yellow stone 6 88 Blue limestone 19 107 White limestone (water bearing) 30 137 --- 137 Travis Peak formation (?): Bluish sandrock 10 147 Blue slate 40 187 Red clay 13 200 Limestone 60 260 Sandstone 31 291 Quartz sand 1 292 Yellow clay 23 315 Limestone 35 350 Sandstone 11 361 --- --- 224 361 361 Strong supply of water rises 250 feet, or to within 135 feet of surface.](figures/txu-oclc-27281517-272-t-a-800.jpg) |
![Record of G. A. Dueller's well, on the Leon, near Aue. [Drilled in the Glen Rose and Travis Peak formations.] Thickness Depth in feet in to bottom of feet. strata. Very hard rock, sandy limestone 2-3 2 Alternations of rock, brown sandstone, inky blue sandstone, etc.. 620 622 Water ---- 622 Same rock as foregoing 51 673 Water rises to within 20 feet of surface, and now stands within 4 feet of surface. Well stopped in yellow sandstone.](figures/txu-oclc-27281517-272-t-b-800.jpg) |
Except at the localities given, we know of no artesian borings of sufficient depth to reach the Travis Peak sands having been made in the plateau canyons west of the Guadalupe, but there is reason to believe that in these sands, lying below all the strata exposed in the deepest cuttings of the canyons, there exists an abundant supply of water which can, in places at least, be made available. On the Nueces there are large springs at Camp Wood, rising through joints and fissures from rocks lying lower than the Glen Rose beds exposed at the surface, and there is every reason to believe that the basal sands are the source of this supply. This water, if it exists, will be found at least 500 feet below the base of the Edwards limestone.
273
MORRIS RANCH WELL.
An instructive boring is reported to have been made at Morris ranch, Gillespie County. This ranch is situated on one of the headwater tributaries of the Pedernales, about 11 miles southwest of Fredericksburg and 91 miles northeast of Kerrville. We are indebted to Mr. Charles F. Morris, of the ranch, for the following information concerning this well: Total depth drilled, 1,100 feet; first water struck at 50 feet; granite struck at 180 feet. At 460 feet more water was reached, and this rose to within 100 feet of the surface. The well was continued and reached another flow of water at 1,000 feet. The water then rose to within 55 feet of the surface. The drilling was continued in solid granite until the depth of 1,100 feet was reached. According to Mr. Morris, "the well was in granite from ISO feet to its bottom. It was all very hard, but would change color at times from red to gray and then to red again. We could only go from 3 to 4 feet in 24 hours."
If the log of this well as given is correct, it shows a remarkable and unusual occurrence of underground water. The specimens sent to this office by Mr. Morris are undoubtedly granitic, but he says that he is uncertain as to the exact depth from which they came. Moreover, the record of this well, if correct, is further confirmation of the belief that the Cretaceous formations are laid down upon an unequal floor of Paleozoic rocks, for the top of the granite in the Morris ranch well, struck at an altitude of about 1,500 feet above sea level, is 1,000 feet higher than the same as met with in Mr. Shriner's first experimental well at Kerrville. This close proximity of the granite to the surface in Gillespie County no doubt also accounts for the intense redness and arenaceous character of the Gillespie beds in the valley of the Pedernales adjacent to Fredericksburg.
The following statement lately appeared in the New York Sun:
Nordenskjold, the Swedish scientist, has shown that water can be found by boring into granite and other crystalline rocks to a depth of from 100 to 170 feet. Briefly, he proceeded on the theory that the variations in temperature ought to cause shearing strains between the upper and lower layers of the rock, in such way causing horizontal crevices into which water from the surface would percolate, and the water would also be fresh. A well was said to have been sunk in the islet of Arko, off the Swedish coast, in 1894, and at the depth of 110 feet fresh water was found, supplying 4,400 gallons a day, and since then six other wells have been bored and water found at about the same descent, the object of the research being to provide lighthouses and pilot stations with a permanent and plentiful water source.
The occurrence of water in granite depends upon the existence of fissures in the granite. The presence of water in mines sunk along veins where fissures exist is an illustration of this principle. But granite call not be regarded as offering conditions favorable for the procurement of underground water, even though in some instances wells sunk in it may have been successful.
274
WATER OF THE RIO GRANDE PLAIN
NONFLOWING WELLS AND GRAVITY SPRINGS OF RIO GRANDE PLAIN.
The nonflowing wells and gravity springs of the Rio Grande Plain maybe classified in two groups:
In the area between the Colorado River and Onion Creek, Travis County, there are many shallow wells which derive their supply of water from the Uvalde upland gravel formation. Similar wells are obtained in the upland gravel beds wherever the latter cover considerable areas in Travis, Hays, Comal, Bexar, Medina, Uvalde, and Kinney counties. In San Antonio they are 40 to 50 feet deep; at Spofford Junction, about 40 feet. The water usually accumulates at the base of the gravel beds, at their contact with an underlying impervious bed rock.
Water is also found in the gravel of the present stream ways of the interrupted and intermittent drainage courses. In Uvalde and Kinney counties, for instance, the beds of such streams, although dry at the surface, usually contain much water below, which can be procured by digging a few feet into the gravel bed. This condition is especially likely to prevail where the channel is filled with gravel. The character of the stream bed is due largely to the fact that the water flows down into the gravel and disappears front view. In those portions of the water way where the gravel has been removed, exposing the formations underlying it, the water accumulates ill large ponds or pools that usually last during the dry season. Such pools may be seen at many places along almost any stream. There is one on the north side of the crossing of the Uvalde San Antonio road over the Frio River. At this place an exposure of basalt, the gravel having been washed off, forms the bottom of a small reservoir.
Just south of the crossing of the Southern Pacific Railroad over the Leona River the gravel is washed off the Shoal Creek limestone and the Del Rio clay. The latter forms the bottom of a large, deep pond, into which the water from upstream accumulates. Below such pools, when the stream way is filled with gravel, the water again disappears. Besides the pools, along the streams there are frequently springs that flow from underneath the gravel when it is underlain by an impervious stratum of any kind. A good example of this kind of spring is the Soldiers' Camp spring on the Nueces River, about 1 mile below the crossing of the Uvalde Nunn's ranch road. The gravel at
275
this place is underlain by the Austin chalk. The water rushes out in a stream of considerable size, and is cool, clear, and pure. Such springs are surrounded by splendid growths of pecan trees. Below the Soldiers' Camp spring the bed of the Nueces River is usually formed by the Austin, Anacacho, or argillaceous Eagle Pass beds, none of which absorb water very rapidly. The result is a flowing stream.
WATERS OF THE LEONA AND KINDRED FORMATIONS.
The terraces above the present flood plains are structurally comparable to the present stream beds, consisting of gravel accumulations deposited in stream valleys upon a bed rock floor.
Water is usually stored in the bottom of these sheets, above their contact with the underlying formations, and wells sunk in them usually
At Uvalde (and perhaps other places) these gravel sheets are many square miles in extent, and the contained water is of great economic
The Leona River below Uvalde cuts down to the contact of the Leona gravel sheet and the underlying floor of Cretaceous rocks, where a large
276
volume of gravity spring water (the springs of the Leona) breaks forth with a discharge of 11 second feet, or 7,000,000 gallons per day. The run off forms large pools below Uvalde, and the water resembles very much that of the typical fault springs, with which it has been confused. This water is drained from the bottom of the Leona gravel sheet and is apparently the same as that obtained in the surface wells sunk in the formation.
Six miles below Uvalde, near the Leona River, there is a well at a Mexican jacal 30 feet deep. Basalt bowlders and rotten basalt were found in the well. The water seems to come from the Leona terrace, and is very good.
The waters of the alluvial deposits are usually abundant and pure.
WATER OF THE RIO GRANDE TERRACES BELOW DEL RIO.
Very little is known concerning the possibilities of wells sunk in the Rio Grande terraces. Below Del Rio to Eagle Pass the country is almost uninhabited, excepting along such creeks as Pinto, Tulio (Las Moras), and a few others, in which there is running water. In a reconnaissance made down the Rio Grande during the summer of 1895 by the junior author and Mr. T. W. Stanton, only one dug well was seen, and there were no settlements except those above alluded to and a few on the Mexican side of the river. The well mentioned was at Upson, about 16 miles above Eagle Pass. It is located near a stagnant pool, from which the water seeps into it. It seems reasonable to expect that wells sunk in the Rio Grande terraces would be successful, but no data are available upon which to base any definite conclusion.
WELLS IN THE FRY PAN DEPOSITS.
Wells are easily obtained in the gravel deposits of the fry pan gorge of Turkey Creek, in western Uvalde County. These have been sunk at Cline Station and at several points on Moore amid Allen's ranch to depths of from 20 to 40 feet, and water obtained just above the contact of the gravel beds with the Austin chalk. The Southern Pacific well at Cline Station is 12 by 12 feet and 40 feet deep, and affords an inexhaustible supply of water, being one of the best wells on that line of railroad. Where the stream bed of Turkey Creek cuts through the gravel down to the Austin chalk, near Cline post office, many springs break forth. These are large in times of rainfall, but fail in dry weather.
WATER OF THE TERRACE FORMATIONS OF THE COLORADO RIVER.
Good surface wells are usually obtained in the terrace deposits of the Colorado River, where they are of extensive area and not too much cut by later erosion. Such wells, however, are apt to vary appreciably with the rainfall, more in the coarser deposits than in those of finer texture.
Small seepage springs can be seen breaking out in many places at the contact of the gravels with the lower formations. This is noticeable after a rainfall in the case of the high Asylum terrace in north
277
Austin. These springs are also quite noticeable along the banks of the Colorado wherever the contact of the terrace and Cretaceous formations is visible.
WATER OF THE ONION CREEK MARL AND SIMILAR DEPOSITS.
Water is obtained from wells in the Onion Creek marl in the vicinity of Delvalle and between Pilot Knob and Buda at depths of from 25 to 50 feet. Shallow wells are also successful in the similar deposits along the Guadalupe River and other streams of that class.
The positions of the wells that have been sunk in the Leona and similar deposits in the vicinity of Uvalde are indicated on the map (fig. 70, p. 275). The depths of the wells are usually from 40 to 60 feet.
Water is obtained along some of the streams by sinking wells near the stream bed through the alluvial deposits. The water from the stream filters through the loose alluvium into the wells. The wells at Eagle Pass, according to reports, belong to this type. The old city waterworks at Austin derived their supply from wells of this class.
Records of wells in the alluvial deposits in Kinney, Edwards, and Uvalde counties.
Mouth of Griffin Gap, head of Pinto Creek. Total depth, 256 feet. This well was through cobblestones and red clay from top to bottom; struck a, running stream.
Nine miles south of east of Brackett, on Turkey Creek. Gravel of white limestone pebbles, 40 feet; basalt, 60 feet; water.
Moore and Allen's well, No. 3, at Cline. Total depth, 47 feet. All gravel.
Cline. Several wells at Cline Station, Cline post office, and in the vicinity of these places; all dug in the gravel of Turkey Creek Flat. Depth, 20 to 40 feet. Spofford. Several wells in the Uvalde gravel. These wells average about 40 feet in depth. The water is alkaline.
Hall's pasture. Well 66 feet deep. Limestone bowlders. Water in gravel. Pinto Flat, north of above, 9 or 10 miles north of Brackett. Yellow clay, red clay, and gravel. Plenty of water.
SURFACE WELLS AND SPRINGS OF THIS CRETACEOUS FORMATIONS.
The outcrops of the various Cretaceous formations of the Rio Grande Plain vary slightly in their water productivity, but in general the yield of all of them is small. This is especially true of those marly beds composed of the so called "joint clays," such as the Webberville, Taylor, Eagle Ford, and Del Rio, in which wells are seldom successful.
The limestone formations are more variable relative to one another, but are in general slightly more productive. Even in these success is not the rule. We have seen a few successful wells in the Austin chalk and the Shoal Creek, Washita, and edwards limestone, but failure is more frequent.
We have not many records of surface wells in the Rio Grande Plain, and the opinions above expressed are largely the result of casual observations. The following records, however, furnished us by Mr. M. C. "
Furnished by Mr. M. C. Ott, of Brackett.
278
Ott, of Brackett, give valuable data concerning the western portion of the region.
IN THE EDWARDS LIMESTONE.
No. 2, east of Las Moras Mountain, head of Guaxillo Creek. Total depth, 255 feet. Starts in Shoal Creek. Limestone to 15 feet of bottom; then 2 feet of slate; then 12 or 13 feet of sandstone. Plenty of water, probably from Edwards limestone.
IN STRATA BETWEEN THE EDWARDS LIMESTONE AND AUSTIN CHALK.
Brackett. Holmes well in Las Moras Flat, Spring street. Depth, 104 feet. Earth and gravel, 9 feet; white limestone, 30 to 40 feet (Eagle Ford); hard black rock, probably basalt, 76 feet. Could not get to bottom of this. Deep water at 18 feet; very bad.
Brackett. C. A. Windus's well. Total depth, 150 feet. Magnesian limestone, 60 feet (Eagle Ford). Bad water (gypsiferous). Water found at 37 feet (in Eagle Ford?). Hard limestone (Shoal Creek) at 60 to 100 feet. Black, magnetic limestone (possibly basalt). Water.
Moore and Allen's well, No. 4. Dug for 50 feet; drilled 100 feet. Water in sandstone at 150 feet. This well is probably on the Nueces, north of Southern Pacific Railroad, east of Cline.
Moore and Allen's ranch. Well No. 1, near head of Turkey Creek. Soil, 4 to 6 feet; "slate," 143 feet (probably Del Rio). Sulphur water at 149 feet. This well is probably in Del Rio.
Moore and Allen's ranch. Windmill well, 2 miles north of Cline. Soil with gravel, 20 feet; "magnesian" limestone, 20 feet (Eagle Ford) ; at 150 feet, Shoal Creek limestone, base of Eagle Ford; slate (Del Rio) at 190 feet. Water in black sandstone at 255 feet (possibly near top of Edwards and base of Washita division). Sulphur water below rock.
New York and Texas Land Company's well, 7 miles from Brackett, on Turkey Creek. Total depth, 404 feet. Earth and yellow clay, 17 feet; blue clay, 3 feet (Eagle Ford); lignite or asphalt, 11 feet (Eagle Ford) ; gray limestone, 12 feet (Eagle Ford); slate, 367 feet (Eagle Ford, Shoal Creek, and Del Rio). First water at 37 feet. Water, 404 feet in sandstone (probably same beds as foregoing).
Near Mariposa ranch, James Dignowity, northwest of Brackett. Depth, 155 feet. Little clay, thin limestone. This well is in a canyon 100 feet deep, and the narrator was uncertain as to horizons.
New York and Texas Land Company's well No. 1, at abandoned windmill 4 miles north of Brackett. Total depth, 269 feet. Hard gray crystalline limestone, 85 feet (Shoal Creek); slate, 184 feet (Del Rio); sandstone, 10 feet. Water rises 131 feet. This water is found in Del Rio.
Four miles east of Brackett, Turkey Creek road. Depth, 105 feet. Gravel on top. Dark blue sandstone struck within 10 feet of water. White sulphur water.
IN THE AUSTIN CHALK.
Fry pan well. Fry pan Valley, Brackett quadrangle. Soil, 10 feet; lime rock, 10 feet (Anacacho); blue limestone, 380 feet (Austin chalk). Most of this well is in Austin chalk.
Palmer's ranch, 8 miles west of Cline, 2 miles southwest of Waldo, north edge of Anacacho Mountains. Soil, 10 feet; white limestone, 10 feet (Austin chalk); blue rock, 280 feet (Austin chalk). No water was obtained.
Moore and Allen's ranch, north edge of Uvalde County, 8 miles west of Cline, 2 miles southwest of Waldo Station. Soil, 10 feet; white limestone, 10 feet (Austin chalk) ; blue rock, 280 feet (Austin chalk). Well quit in last. No water, but a seep.
On the southwest side of Pilot Knob, Travis County, along the branch that flows north into Onion Creek, there is a well sunk into the Austin chalk, 28 feet deep, that yields a constant supply of clear, pure water.
279
IN ANACAHO AND EAGLE PASS FORMATIONS
James's ranch well. Depth, 170 feet, through top of Anacacho limestone. Water salty and sulphurous.
Furness ranch, southwest corner of Anacacho Mountains. Sandy, muddy rock to sulphur water, 220 feet (Anacacho or Eagle Pass). Last water in white sandstone rises to 11 feet of the surface.
Moore and Allen's well No. 5, 12 miles east of Cline, in Uvalde County, near old California road, south of Nueces River, west of Big Mountain. Total depth, 250 feet. Green marl all the way (Eagle Pass beds). Water at 70 feet.
Another well 4 miles north of above. No coal or hard limestone. Black, clayey material.
Creek 3 miles east of Nueces River, on the road from Uvalde to Carrizo. Black sulphur water. Soft rock, species of slate or shale (Eagle Pass). Struck water first at 170 feet. Struck coal in three places; one vein 5 feet at 150 feet depth.
There is a well on the Frio River, below the Southern Pacific Railroad, near Connor's ranch, sank into decomposed basalt or phonolite. The water is sulphurous and smells very bad.
From the foregoing data the following generalizations can be formulated concerning the nonartesian wells of the Cretaceous formations of the western portion of the Rio Grande Plain : Water is obtained in the Edwards limestone on the downthrown side of the fault. Water can be obtained in the Del Rio clays, but we do not know its quality. That in the Eagle Ford shales is very bad. The wells in the Austin chalk are often entire failures. The water obtained in the Anacacho formation may be salty or contaminated by sulphur. Good water may be obtained from the Eagle Pass formation, but is sometimes salty. The water obtained in wells that are sunk into the decomposed igneous rocks is likely to be very bad, but that obtained nearby where such rock material is not penetrated does not seem to be especially contaminated.
ARTESIAN WELLS.
The Rio Grande Plain, so far as the inclination of the strata is concerned, with certain important local exceptions to be mentioned, presents conditions favorable for obtaining artesian water. The strata dip with the surface slope from the interior margin of the plain toward the coast at a very gentle angle. The plain as a whole is underlain by several artesian systems of different geologic ages. Two of these systems underlie the portion of the plain discussed in this paper. These are the water bearing sheets of the Edwards beds and the Travis Peak sands. Water also occurs in some of the Glen Pose beds.
The nature and availability of these reservoirs can best be understood by an examination of the data at hand concerning the various artesian wells, especially those at Austin, Manor, San Marcos, and San Antonio.
AUSTIN WELLS.
At least seven artesian wells have been sunk in the vicinity of Austin one at the State Institution for Colored Dependents, altitude 650 feet; the Groom well, altitude about 615 feet, and one at the Lunatic
280
Asylum, altitude 635 feet, both in the northern suburbs of the city, beyond the university; two on. the capitol grounds, altitude 520 feet; one on Fifth street east, altitude 500; and one at St. Edward's College, altitude 600 feet, 2 miles south of the river. They all pass through similar formations, as far as they go, but the Asylum and St. Edward's wells are the only ones which have been drilled to the basal Trinity beds.
The Groom and Capitol wells at the present time have only a small discharge, 5,000 or 6,000 gallons a day. The Asylum well had a discharge of 150,000 gallons a day and threw the water to a height of 40 feet. In St. Edward's well, which is 25 feet higher in elevation than the latter and obtains its supply from the same source, the water comes within about 5 feet of the surface and has to be pumped.
The Asylum well is 1,975 feet deep, and the formations, in order downward from the surface, are as follows:
Log of well at Lunatic Asylum (altitude 635 feet), Austin, as given by Mr. McGillvray, the driller. |
Mr. McGillvray states that stratum "b" is very hard limestone and is easily recognized whenever encountered; stratum "d," of 1,105 feet of limestone, contains occasional streaks of shaly water-bearing sand. In the Capitol well, at 400 feet, a small flow having disagreeable odor was encountered. The beds marked "i," or the 315 feet of Trinity (Travis Peak) sands, consist of alternate bands of from 25 to 35 feet of thin sand and from 5 to 6 feet of brown and reddish shale.
The strata enumerated may be converted into geologic terminology as follows:
(a) The lower part of the 80 feet of dark shale called "a" is the Eagle Ford shale, locally known as the "Fish beds." The surface portion is composed of Pleistocene terrace material.
(b) The 25 feet of very hard limestone represents the upper portion of the Shoal Creek beds.
"Statistics collected by Mr. Cyrus C. Babb, of the U. S. Geological Survey.
e to i inclusive = 490 feet.
It should be borne in mind that the thickness of strata as reported front churn-well drillings is never accurate within 10 feet or more.
281
(c) The 90 feet of blue marls are the Del Rio beds.
(d) The 1,105 feet of limestone marked "d" includes the Fort Worth limestone, 70 feet; the Edwards limestone, about 250 feet; the Comanche Peak and Walnut beds, about 60 feet; 450 to 500 feet of the Glen Rose formation, and 275 to 225 feet of the Travis Peak.
(e -i) The beds from "e" to " i," inclusive, 490 feet, are probably the basement beds of the Travis Peak formation.
(j) May or may not belong to formations older than the Cretaceous. The Groom well, altitude 615 (?) feet, is 1,300 feet deep, and is said to receive its supply of water from stratum " e." The total depth of the new Capitol well is about 1,450 feet. Its water is supposed to have its source in a stratum similar to that of the Groom well, but 100 feet deeper from the surface. The surface of the Capitol well is 65 feet lower in altitude than the Groom well. This would give a difference of 165 feet in the depth of stratum "e" between one well and the other. This difference may be real, and may result from the fact that the Groom well is geologically lower at the surface, from downthrow by faulting between the two localities.
The St. Edward's well, sunk in the winter of 1892-93, is 2,053 feet deep and obtains its supply from "i." At this point the surface is near the top of the Austin chalk, and is 500 feet above the bottom of the blue marl, or "c."
Two artesian wells have been bored by the State authorities on the capitol grounds. The first of these was in the year 1858 and was carried to a depth of only 471 feet. The log of this well, as published by Dr. B. F. Shumard, the first State geologist of Texas, is much more detailed than any other in the neighborhood, and is here given. The character and thickness of the different strata passed through, as shown from an examination of the borings preserved by Mr. Peterson, are given in the section, fig. 2 of Pl. XLI (p. 286).
 |
Texas Almanac, Richardson and Co., Vol. III, pp. 161-162 Galveston, 1859.
282
 |
The foregoing section of Dr. Shumard's may be interpreted as follows:
Dr. Francis Moore, in The Texas Almanac for 1860, page 96, has also
283
given a section of this well, which disagrees with the one given by Dr. Shumard both in minor details and in total depth. Dr. Moore's section is as follows:
 |
Our interpretation of these beds is as follows :
We have not been able to obtain a complete log of the second well bored by the State, six or seven years ago. It is reported that it was drilled to about 1,450 feet, and stopped just as it reached, and before penetrating, the greater basement group of water bearing strata. Hence its flow is feeble.
The well at the colored asylum, 2 or 3 miles northwest of the State capitol, commences in the Shoal Creek limestone and passes through the Del Rio and Fort Worth beds and penetrates the Edwards limestone. The water does not rise to the surface.
The following record of a well on East Fifth street, in process of boring oil May 1, 1897, which was obtained for us by Mr. E. W. Parker, of this Survey, will also afford an idea of the nature of the rocks encountered.
 |
Flowing 20,000 gallons of sulphur water per clay. Main Trinity flow expected at 1,400 feet. First sulphur water, 50 feet below summit of Edwards beds.
284
Further particulars of this well, since completed, have been furnished as follows by Mr. H. McGillvray:
 |
The flow is about 250,000 gallons a day, and is increasing; temperature, 100°. Pressure has not been tested, but is probably between 70 and 80 pounds. Casing, 300 feet 10 inch pipe, 600 feet 8 inch pipe, 300 feet 7 inch pipe, 200 feet 6 inch pipe. The medicinal vein has been separated from the freestone vein b3 letting the flow come up between the 8 and 10 inch pipes. This water will be used for a natatorium and sanitarium.
There is a discrepancy between the two records given above which we can not explain. So much as is given in Mr. Parker's log is more in harmony with the stratigraphic measurements made in the region. The strata from 1,915 to 2,025 feet may be Paleozoic.
THE MANOR WELL.
A valuable contribution to the extent of the artesian field in Travis County was made by the drilling of the well at Manor, in 1895. This well is situated about 20 miles east of Austin, near the extreme eastern
285
margin of the Black Prairie and on the outcrop of the base of the lower portion of the equivalent of the Eagle Pass beds. Through the kindness of Mr. E. W. Parker we have been enabled to obtain the following accurate log of this well, which enables us to locate the water vein with definiteness and also to ascertain the thickness of the Taylor and Austin beds of the Upper Cretaceous, which had not until then been accurately measured:
![Log of well at Manor. (See fig. 7 of Pl. XLI.) Thickness in feet. Depth in feet to bottom of strata. 16. Black soil 6 6 15. Yellow clay 11 17 14. Flint rock and gravel (surface water) 3 20 13. Yellow and joint clay 30 50 12. Blue clay 540 590 11. Soft white rock 74 664 10. Indigo blue clay 1 665 9. Rock, hard and soft alternately 335 1,000 a8. Shale (caves badly) 25 1,025 b7. Hard rock 50 1,075 c6. Blue clay (caves) 60 1,135 d{5. Lime rock 115 1,250 d{ 4. Water-bearing rock (water flows at 1,250 feet) 150 1,400 d{3. Hard rock 20 1,420 [From this point downward we have no detailed record.] Quantity of water discharged per hour, 4,166⅔ gallons. Size of discharge pipe, 6 inches. Temperature of water, 93°.](figures/txu-oclc-27281517-285-t-a-800.jpg) |
There are several records of this well, which show slight discrepancies. The one presented gives the most detail and appears to be most nearly accurate.
Having very recently studied the section of the rocks through which the Manor well passes, we can easily identify the different strata as follows:
Furnished by Calcasieu Lumber Company. Exact location of well: Middle of lot No. 13, in block No. 25, according to map and plot of Manor.
Three separate accounts give this strike at 1,250, 1,265, and 1,280 feet, respectively.
286
Water will rise in pipe about 30 feet above the surface. The first effort to dig this well failed, as it caved in at the depth of about 1,100 feet. The present well, bored by Mr. G. J. Eppright, was finished about February 13, 1896, and cost $4,060.
All the information we have been able to obtain concerning the wells of Travis County is plotted on Pl. XLVI (p. 282). In this we have endeavored to show the geologic position of the surface at the location of the well, and have referred all the wells to a common geologic datum the top of the Shoal Creek limestone, which seems to be recognizable in most of the well records.
From the records thus plotted valuable generalizations can be made, as follows : There are at least three important water bearing horizons in the Lower Cretaceous series beneath Austin. The first of these, which is conspicuously illustrated by the Manor flow of 1896, is recognizable in the old and new Capitol wells and in the Sixth street well now under construction. This water was also probably struck in the other three wells, but we have no detailed record thereof. It is highly charged with sulphur and injurious mineral ingredients, as will be described later, and in all instances where encountered should be cased off. This water is undoubtedly obtained in the Edwards beds, from strata which, according to the best computations we can make at present, lie from 21 to 50 feet below their summit, or 90 to 140 feet below the summit of the limestone group, commencing below the Del Rio clays.
The next flow occurs about 1,000 feet lower in the series, in strata which may be considered as practically the transition ground between the Glen Rose and Travis Peak beds. This water is the uppermost of the water bearing strata characterizing the base of the Cretaceous.
In the Asylum well the above mentioned flow is separated by about 160 feet of limestone and shale from the third and lowest of the water bearing beds, the basement, Travis Peak, or "Trinity" sands. These produce the purest water, and ordinarily the most, of all the water bearing strata of the series. According to Mr. McGillvray's record, these water bearing beds are 315 feet in thickness, and are marked by thin bands of reddish clay.
It will be seen that only one of the wells has positively penetrated the entire series of beds composing the Cretaceous system, thereby exploiting its fullest capacity and reaching into the underlying impervious Paleozoic formations. This is the well at the Insane Asylum. It is very probable that the well at St. Edward's College has also penetrated the entire thickness of water bearing strata, but we have been unable to obtain an exact log of it. All the other wells are incomplete, inasmuch as they have not reached the best water of the district that which lies close to the base of the Cretaceous system, "
According to the best computations we can at present make, sulphur water was struck in the Edwards formation at a depth of 120 feet below the bottom of the Del Rio clays in the Sixth street well, 78 feet in the old Capitol well, 50 feet in the new Capitol well, and 35 feet in the Manor well.
287
at a depth of from 1,400 to 1,750 feet below the top of the Shoal Creek limestone.
The fact that the water does not flow out at the surface at St. Edward's College (altitude 660 feet) indicates that the 650 foot contour on the Austin geologic sheet marks the possible limit of flowing wells in the vicinity.
THE SAN MARCOS WELL.
Two records of an artesian well drilled for the United States Fish Commission at San Marcos in the year 1895 afford the most complete well section we have been able to obtain in the Rio Grande Plain. The records consist of
The well was drilled to a depth of 1,490 feet, and when stopped was still in the Cretaceous formations, about 175 feet above the estimated base of the Asylum well, at Austin.
![Log of the San Marcos well; furnished by Mr. Judson, the contractor. [See fig. 8 of Pl. XLI.] Thickness in feet. Depth in feet to bottom of strata. Soil, wash, and gravel 30 30 Blue clay and limestone (Del Rio beds) 20 50 Hard gray limestone (Fort Worth) 78 128 Cave with abundant water 2 130 Brittle white limestone (Edwards) 72 202 Black flints (Edwards) 202 Blue shale (Edwards) 12 214 Black flints (Edwards) 6 220 Soft white limestone (Edwards) 106 326 Hard white limestone (Edwards) 57 383 Hard gray limestone 37 410 Rotten limestone 41 451 Hard gray limestone 35 486 Trace of blue clay 486 White limestone 47 533 Hard white limestone 92 625 Hard blue clay 27 652 Hard white limestone 78 730 White clay 5 735 White limestone 25 760 White clay . 5 765 Limestone 34 799 Hard blue limestone 11 810 Brittle white limestone 225 1,035 Gray limestone 77 1,112 Blue clay 7 1,119 Hard gray limestone 59 1,178 White limestone 71 1,249 Hard yellow limestone 42 1,291](figures/txu-oclc-27281517-287-t-a-800.jpg) |
288
 |
 |
289
 |
The San Marcos well commences in strata about 150 feet lower geo- logically than the Austin wells, the former commencing near the base of the Del Rio and the latter near the base of the Austin chalk. If the figures for the Insane Asylum well are correct, the San Marcos well has not yet reached the bottom of the Cretaceous strata by some 100 feet. In drilling the San Marcos well seven horizons of underground water were encountered between 128 and 1,450 feet, as follows:
 |
The first of these horizons (No. 1), struck at 78 feet below the summit of the Fort Worth limestone, or about 8 feet below the estimated summit of the Edwards beds, is the same general geologic formation as that giving the first flow of the Austin, Manor, and San Antonio wells.
No. 2 is probably a lower horizon in the Edwards beds, and its freedom from sulphur is a valuable quality.
18 GEOL, PT 2-19
290
The sulphur impregnation in No. 3 is probably a local layer in the Glen Rose beds.
No. 4 seems to correspond almost exactly with flow No. 2 of the Austin wells, that from which the new Capitol and Groom wells are supplied.
Nos. 5, 6, and 7 are, beyond any reasonable doubt, from the basement sands.
There are one or two elements of perplexity concerning the Sail Marcos well which also present themselves at San Antonio. It is located almost upon the line of a great fault, which may be seen on the west side of the spring pool a short distance from the well. It is possible that the drill hole may cross this fault line not far below the surface, and that the so called caverns are its waterworn fissures. The fact that this water was full of peculiar cave inhabiting animals indicates that there are cavities beneath the ground, the extent of which, however, can not be stated. These may be pockets, such as are seen in the outcrops of the Edwards limestone, or they play be extensive caves, like the Hillcoat caverns of Edwards County. The fact that the drill passed through only 2 feet of cavity rather opposes the latter hypothesis.
SAN ANTONIO WELLS.
A large number on wells have been drilled in and around the city of San Antonio. They occur in nearly all parts of the city and its adjacent suburbs, as shown in fig. 73 (p. 310)
These wells, like the well at San Marcos, are all drilled close to the lines of faulting, and hence present many anomalies. It is very possible that the drill often crosses the lines of faulting, producing inconsistencies in the record.
The following list is incomplete, but it will afford alt idea of the approximate number and location of the wells.
 |
Described by Messrs. Stejneger and Benedict, in Proc. U. S. Nat. Mus., Vol. XVIII, April, 1896, Nos. 1087 and 1088.
291
 |
292
The underlying geologic structure of San Antonio and immediate vicinity is difficult to discover, owing to the fact that the region is mostly covered by a sheet of superficial gravel and marl (the Uvalde formation). The depth of this alluvial sheet can not be stated or even approximated with accuracy. In some wells it is 40 feet in depth, in others more than 100. The variation in thickness is caused by its having been deposited upon the unevenly eroded surface of the Austin chalk. The Austin chalk outcrops near the San Antonio and the San Pedro springs, striking in a northeast southwest direction at those points. It is probable that southeast of its outcrop the Taylor formation underlies the Uvalde, and that all the Cretaceous beds descend gently toward the southeast. The concealed strata are much jointed and possibly faulted in this area. The limestones are also very cavernous.
We have been unable to obtain detailed records of many of the San Antonio wells, but the following data will give a general idea of their character.
Mr. C. S. Austin informs us that the wells in the valley of Alazon Creek, just west of the center of the city, range from 225 to 450 feet in depth. Their flow is about as strong as that of the wells in the valley of the San Antonio.
 |
Gustave Jermy makes the following notes concerning artesian wells in the vicinity of San Antonio:
One, of a depth of 225 feet, in the western addition to the city of San Antonio, with a fine quality of drinking water which forces its way to the surface. Another, 4 miles east of San Antonio, near the Salado, which was sunk to a depth of 450 feet and also brings a constant stream of water, containing hydrosulphuric acid, to a considerable height. It is clear and is being utilized for the ordinary wants of man and beast, but could be made more useful on account of its medicinal qualities.
The following record has been published by Roessler:
 |
Texas Geological and Mineralogical Survey, First Rept. of Progress, for 1888, 1889, p. 64.
Report of F. E. Roessler, division field agent for Texas, Fifty first Congress, first session, Ex. Doc. No. 222, Washington, 1890, pp. 243-319.
293
Other wells have been reported as follows:
 |
Eyeless salamanders were found in the water. It is located in a faulted district. A well one-third of a mile southeast of this struck water at 540 feet and went 90 feet through a cave.
 |
Judson Brothers, who bored the wells for the Waterworks Company, furnish us the following record:
 |
Total depth of well, 880 feet. Has a strong flow of water, struck at a depth of 880 feet from the surface. This flow fluctuates slightly, but has a maximum head of 51 feet above the river. The flow is estimated at 5,400,000 gallons per diem. Water slightly impregnated with lime.
The water came from a crystalline limestone out of a fissure 3 feet in depth. We have sunk nine flowing wells for Mr. Breckenridge, varying in depth from 630 to 883 feet. All passed through about the same formation.
The following information concerning the well of F. F. Collins Manufacturing Company was furnished through the owners by Mr. James Brown, who drilled the well. (See P1. XLIII.) The well is located 900 yards northeast by north from the new court-house of Bexar County, on the banks of the San Antonio River. It is 20 feet from the water line of the river; the level of the surface is 14 feet above the
294
river. The well was first sunk to a depth of 40 feet, 10 inches in diameter, and cased, then continued to a depth of 440 feet, or until the sulphur and iron water was reached. It was then cased with 6 inch casing from the top, carried down that size through 280 feet of a light colored limestone rock, and through 75 or 80 feet of a blue clay, and sufficiently far in the next formation to be secure. From this point the boring was continued through this rock, which was firm enough to stand up without any casing, until a final depth of about 1,100 feet was reached. The "sweet" water was found in this rock at a depth. of 880 feet from the surface, and gave a flow of 200,000 to 250,000 gallons in twenty four hours. This water rises in a standpipe to about 40 feet above the level of the San Antonio River. The sulphur iron water struck 440 feet below the surface gave a flow of 5 to 10 gallons per minute; associated with it was gas, given off at the rate of 30 to 40 cubic feet per minute. The gas burns well in an Argand burner with Welsbach attachment.
The flow of "sweet" water comes from the upper part of the Edwards limestone, 50 to 70 feet below its top.
The most complete data we have been able to receive concerning the strata penetrated at San Antonio and vicinity are those furnished by Col. C. M. Terrell, United States Army, concerning the well drilled by him.
This well is 2 miles due south of the city limits, on the west side of the main road to Pleasanton and on the divide between San Antonio and the Medina River. It is on a summit, and so situated that the water is now carried for purposes of irrigation to all parts of the land through 61 miles of surface acequias, and the surplus water can be turned into the San Antonio River, 4 miles east, or into the Medina River, 5 miles south.
The well is about 1,900 feet deep; the yield is estimated at 800,000 gallons per day. Temperature, 106° F. The water overflowed with force from the top of a pipe 68 feet above the surface. When a 22 foot joint of pipe was added. the water rose in the pipe to about 84 feet above the surface. This well at the surface of the ground is 51 feet 6 inches below the level of the main springs of the San Antonio River, and at its greatest depth is 1,270 feet below the level of the Gulf of Mexico. The elevation above the surface to which the water will rise is 84 feet, or 32 feet 6 inches above the level of San Antonio springs. Colonel Terrell says that this fact convinces him "that the confined basin from which it comes has no direct connection with the source of supply of the other flowing wells in this vicinity," but in this opinion we are inclined to differ from him, for there is every evidence that the sources of the waters are identical. While lower in altitude than the San Antonio springs, the outlet of this well is some 650 feet higher in the geologic series. These differences, and similar conditions in all the wells east and south of the city, such as Kampmann's, Dullnig's,
295
and that at the State Lunatic Asylum, are due to the dips of the strata.
Colonel Terrell gives the following log of the well, a graphic record of which is given in P1. XLIII, fig. 12.
Formations: Ordinary surface soil, 6 feet; coarse gravel, 5 feet; yellow joint clay, 50 feet. From this the drill passed into the blue marl. At 140 feet below surface it passed through about 10 inches of brown coal in the blue marl; the same at 240 feet; the same at 280 feet. Six hundred feet below the surface it passed through 20 feet of very hard shell conglomerate composed of Gryphæa vesiculcaris (identified by Robert T. Hill), the largest shell of which is 4 by 3½ inches, thickness of walls one half to three fourths inch. The blue marl cement of this shell conglomerate was hardened into rock.
At 1,000 feet below the surface the drill struck limestone rock, supposed to be the Austin chalk, which crops out northwest of San Antonio. At 1,310 feet a small seep of sulphur water was struck in this rock. This was the first water found in the well. The drill passed through the upper limestone at 1,380 feet and into a bed of yellow joint clay, resting on about 10 feet of shells, together 45 feet. Immediately below this, at 1,425 feet, it struck limestone, supposed to be the Caprina (Edwards) limestone. At 1,535 feet a small overflow of water was struck. When the second contract was completed, at 1,758 feet, there was an overflow equal to about 18 gallons per minute. At this depth, having completed his second contract on the well, the contractor declined to make another contract for depth, and was employed thereafter by the day. From 1,625 to 1,758 feet, Colonel Terrell had noticed an increase of temperature of the water, equal to 1° for each 25 feet of descent. This rapid increase led him to believe that the strata were porous for at least 100 feet below the then bottom of the well, and that the water was working up from below. If his reasoning was correct in this, then there would be no difficulty in drilling deeper with satisfactory daily progress, and the result demonstrated the correctness of his view. Rapid progress was made 142 feet in nine days with a less increase of beat for corresponding additional depth.
It was intended to sink the well to 2,000 feet, but the contractor quit at 1,900 feet. At intervals of about 70 to 75 feet below the first water that overflowed, and after passing a very hard stratum in the rock, the drilling became easier, and there was in increase of the overflow, each of such sections giving an increase over the preceding. The greatest increase, however, was below the depth of 1,885 feet.
This well must have been very nearly through the Edwards limestone at the time when the drilling ceased. It is probable that the flow and pressure would have continued to increase if the well had been sunk deeper.
The flow from the well seems to have slightly increased, notwithstanding two years of drought since it was completed, and the temperature
296
has gone up from 103.8° to 106° F. Some claim to have found it 108° and 109°. This increase in temperature may be accounted for in part by heat being abstracted, for some time after the well was completed, to warm up the 1,900 foot channel.
The estimate of flow, 800,000 gallons in twenty four hours, was made by the contractor. Colonel Terrell's own estimate is slightly less. Other wells near this city having less minimum diameter of bore and less heat are estimated at 1,200,000 gallons in twenty four hours, from which fact Colonel Terrell concludes that there is a tendency to overestimate the flow from artesian wells. Colonel Terrell thinks that the flow of water is slightly influenced by atmospheric pressure, a low barometer causing an increased flow.
An analysis of the water was made some time after the well was completed, and is given on page 301. It will be seen from the analysis that the water contains no chloride of sodium and no organic matter. The carbon dioxide is equal to 34 cubic inches of carbonic acid gas to the gallon, and the hydrogen sulphide to 8 inches of sulphureted hydrogen gas to 1 gallon of the water.
When the surface flow in the acequia is obstructed, a floating scum accumulates. This scum has not been analyzed. The acequia terminates in a lake 2,200 feet long on the tract of land. The lake is stocked with rock and black bass, striped perch, cat, minnows, turtles, etc. Minnows have been seen in the acequia within 100 feet of the well, where the water has nearly its maximum heat.
The water has been used for two years for irrigation. A small orchard, 1,700 fruit trees, depends almost entirely on this water, and the trees are doing well. Garden vegetables, corn, cotton, oats, and grass seem to do well when irrigated with the water. Stock drink the water freely for months at a time having no other and it is said to free them from ticks. In one case, when given for the purpose, it entirely freed a horse from bots and other intestinal worms.
Many sick people have visited the well to use the water, both for drinking and for bathing. Afflicted persons who have tried it claim that by drinking and bathing in the water they have been completely cured of many diseases.
No commercial use has been made of the water, but it is daily used at the well and carried away for use, by and for sick people, without charge. We have plotted upon one plate all the well data which. we have been able to procure relating to the vicinity of San Antonio. (See Pl. XLIII.) Many of these records are meager, giving only the depth at which water was found, unaccompanied by any description of the rock material passed through. The Terrell and Collins wells, however, may be considered the best available standards of reference, to which the others may be compared. We have referred all these sections to a geologic datum plane, which approximately corresponds with the top of
297
the Shoal Creek limestone. Upon examination of these sections it will be seen that there is a notable absence of sandstone strata, and that the water occurs in limestones and marl. Furthermore, there are two persistent occurrences of water in the general geologic section as far as explored, accompanied probably by one or two minor horizons. The uppermost of these consists of a stratum off sulphur water, which is struck in many wells in a geologic horizon near the base of the Austin chalk, possibly in the Eagle Ford shales. This horizon is accompanied by gas and oil. These impurities are such as one familiar with these rocks would naturally expect to find in them, owing to the bituminous character of the shales and the excessive accompaniment of iron pyrites, which no doubt causes the element of sulphur to enter into the water. It is very probable that there are more than one of these horizons in the lower part of the Gulf series, as will be seen in some of the sections given.
The "sweet water," as it is locally called, however, lies about 200 feet lower than that at the base of the Austin chalk, and undoubtedly occurs in the upper part of the Edwards limestone, or about 100 feet below the bottom of the blue black clay of the Del Rio beds.
In Terrell's well the pressure and volume of this water increased as the drill was pushed for a distance of over 350 feet below it, being strongest at a depth of 1,850 feet. It is very probable that the drilling was stopped just as the Glen Rose beds of the Trinity division were being encountered, and we possess no record of wells drilled into the basement sands of the Trinity division, which supply some of the Austin and Waco wells. We have heard rumors that certain wells have been drilled to a depth of 3,000 feet in the city of San Antonio and failed to obtain water from the base sands, but we have no positive facts bearing upon this subject.
From our knowledge and observation of the thickness of the strata in adjacent regions, we estimate that the basement sands of the Comanche lie from 1,000 to 1,300 feet below the sweet water flow at San Antonio. We can not predict whether these will prove water bearing or not.
These records, together with the depths given in other cases, all show that the lower or "sweet" water is obtained in the foregoing wells in the city at depths varying with the dip, from 500 feet in the extreme northwest to 880 feet at Alamo Plaza.
In general, the wells deepen toward the south and east, as is illustrated by the depths of the wells belonging to the San Antonio waterworks and those at the State Lunatic Asylum and at Terrell's.
The records in the northwestern part of the city show that the water "
The exact thickness of the Fort Worth limestone at San Antonio is not known. Estimating it at 50 feet, the water in the Edwards limestone (the San Antonio flow) is about 50 feet below the top of the formation, corresponding with the Austin, Manor, and San Marcos records.
298
was struck at about 350 feet; the wells to the east, about one-half mile below the head of the springs, are 630 feet deep; 1 mile down the river they are about 780 feet deep, and the five wells in the city, about 1½ miles still lower down, are about 880 feet deep. These records show a progressive deepening of the wells of 250 feet in 3 miles, or about 83 feet per mile. Colonel Terrell's well, the exact distance of which from the city wells can not be ascertained, but is about 4 or 5 miles, did not secure flowing water until over 1,500 feet had been drilled.
This deepening of the water bearing rocks is in accord with the geologic conditions previously explained. The limitations of the available belt at San Antonio have not been fully ascertained. We know that Cable's ranch well, 8 miles north of west of San Antonio, is probably west of the line of success, an unsuccessful well having been drilled there to a depth of 2,000 feet. This locality is probably west of the fault line, which runs from Helotes northeast, via Leon Springs, to New Braunfels. West of this line no flowing wells are obtainable, except in the canyon valleys of the Edwards Plateau, as previously explained.
Colonel Terrell's well shows that the available water is deepening rapidly to the east and south. Mr. John Wickland, 12 miles east of San Antonio, drilled 700 or 800 feet through the blue clays of the lowermost Tertiary and uppermost Cretaceous. He would probably have to go at least 1,000 feet lower than at San Antonio to obtain the sweet water.
There are many resemblances between these wells and those at Austin and San Marcos. By comparison with the log of the old Capitol well, given on page 282, it will be seen that the San Antonio "sweet water" corresponds approximately in its depth and geologic position to the sulphur water obtained in the old Capitol well at 323 feet. The geologic horizons of the surface are slightly different, but the details of the section below the Austin chalk are similar. The water in both is obtained in the upper part of the Edwards limestone. The geologic position of the "sweet water" is very similar to the second water struck in the San Marcos well.
At the well of the Crystal Ice Company it is said that the " water is apparently an underground stream, the head of which is covered with a red or dark yellow clay, mixed with decomposed clay." This description suggests the red clay and fossils so frequently found as residuum in the caverns of the Edwards limestone , as can be seen west of Austin.
The occurrence of underground caverns and eyeless cave animals, and alleged underground streams, are also phenomena peculiar to the Edwards limestone of the Austin and San Marcos regions.
The only limestones of the Cretaceous series which leave a bright red clay residuum are found in the Edwards beds.
299
THE DEL RIO WELL.
From San Antonio west to the Rio Grande, we possess but few records of well experiments, and know of but one flowing well in that direction. A mile or two south of Del Rio, Valverde County, a well was bored to a depth of 460 feet which struck a flow of unpalatable sulphur water. This well was commenced at a horizon near the top of the Del Rio clays, and the water was obtained from one of the upper beds, probably the Edwards limestone.
Mr. John Wood, of Del Rio, put down a well in the town 760 feet in depth in which water was struck at about 565 feet. This contained alum, sulphur, and iron, and there is little doubt that it came from a mineral horizon of the Edwards beds. The water rose to within 60 feet of the surface. It is to be regretted that this well was not continued lower, for it is reasonably certain that the more potable and abundant waters which supply the adjacent San Felipe springs would have been encountered within the 1,000 feet or more of the Cretaceous strata underlying the present bottom of the well.
An unsuccessful experimental well was also drilled at Spofford Junction some years ago by the Southern Pacific Railway Company. This is further discussed on a succeeding page (p. 318).
CHEMICAL QUALITIES OF THE WATERS.
Each of the various water bearing beds enumerated in the foregoing pages possesses peculiar chemical qualities. It is exceedingly difficult, for various reasons, to determine the properties of each particular bed. In wells penetrating one or more of these the water is invariably mixed, so that when it reaches the surface it is not representative of any single bed. In the second place, such analyses as have been made were mostly for individual parties and are inaccessible. No systematic comparison of the waters as a whole has been made. It would be an interesting experiment to collect, analyze, and compare these various waters with one another and with those of the spring rivers, and the writers hope that it will yet be done.
From our knowledge of the rocks through which these wells are drilled, we know that the different strata vary greatly in chemical character, some being comparatively free from all mineral ingredients except lime carbonate, which is nearly always present, while others carry various accessory impurities. The Austin chalk, miscalled magnesian limestone, contains no magnesia, and hence the magnesian constituent of the water must originate below that formation. It does contain considerable pyrites, however, sufficient to supply the iron and sulphureted hydrogen derived from this horizon at San Antonio.
The Eagle Ford shales are somewhat similar to the Austin chalk, so "
See analyses in Third Ann. Rept. Geol. Survey Texas, pp. 351-352,354.
300
far as accessory minerals are concerned, containing, however, more pyrites and bituminous and lignitic matter.
We have never been able to obtain a complete analysis of the Shoal Creek limestone, but there is reason to believe that water penetrating it would take up sulphur, iron, and saline impurities, such as can be seen incrusting it at Austin, where we have seen efflorescences of salt and magnesium sulphate. We have little doubt that any water transmitted through these beds will be strongly "mineralized."
The Del Rio clays are very impervious and not apt to affect underground waters seriously, unless they percolate through the pyritiferous fossiliferous beds, in which case much sulphureted hydrogen will be present.
The waters from the Fort Worth limestone and the upper part of the Edwards are highly impregnated with mineral matters, somewhat analogous to those of the higher Shoal Creek limestone and lower Glen Rose beds. The minerals form an efflorescence on the surfaces of the protected rock ledges, or in the bluff caverns, as can be seen in many places west of Austin.
The waters from the middle and lower parts of the Edwards beds are singularly free from any mineral accessories except magnesium and a trace of sodium, and this condition harmonizes with the composition of the San Antonio "sweet water."
In the upper Glen Rose beds, as can be seen in the bluffs of Mount Bonnel, west of Austin (P1. XXVIII, p. 222), there are certain strata that contain strontium, magnesium, and sodium, which would materially affect the water. Fortunately we know of no wells in this province affected by them.
Another magnesium horizon occurs near the top of the Travis Peak beds, but below that these beds are very free from any unpleasant ingredients, as is attested by the analyses of the waters and certain of the beds.
The following table shows all the analyses we possess of these rocks. Many of the materials deleterious in water have not been determined, but the variation of some of them is clearly shown.
301
We present also, in the table on the next page, for what they are worth, a few analyses of the water from the various beds, such as we have been able to gather from miscellaneous sources. These indicate that there are four or five broad classes of water, varying in chemical impurities as much as in geologic occurrence.
The analyses are expressed in parts per million. Analysis 1 was made by Prof. W. A. Noyes and is stated in grains per liter. The other analyses are stated in grains per gallon. No. 2 is taken from a circular furnished by Colonel Terrell and bearing the signature of Professor Noyes. No. 4 was made by Prof. H. W. Harper, of the University of Texas. Nos. 5 and 6 are taken from Roessler's report, page 271, in "A report on the preliminary investigation to determine the proper location of artesian wells, etc." (Fifty-first Congress, first session, Ex. Doc. No. 222,1890.) The name of the analyst is not given. All of the analyses have been recalculated and the mode of statement has been changed. The hydrogen is derived from the bicarbonates;
302
 |
We present here, also, the following sanitary analyses of some of the spring waters from near Austin and of the Colorado River water at the power house at the dam.
303
![Analyses of waters from Austin and vicinity. [Dr. Henry Winston Harper, University of Texas, analyst. Samples collected March,14, 1896. Parts in 1,000,000.] Spring in Spring on Large spring River water power beach half south side from pump house. mile below of river in power power house. (Barton's spring). house. Total solids 298.0280.0 325.00 296.0 Temporary hardness72.0 hardness 83.06 50.00 66.13 Permanent hardness 34.4 37.90 44.08 50.53 Total hardness 106.4 120.96 94.08 116.66 Chlorine 28.28 12.62 22.22 29.29 Nitrogen as nitrates 0.125 1.000 0.667 0.080 H2S. none none none none Sulphides none none none none Sulphites none none none Dr. Harper states that the total solids fluctuate from 260 to 300.](figures/txu-oclc-27281517-303-t-a-800.jpg) |
The similarity of results of analyses of water from the spring in the power house and of the river water from the pump in the power house is suggestive of a relationship between the sources of the two waters. This feature presents peculiar interest because the results differ very widely from the analyses made of these two waters during the spring of 1895, and the limit of error does not account for the variance. This feature is worthy of further study.
![Chemical analyses of waters from Austin. [Harry W. Clark, chemist in charge of the Lawrence Experiment Station of the Massachusetts State board of health, analyst. Samples collected March 21, 1896; examined March 30, 1896. Parts in 1,000,000.] River water from pump in power house. Spring in power house. Sprin on south side of river (Stern sprin). Turbidity decided very slight none Sediment decided very slight very slight Color 3.1 1.0 Total solids 289.5 264.5 Free ammonia 0.108 0.002 Albumenoid ammonia 0.112 0.048 Chlorine 28.300 29.800 Nitrogen as nitarates 0.320 0.280 Nitrogen as nitrites 0.008 0.004 Oxygen consumed 1.400 0.600 Hardness 140 130](figures/txu-oclc-27281517-303-t-b-800.jpg) |
304
![Analysis showing saline constituents of the water from the artesian well located near San Jacinto and Fifth streets, Austin, Texas. [Dr. H. W. Harper, associate professor of chemistry, University of Texas, analyst.] Sulphureted hydrogen gas, 6,597 cubic inches per United States gallon. Carbonic acid gas, 43.971 cubic inches per United States gallon. The gases were measured at 16 C., and 750 mm. barometric pressure. Temperature of water as it issues from the well, 21.2 C. Depth of well, 182.88 meters (600 feet).](figures/txu-oclc-27281517-304-t-a-800.jpg) |
From the analysis last given, it will be seen that this well yields a sulphur water resembling the waters of
With the above statements and analyses in mind, we can now briefly consider the character of the water of the various horizons reported.
We know from the well records that at San Antonio certain ferruginated and sulphureted waters are met with in the Austin chalk and Eagle Ford clays. This water is also accompanied by or is closely adjacent to oil and gas, and is usually piped off so as not to contaminate the purer water below.
We possess no data proving that water has been obtained from the mineral charged Del Rio and Fort Worth beds, although we have suspected that in Kinney County several sulphur waters were derived from these horizons.
Concerning the Edwards: limestone, however, we feel justified in speaking with more positiveness. The upper strata are strongly impregnated with mineral ingredients which the waters take up in passing through them, as shown by wells at Austin, Manor., San Marcos, San Antonio, and possibly Del Rio.
305
The vein of mineral water struck at the old Capitol well in the Edwards beds at a depth of 325 feet from the surface, according to Dr. B. F. Shumard, had a saline taste, and was strongly impregnated with sulphureted hydrogen gas. A qualitative chemical analysis of this water by Prof. W. P. Riddell, chemist of the Texas geological survey, showed it to contain the following constituents, named in the order of their relative abundance:
A quantity of the Edwards limestone water from the Manor well was sent to the chemical department of the State university at Austin for analysis, but a detailed report had not been received at last advices. Dr. Stromberg made the following qualitative analysis of the water:
Dr. Stromberg says that the above ingredients correctly proportioned would represent approximately an analysis of the water.
The following partial analysis showing the chief ingredients of the Manor well has been kindly furnished us by Prof. J. C. Nagle, of the State agricultural and mechanical college:
This water is strong in salt, magnesia, and gypsum, while the strontium and lithium are quite high.
A small flow of sulphur water corresponding to the foregoing was also struck in the San Marcos well at a depth of 162 feet, or 32 feet below the estimated top of the Edwards beds.
The mineral water of Terrell's well at San Antonio, according to the record, also appears to be from this horizon.
The highly impregnated mineral waters of the Del Rio well may also
306
come from the same upper horizon of the Edwards. We have records of several strong sulphur wells (nonartesian) in Kinney County apparently from the same source.
A variation is noticeable in all these analyses, which, notwithstanding their imperfection, are sufficient to show that the waters of approximately the same horizon are chemically dissimilar in different localities. For instance, the strontium of the Manor well has not elsewhere been reported from the Edwards beds. Furthermore, in the vicinity of San Antonio the waters vary greatly in different places.
The purer or "sweet water" of the San Antonio wells probably comes from a horizon only a short distance below the mineral horizon of Terrell's well. We have been unable to procure an analysis of this water, although we suspect that the analysis of the Kampmann well given in the table represents these potable Edwards limestone waters. This has magnesium and lime as its chief ingredients the normal material of the Edwards limestone. The record of the San Marcos well also indicates that two classes of water may be obtained from the Edwards limestone, one of which is soft and potable while the other is highly charged with sulphur and various chemical ingredients.
The minerally impregnated beds of the Glen Rose, containing epsomite and other deleterious substances, have not been encountered in the San Antonio wells, although we strongly suspect that they have contaminated certain of the Austin wells. In the still lower Travis Peak formation the waters are softer, more potable, and freer from sulphureted hydrogen, containing mostly bicarbonates in solution. The waters of the latter formation have been utilized, so far as we know, only in the State Insane Asylum and St. Edward's College wells at Austin. These are so much purer and better than the overlying "mineral" waters that one wonders why all the other wells of that city have not been driven down to them.
From the various analyses given it will be seen that the chief mineral impurities of the mineral waters derived from the Edwards beds are chlorides and sulphates, calcium, magnesium, and sodium. In this they differ greatly from the potable water of the lower Trinity (Waco) beds, in which the chief ingredients are bicarbonates and sulphates of sodium, while the total solids in the former are nearly five times as great as those in the latter.
The fact that the waters of the fissure springs appear to have the properties of the lower Edwards and Travis Peak artesian waters leads us to accept the hypothesis that the latter waters lie below much of the region in which the fissure springs occur, west of San Antonio, and have not yet been penetrated by the artesian drillings.
From these studies of the chemical relations of the rocks and waters we think some useful deductions can be made. Bad waters can and should be cased off wherever encountered, and the well continued until a purer flow is struck or the Cretaceous system entirely passed. This
307
is done in many instances, such as the principal wells of San Antonio. In our opinion, no matter how firm the drill hole, it would pay in all instances to drill wells to the lower waters and case up the well above them in order to insure against the seepage of these mineral waters. In the light of the facts set forth in this report it will hardly be excusable hereafter to allow such waters to flow as those issuing from the State Capitol well at Austin, the State Insane Asylum well at San Antonio, and many other similar wells.
FISSURE SPRINGS.
At intervals along the interior boundary of the Rio Grande Plain from Austin to Devils River, through a distance of 300 miles, there is a series of remarkable springs which rise out of the ground. They do not break out from bluffs or fall in cascades, but appear as extensive pools, often in the level prairie. These pools or small lakes of limpid blue water find their outlet in swift and silently flowing streams.
The pools are carpeted with rare water plants, among which many fishes may be seen swimming. So transparent are these waters that objects 15 to 20 feet below the surface appear to be only a few feet away. They have been filtered by passing through the pores of the rocks for many miles.
The most conspicuous of these springs are near
Several groups of springs break out in the vicinity of Austin along the line of the secondary faults accompanying the great fault zone which extends approximately north and south through the east foot of Mount Bonnel. The principal are Mount Bonnel and Taylor springs, east of the foot of Mount Bonnel; Sand springs, between the dam and the city; Bee springs, Barton springs, and several unnamed springs breaking out at river level beneath Deep Eddy Bluff, west of the river. Sieder's spring, on Shoal Creek, in the northwest part of the city, also belongs to this category. Although small in volume, it is educationally instructive, because its relation to the faulting is clearly visible. The other springs are of larger volume.
The Mount Bonnel and Taylor springs are now covered by the back water of the lake. Barton springs occur in and on each side of Barton Creek, about one quarter mile above its confluence with the river, and give forth large volumes of water. The head lake of these springs is partially shown in Pl. XLIV. They have a discharge of 25 second feet, or about 16,000,000 gallons per day. The chief springs in the bed
308
of the creek can be seen welling up out of the clean cut fissure of a fault line just above the old dam site. A mill was until recently run by the water, but the power is now unutilized. These springs are beautifully situated and are the favorite resort of the people of Austin; they are surrounded by fine groves of pecan and picturesque rocks. Their aggregate volume must reach thousands of gallons per minute. The related faulting of the strata is shown particularly well in the vicinity of these springs.
Almost due north of Barton springs, beneath the highest bluff of the river at Deep Eddy, along other fracture lines, there is another group of fissure springs, but, owing to the fact that they are at the base of a high bluff and accessible only by boat and at the low water level of the Colorado, few people have seen them. They discharge a large volume, but as they break out in the river's edge it is impossible to gage them. The aggregate flow of the springs near Austin is so great that the volume of the river is materially increased.
Manchaca springs, about 13 miles south of Austin, on the old San Antonio road, burst out of a fissure in the Austin chalk, and the run off
Southward the next conspicuous springs of this character are in the Blanco River above Kyle. At the village of San Marcos a great group of springs breaks out at the foot of a north south line of bluffs making the Balcones scarp line in this region, and form the source of a beautiful river flowing 57,000,000 gallons per day. This has long been a famous resort in Texas on account of the exquisite aqueous flora and the beauty of the water. The springs form a lake nearly half a mile long, and its run off forms the San Marcos River. At the lower end of the lake a mill and an ice factory are run by its water, and the United States Fish Commission has established a culture station here. While none of the water is utilized in irrigation, there is no reason why such a volume of water should not be used to irrigate considerable areas of the fertile Black Prairie lands.
The springs of the Guadalupe and Comal rivers near New Braunfels also belong to this class. Those of the latter stream have a flow of 328 second feet, or about 200,000,000 gallons per day, and are the
309
largest of the whole group. The water is utilized to run a mill, and could irrigate many thousands of acres. Dr. Evermann has published the following notes on these springs:
Like most of the streams of this part of Texas, the Guadalupe is fed chiefly by numerous springs issuing from the Cretaceous limestone along its course. As a consequence, the water is usually exceedingly clear. During heavy rains, when there is considerable direct surface drainage, these streams, of course, become muddy for a short time. The temperature of the water at 4 p. m., December 3, was 68°, the air being 58°.
The Comal springs. There are a great many springs in the vicinity of New Braunfels, the principal group being known as the Comal springs. There are several springs in this group situated upon the land of Mr. Joseph Landa, a little over a mile northwest of New Braunfels. The largest of these flows, perhaps, as much as 50,000 gallons per minute, and is certainly a magnificent spring. The other springs of the same group flow at least as much more.
The main spring comes out near the foot of a limestone hill, and after running rapidly for a short distance over a pebbly bottom and in a narrrow channel, it widens out into quite a pond with mud bottom and filled with vegetation. This pond also receives the water from numerous other springs, and has its outlet in Comal Creek (or the Rio Comal), which, after a course of 2 or 3 miles, joins the Guadalupe River. The water of these springs is, of course, very clear. The temperature is 75°.
About 2 miles north of the town is another group of springs, smaller than that just described. The amount of water is abundant, however. . . .
Many of the large springs, and some of the most noted of their class, occur in the vicinity of San Antonio, the largest being at the head of the San Antonio River, a few miles north of the city. Until recently these flowed out of the ground in great volume 27,000,000 gallons per day forming an exquisite lake, the run off of which is the San Antonio River, which flows through the heart of the city of San Antonio and supplies it with water.
Below this group of springs and upon the banks of its outflow was situated one of the most ancient Indian settlements, or pueblos, of Texas. The early Spanish priests, appreciating the beauties and natural advantages of the place, located several missions there within a short distance of one another. The natives were employed in the cultivation of farms and gardens irrigated by the spring waters. The ancient acequias or ditches, followed by the older streets, shape the present outline of the city.
The spring fed river furnished, until recently, water for the city of 48,000 inhabitants without very appreciably diminishing its volume. Many acres of gardens and farms were irrigated, and there was sufficient water to irrigate many more. As elsewhere shown, the flow of the river has been recently seriously diminished by the drilling of numerous wells around San Antonio.
The San Pedro springs are about 2 miles southwest of those above mentioned, at the head of the river. Besides supplying an irrigation ditch they constitute the nucleus of handsome pleasure grounds. The "
Bull. U. S. Fish Commission for 1891, pp. 72-73.
310
springs here break out of fissures in the Gryphæa aucella beds of the Austin chalk. Their flow is estimated at 9 second feet, or 6,000,000 gallons per day.
The Los Moras springs, at Fort Clark, 125 miles west of San Antonio,
311
beautiful, limpid stream, which is largely used for irrigation a few miles below Fort Clark. Mr. Babb has measured the flow of these springs and estimates it to be 21 second feet, or over 13,000,000 gallons per day. About 10 miles north of Los Moras springs are the springs of the Pinto, which probably belong to the same class. They are much smaller than most of those herein enumerated.
The westernmost of the line of fault springs are the San Felipe springs near Del Rio. They break out at the edge of the Edwards Plateau, 2 miles northeast of Del Rio and about 5 miles from the Rio Grande. The pool is almost as large as that at the head of the San Antonio River. From the deep seated rock at its bottom the water can be seen welling up in a great column, and has the same peculiar greenish blue color as that of the other streams of this class. No trees surround it; it is alone a fountain in the desert. The rocks from which it bursts the Fort Worth limestones have the same kind of joints and faults as are found at San Antonio and Austin. The outflow from the pool forms a bold, rushing stream that runs off to the Rio Grande, some 5 miles distant. This spring stream, in addition to running a mill and supplying the village with water, is partially utilized to supply 15 miles of irrigation ditch and to irrigate 5,000 acres, and can furnish water for the irrigation of several thousand acres more. Mr. Babb's measurements make a total discharge of 19 second feet, or about 12,000,000 gallons per day.
The flow of these various springs has never been measured through a period of time sufficiently extended to give their variation. At the writers' request, the Division of Hydrography of this Survey, in 1895, made measurements of the principal spring rivers, and the results as tabulated were as follows:
 |
Bull. U. S. Geol. Survey No. 140, 1895. p. 86. Measurements made by Mr. C. C. Babb.
312
 |
It is a significant fact that the trend or line of these great springs along the northern margin of the plain coincides almost exactly with that of the Balcones fault line. A study of the rocks in the vicinity of the springs has shown that the springs are associated with the system of joints and fractures accompanying the fault line, and that their waters ascend to the surface along these fissures. In other words, these waters come from the deep seated rocks, and are forced to the surface by hydrostatic pressure. Hence they are artesian in nature and constitute natural artesian wells.
PROBABLE IDENTITY OF SOURCE OF ARTESIAN AND FISSURE SPRINGWATERS.
From the similarity of color, taste, temperature, etc., of this chain of springs extending in a continuous line 250 miles between Austin and Del Rio, and from their association with the line of Balcones scarp and faults, there can be no doubt that they are all of similar nature and origin. The temperature of the water brought by the springs from their subterranean source, about 750, does not differ greatly from the mean annual temperature of the air in this part of Texas 68° to 69°; and as the normal downward temperature change requires only 50 or 60 feet of depth for one degree of temperature, the formation from which the water comes call not be many hundred feet below the surface. The great volume of the springs shows their chief source to be a formation transmitting water freely. Their freedom from sulphureted hydrogen and other ingredients that would be detected by taste or smell excludes from consideration the higher water horizons of the Edwards limestone as sampled by artesian wells. These various facts leave no reasonable doubt that their water is derived from either the "sweet water" horizon of the Edwards formation or the Travis Peak sands; that is, they have the same source as the purer waters of the artesian wells.
The fact that the flow from the springs is slow in showing sympathetic variation with drought or rainfall is evidence that the reservori supplying them is of vast extent.
313
The identity of the source of the fault springs and the artesian wells is further confirmed by their relationship at San Antonio. Upon information furnished by Col. G. W. Brackenridge, who owns the city water works, which are supplied by both the spring at the head of the San Antonio River and a large number of auxiliary artesian wells, we learn that when the wells are allowed to flow the springs diminish in volume and the river is greatly lowered, being at times almost completely emptied. On the other hand, when the wells are stopped by valves the springs furnish their usual flow volume.
It is also an interesting fact that although the waters of the fissure springs of the Rio Grande Plain and the gravity springs of the Edwards Plateau, respectively, differ in mode of outflow, they are both derived from the same geologic horizons the rock sheets of the Edwards beds and Trinity division. The difference in mode of outburst is due to the difference in structural arrangement which these horizons present in the regions of their occurrence.
SOURCE OF THE UNDERGROUND WATERS OF RIO GRANDE PLAIN.
The question naturally arises in the minds of all who reflect upon the phenomena we have described, What is the origin and source of the underground waters of the Edwards Plateau and Rio Grande Plain? The customary explanation is that the waters supplying the artesian wells and spring rivers come from the distant Rocky Mountains. This is impossible, because the continuity of the strata between these rivers and the mountains is completely severed by the drainage valley of the Pecos, the strata being eroded away over thousands of square miles. The real source of the water is the rainfall of the Plateau of the Plains and its adjacent borders, as will now be explained.
Let us first consider the mode of catchment. Much of the rain water is caught directly upon the edges of the Glen Rose and lower beds which outcrop along the western and northern summits, breaks, and margins of the Plateau of the Plains at an elevation higher than that of their embedded continuation along its eastern and southern margin. These outcrops on the higher surface and slopes of the western part of the plateau, between the New Mexican line and the Pecos at the thirty first parallel, are between 3,000 and 5,000 feet high, while a part of the same strata along the eastern and southern margin of the plateau are less than 1,000 feet above sea level. Gradual as is the dip from Castle Mountain, on the Pecos, to San Antonio, it would not be sufficient to embed the waters at the latter locality below the altitude of their outcrop were it not for the sudden faulting. No doubt some water also enters the basement beds along that portion of the eastern margin of the Plateau of the Plains which constitutes the northern border of the Edwards Plateau. The outcrop of the water bearing basement beds in this escarpment along the south breaks of the Concho have an altitude of between 1,500 and 2,000 feet, exceeding in height
314
the embedded strata at the southern and eastern margin of the plain at Austin by about that much.
Another large part of the rainfall on the surface ofthe Edwards Plateau percolates downward through thelimestone itself to the water bearing beds. In this manner water often reaches the embed through intricate conduits from the surface, such as fissures and caves and honeycombed spaces in the limestone strata. (Fig. 75.)
The disappearance of the Nueces River into a cavernous fissure at the lower end of the Kickapoo or Four Mile water hole, as described on page 269, is an example of the direct transmission of water underground through the aid of caverns and fissures. Here a large volume of water can be seen passing directly into the "bowels of the earth" so to speak. In the adjacent massive limestone bluffs of the Nueces, vertical sections of old caverns can be seen, as shown in Pl. XLVIII. These are no doubt similar in character to those into which the stream now disappears, and at a time before the stream bed had been lowered by erosion to its present level the water disappeared down them. These caves are probably of the same nature as those penetrated in the San Marcos and San Antonio drill holes. The presence of peculiar cave animal life in the wells of these places also demonstrates that some of the underground water comes through cavernous passages.
W e do not possess sufficient data to estimate how much water the embed of the Edwards limestone receives throughout the vast extent of the thousands of square miles constituting the Edwards Plateau.
Such are the conditions explaining the catchment and transmission of water in the embedded rocks of the plateau. These conditions are entirely different in the Rio Grande Plain, where the water bearing beds have no outcrop open to the rain, and where the jointed limestones of the Edwards formation lie beneath impervious clays and shales.
The embedded Edwards limestones of the Rio Grande Plain are charged by the peculiar mechanical arrangement produced by the Balcones system of faulting. The dislocation or throw of the fault or faults breaks or disconnects the continuity of the water bearing strata. This
315
does not relieve the hydrostatic pressure, and the water is powerfully urged either to push its way to the surface through the fracture or to enter the stratum which is brought into juxtaposition with it and continue
316
strata under the plain receive their water supply, and it is forced into them with the same pressure that carries another part of it to the surface in the fissure springs.
The same explanation does not seem to apply to that part of the Travis Peak formation which is embedded under the Rio Grande Plain. The fault wholly separates it from the part underlying the plateau, and that which is opposed to its cut edges is probably some portion of the deep lying Paleozoic series; and we have no reason to suppose that those rocks could furnish it either a large quantity or a good quality of water. It seems possible that a part of the water from the Travis Peak sands under the plateau may pass downward along the fault plane to the Travis Peak beds under the plain, just as some passes upward along the fault plane to the surface of the ground; but it is perhaps more probable that the passage from west to east takes place farther north, where there is no fault and the basement sands are continuous.
AVAILABILITY AND LIMITATIONS OF THE UNDERGROUND WATERS.
Having now explained the general conditions controlling the occurrence and distribution of artesian water along the interior margin of the Rio Grande Plain, a more definite word or two concerning the limitations of the possible success of wells may be said. It should be remembered that owing to the lack of good topographic and geologic maps it is impossible to discuss the details of the respective districts with that degree of accuracy which is desirable, and owing to these reasons all opinions herein expressed are tentative and based upon observations largely of the nature of reconnaissance.
For the purpose above mentioned the Rio Grande Plain may be subdivided into four districts, as follows :
In the first of these districts the northernmost locality of the occurrence of flowing artesian water derived from the Edwards limestone is at Manor, in Travis County. The most southern locality is Colonel Terrell's well, south of San Antonio. As the same structural conditions which control the success of these wells prevail from north of the Colorado to San Antonio, it is a logical deduction that similar water can be obtained throughout the intervening distance of 90 miles. The only recorded well throughout the distance is the one at San Marcos, and this supports our hypothesis.
From the experiments at Manor and Marlin it is evident that artesian wells can be obtained at least as far east as the eastern margin of the Cretaceous Black Prairie. In view of these facts, it is reasonable to believe that water can be obtained at depths of from 200 to 3,000 feet (the depth increasing to the eastward) in all the region between San
317
Antonio and the San Gabriel, in a belt of country averaging 20 miles in width, lying between the post oak timbered region on the east and the International and Great Northern Railway on the west.
In the second district mentioned, from San Antonio westward to the Sabinal, the same general conditions prevail, with some slight but important modifications, in the structure. It is our opinion that that portion of the belt lying north of the boundary between the Eagle Pass and Taylor formations, and south of the Balcones fault, except where disturbed by igneous intrusion, presents favorable conditions for procuring artesian wells.
This belt may be limited to the coastward by a line arbitrarily drawn parallel with the Southern Pacific Railroad and 5 miles south of it. We have in our possession no record of any well experiments having been made in this particular region, nor have the structural conditions been minutely studied, but so far as known they are favorable for the securing of artesian wells of the Austin, San Antonio, and San Marcos type.
It would be well for those who contemplate drilling wells to ascertain positively whether any such experiments have been made. The waterbearing strata deepen more rapidly away from the Balcones fault in this region than to the northward, and hence the width of available area is less. It is doubtful it wells of less than from 1,500 to 2,500 feet in depth can be obtained along the line of the Southern Pacific.
The third district is one concerning which we can not express an opinion favorable to the procurement of artesian waters. In prospecting for water in this region it is necessary to consider the effect of the numerous igneous intrusions. Owing to their presence it is not probable that flowing wells can be obtained in this portion of the Rio Grande Plain. These igneous rocks break the continuity of the strata, and how much they may affect the occurrence of underground water is an important question.
The presence of masses of igneous rocks also renders the success of artesian experimentation doubtful in a small section of country belonging to the first district, i. e., the immediate vicinity of Pilot Knob, in Travis County. The recent experiments at St. Edward's College, Manor, and Austin prove that the presence of intrusions of the Pilot Knob group does not affect the artesian conditions for any great distance to the west of them. No experiments have been made to the east.
In how wide an area outside of the immediate occurrence of the Uvalde group of laccoliths the artesian prospects are influenced is also "
Since this report was prepared the writers have learned that Mr. William Blocker, living about 2 miles southeast of Pilot Knob, has commenced an artesian well. This well begins in the Taylor formation, and necessarily would have to go nearly as far as the depth of the San Marcos well to reach the Edwards limestone, or 1,000 feet lower to the Travis Peak flow. If this well should prove or has proved successful, it will demonstrate the possibility of success for a large area of country between the Colorado and Mayhard Creek as far east as Bastrop. No more important experiment than this well has as yet been undertaken, and the writers regret very much that they have not means at their disposal to study its progress.
318
problematic. From experiences around Austin, and the occurrence of artesian springs at Fort Clark, just beyond the volcanic outcrops, it seems probable that those intrusions do not seriously affect the artesian conditions for any great distance outside of their own immediate area, i. e., the Uvalde laccolithic region of the Rio Grande Plain between the Sabinal and the east end of the Anacacho Hills.
In the fourth district named, we again meet some evidences of the possibility of obtaining artesian water. Two artesian well experiments have been made west of the Uvalde laccolithic area, at Spofford Junction and Del Rio. The experiment at Spofford Junction was made many years ago by the Galveston, Harrisburg and San Antonio Railway Company. We have not been able to obtain a trustworthy record of the boring. The depth to which this well was dug has been variously reported at from 1,160 to 1,800 feet. From our geologic studies of this region we know that Spofford is underlain by the elsewhere water bearing beds of both the Edwards and Travis Peak formations. Whether the drill ever penetrated one or both of the water bearing beds is uncertain. A report in our possession makes it seem probable that water was struck at a depth of over 1,800 feet, and rose within the well some 600 feet, or to within 1,200 feet of the surface. We have been informed that the fossil Exogyra arietina, which is peculiar to the Del Rio clays, was found at a depth of 1,800 feet. If this is true the water obtained was probably the top water of the Edwards beds, and the well should have been continued at least 500 feet before being abandoned. The fact that the water rose 600 feet in this well demonstrates that beneath the place there is water under hydrostatic pressure. The position of Spofford relative to the beds of the Cretaceous section is problematic, owing to the fact that the surface at that place is obscured by the post Cretaceous Uvalde conglomerate, and the rocks have been faulted in the neighborhood. The geologic position of the place is certainly higher than the Austin chalk, and we are inclined to believe that it is well up in the Anacacho formation, or toward the base of the Eagle Pass. If these hypotheses be correct, then the water alleged to have been struck at 1,800 feet must have been in the Edwards beds, and it is assumed that the basal sands have not been penetrated here as yet. Furthermore, artesian water does rise at the Las Moras springs, 100 feet above the altitude of Spofford; and hence we believe that if a well should be continued to the base of the Cretaceous series at Spofford, it is probable that an artesian flow would be struck at a depth of less than 2,500 feet.
Both Spofford and Del Rio are typical localities of that portion of the Rio Grande Plain lying west of the Anacacho Mountains, and are situated on the downthrown side of the Balcones monocline, where the stratigraphic conditions necessitate the embedding of the water bearing . strata. Furthermore, both localities are immediately south of great artesian springs, those of the Las Moras and San Felipe, respectively, which show the presence beneath the region of water bearing strata of
319
great capacity, identical in character with those of San Antonio, San Marcos, and Austin, and the waters are undoubtedly of the same origin, coming up through natural fissures from an embed which is equally accessible to the artesian drill.
Among the data in our possession there is no record of any drill hole in the Rio Grande Plain country on the downthrown side of the Balcones fault between San Marcos and the Devlis River which has as yet penetrated to the water bearing basement beds of the Travis Peak formation. From the records of the wells and study of the geologic sections of the canyons of the plateau region, we know that these beds are well developed up to the very margin of the Rio Grande Plain, and must exist beneath it. They have been penetrated and are water producing in the Austin, Kerrville, and San Marcos wells. In view of these facts, we are inclined to the belief that the artesian experimentation in this subregion has not been carried to a sufficient depth to reach the Travis Peak sands. This opinion is not to be taken as a positive prediction of success, and might be changed if we possessed more accurate logs of the wells already bored.
PRACTICAL SUGGESTIONS.
While this paper does not propose to deal with other than the geologic side of the question of underground water, it may not be inappropriate to offer a few direct suggestions bearing upon the location and drilling of wells.
The question of interest to the well seeker is, How can I estimate the depth of the water bearing sheet below the spot where I propose to sink my well? In a general way there are two methods. If there is another well within a few miles of the locality, whose log shows the local depth of the desired water, and if the rocks are not faulted, then by determining the dip of the rocks lie can compute from dip and distance the difference in absolute altitude of the water horizon at the two places; lie can also, by actual measurement with the surveyor's level, determine the difference in height of the surface of the ground at the two places. These two differences can then be applied as corrections to the depth of the water horizon in the existing well, and the result will give an estimate of its depth at the place where the well is desired.
The second method demands a knowledge of the geologic column of the region that is, of the various formations, their order of superposition, and the thickness of each. Which formation occupies the surface of the ground at the locality where boring is projected must also be known. Then, by adding the thickness of all the formations between the one at the surface and the one carrying the water below, an estimate of the depth to which it will be necessary to drill may be obtained. Accurate geologic surveys, such as have been made of two typical portions of "
The Austin and Brackett quadrangles have been surveyed in detail and the data are now in process of publication.
320
the region, would give this information. Unfortunately, however, only reconnaissances have been made over the rest of the region, and in the absence of maps these determinations for each particular locality should be referred to a professional geologist.
Paleontology is the most reliable guide in determining the position of any bed in the geologic series, in order to ascertain the depth from any particular portion of the surface of the underground waters in the Cretaceous regions of Texas. If a dozen fossils, such as can be found in nearly any locality, be sent to a geologist familiar with the sequence of the beds, he can predict within a few feet the depth below the surface of any particular stratum in the series.
The specimens of the rock sheets as brought up from the drill hole differ quite materially in color, and sometimes in hardness, from the surface outcrops of the same rock, in which the color is changed by oxidation and the texture by induration or decomposition. Thus it is that limestones, marls, and clays brought from considerable depths in these wells are usually bluish or blue black in tint, while the surface outcrop of the same beds, owing to drying, will be lighter, or if they contain iron, yellow, or occasionally red.
In general, if one boring an artesian well should commence at the uppermost of the Cretaceous formations and penetrate the entire system, he would be able to recognize the different beds as they were encountered by the following characters:
North of San Antonio the Webberville, Taylor, Eagle Ford, and the Del Rio formations are composed of marls, locally called "joint clays." These can be distinguished from one another as follows : The Webberville joint clays always have minute specks of the greenish black mineral called glauconite. The Taylor marls are very blue and possess no accessory mineral except lime; they are especially free from grit. They weather into lumps with conchoidal fracture. The clays of the Eagle Ford beds are blue black in color, impure in composition, and have a very finely laminated structure, whereby on the least weathering they separate into leaf like layers. They also have occasional layers of impure laminated limestone of blue or brown colors. The Del Rio beds are usually blue clays containing much iron pyrites and the characteristic oyster, Exogyra arietina, which almost invariably appears in the well debris when the drill penetrates this horizon.
Still other clays are encountered after passing through great thicknesses of limestone beneath the Del Rio clays. The first of these will be the Walnut clays, which contain a different species of oyster, Exogyra texana, and have a somewhat granular character. The clays in the upper Glen Rose beds are also very granular, and often contain much minute shell debris. The lowest clays of the Cretaceous formations found intercalated in the sands of the basement beds have faded red colors, even when brought up from these great depths.
The first limestone of much thickness encountered by the driller will be the Austin chalk, which, under the churn drill, appears as an
321
unctuous blue mud. This lies between the Taylor and Eagle Ford marls and is from 400 to 500 feet in thickness.
The few limestone layers of the Eagle Ford beds can be recognized by their laminated character, which distinguishes them from all other limestones of the region.
The Shoal Creek limestone is intensely hard, and the well driller will have no difficulty in recognizing it when he encounters it.
The Fort Worth, Edwards, and Comanche Peak limestones will constitute practically one formation to the well driller, having an aggregate thickness of about 450 feet in the Colorado Valley and about 640 along the Nueces. This horizon can be recognized principally by its thickness and hardness. The Edwards limestone is easily distinguished by the flints that it contains.
The Glen Rose beds will be recognized by their consisting of alternations of soft flaggy limestones and calcareous clays.
Probably, in boring, the most easily recognized geologic horizon is that of the Del Rio clays. The Travis Peak water in the vicinity of Austin is 1,300 to 1,600 feet below them. The water bearing sands are slightly more than 300 feet thick. The "sweet water" at San Antonio occurs only 80 to 100 feet below the Del Rio.
The following table gives the variation in thickness, the approximate position of the water horizons, and the most noteworthy characters of each formation.
 |
