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somewhat similar in character of soil and vegetation, being usually composed of red loam and gravel derived from the central provinces of the State and covered by a growth of post-oak timber. The bottoms and terraces of the larger creeks are usually composed of calcareous soils and debris derived from the Cretaceous formations of the adjacent Edwards Plateau.
The old terraces may be divided into two classes:
- (1) high gravel terraces, which rise from 100 to 200 feet above the level of the streams, and which are now largely of a stony nature;
- (2) wide, flat terraces elevated only from 40 to 50 feet above the streams, usually of good soil and devoted to agriculture.
Westward, in the Edwards Plateau country, the terraces narrow in width and are reduced in number, although they continue to be a marked feature in the canyon of the Colorado.
Onion Creek basin . — Along Onion Creek, extending from a little above Buda to Pilot Knob, is a wide, flat constructional valley plain standing about 100 feet above the present stream-way, terminating in a deep gorge through the Pilot Knob hills. The surface of the plain is very level, its elevation rising from about 500 feet at Pilot Knob to a little over 700 feet at Buda. This plain appears once to have been an old lake basin, which has been drained by the canyon later cut through the northern side of the Pilot Knob dome.
Upland gravel plains .—There are several high divides and small upland plateaus within the Coastal Plain area which are covered with old gravel deposits. These are so distributed as to indicate that nearly all of the area of the Coastal Plain in the Austin quadrangle was once veneered with alluvial debris from the Edwards Plateau. One of these peculiar gravel deposits is in the vicinity of St. Elmo post-office, and hence has been named the St. Elmo Plateau. Its surface is level and the soil consists of black, waxy, calcareous clay containing gravel. It is mostly open prairie, except a few mesquite trees. The elevation of this plateau is from 550 to 700 feet. Another high divide that reaches across from the rolling black prairies and into the sandy Lytton Springs area is Mustang Ridge, which stretches from the confluence of Mahard and Cedar creeks, in the southeast section of the quadrangle, in a westerly direction to the International and Great Northern Railroad about 2 miles south of Buda. This ridge is covered with gravel, which in the areas of the Taylor and Webberville formations is embedded in calcareous clay; in the Eocene area, in loose sands or loamy clays. The vegetation consists mostly of mesquite, or the land may be prairie in the Black Prairie region. In the Eocene area post-oak timber may be abundant. The summit of the ridge is very nearly level, the elevation being from 650 to 800 feet. Bald Knob, a conspicuous hill west of Manor, is also capped with this material.
Volcanic hills— exceptional feature of the country is the small group of volcanic hills known as Pilot Knob, about 7 miles southeast of Austin. These occupy an area of about 3 square miles and consist of three low summits rising to a height of about 100 feet above the adjacent prairie. They are covered by a peculiar species of grass, which is apparently limited to their surface, growing on a black soil from which protrude many fragments of hard, basaltic rock.
At various other places in the region, as shown on the geologic map, there are small outcrops of this material, constituting exceptional features within their areas.
DRAINAGE.
The waters of the Austin quadrangle reach the sea principally through the Colorado River, although a few streams on the northern edge lead to the Brazos, and some on the southern edge to the San Marcos. The Colorado drains over nine-tenths of the total area.
The streams, except in time of flood, are not copious; in fact, they present peculiarities of flow which may be classified as perennial, interrupted, and intermittent. The Colorado is perennial or constant in flow, although variable in volume, being subject to great floods. Other streams, like Barton and Onion creeks, are interrupted, presenting alternate stretches of running water and dry bottom. Still others, such as the secondary streams of the eastern margin, are intermittent, having no water except in time of rainfall.
The streamways have certain distinctive characters in each of the two greater topographic divisions. In the Edwards Plateau they are usually encased in deep and narrow canyons, like those of the Colorado and Barton Creek west of Austin. In the Coastal Plain the valleys are usually broad and terraced by older alluvial plains leading up to the rounded divides described under a separate head.
It is interesting to note that the courses of the larger streams are sinuous in all the topographic divisions. Thus the Colorado River flows in great sweeping curves, sometimes almost joining, in both the country above the Balcones escarpment and in the plains country southeast of Austin. The meanders are as thoroughly established in the canyon part of the Colorado as in the prairie country in the eastern portion of the quadrangle.
The Colorado, a great through flowing river (i. e., one that rises beyond the province and runs through it), crosses the quadrangle from northwest to southeast. It rises in the breaks of the Llano Estacado and empties into the Gulf of Mexico. It meanders across the quadrangle in great oxbows, and occupies a deep valley some 500 feet below the present summit level of the Edwards Plateau and 250 feet below the summit levels of the Coastal Plain. This stream, the most important drainage channel of the region, gathers little perennial water from the area, although it receives water from many intermittent streams in time of rainfall, from a few gravity springs in the northwestern portion of the quadrangle, and from some fissure springs between the foot of Mount Bonnel and the city of Austin.
The principal secondary streams are Barton and Onion creeks, which rise along the western border of the quadrangle south of the Colorado and enter the latter stream to the eastward.
Barton Creek is peculiar in that it runs closely parallel to the Colorado River and has incised deep canyons into the Edwards Plateau, Barton Creek almost equal to those of the Colorado itself. Furthermore, this stream, except in time of flood, is eccentric in the intermittent character of its drainage. Within the Edwards Plateau its bed is at first a dry arroyo for several miles, which is succeeded by 6 or 7 miles of running water derived from gravity springs; then follows a dry stream bed for 5 or 6 miles, to within a mile of its mouth, where magnificent artesian springs (Barton Springs) arise from fissures within its bed and supply a beautiful and constant stream which empties into the Colorado. Barton Creek is also notable for the relatively small amount of country drained by it laterally.
Onion Creek rises in the Blanco quadrangle, west of the Austin quadrangle, and flows nearly the entire distance across the latter, emptying into the Colorado River near the eastern margin. It is somewhat different in physiographic character from Barton Creek in that it is apparently not so deeply incised and receives the floods of many laterals, such as Bear, Slaughter, and Williamson creeks, thereby draining a very large area of the quadrangle. Its headwater portion is normally a long dry arroyo, with occasional pools rising in the hills of the Edwards Plateau of Blanco County. As it enters the Austin quadrangle it shows some long and beautiful pools of permanent water derived from gravity springs. Crossing the Bear Creek country it is a dry and stony arroyo inclosed in a rocky canyon. From Buda to the Pilot Knob uplift it occupies a more modern valley within the higher and older alluvial plain elsewhere described as the Onion Creek Basin. On reaching the western side of the Pilot Knob uplift it enters a steep-walled canyon cut directly across the uplift for a distance of about 3 miles. After passing the latter obstacle it continues its way through the Taylor Prairie and the Colorado bottom to the Colorado River.
In the extreme southwest corner of the quadrangle is found a small section of the Blanco River, a tributary of the San Marcos, which enters and leaves the quadrangle from the south. The portion of this stream here shown is embedded in a steep and almost impassable vertical canyon nearly 300 feet deep.
The secondary streams in the eastern half of the quadrangle, such as Cottonwood, Wilbarger, Gilleland, Walnut, Cedar, and Mahard, are usually dry arroyos which have an occasional pool of standing water and are incased in valleys with rounded clay or gravel slopes. Antecendent and consequent streams
These stream courses show peculiar and interesting adaptation to the geologic structure and changes of level, elsewhere discussed in this paper. In technical terms the Colorado River is antecedent to the Edwards Plateau and consequent to the Coastal Plain. Barton Creek and the Blanco relative to the plateau are also antecedent and inherited, but of later origin than the Colorado. Onion Creek and the numerous smaller tributaries are still newer consequent streams which have adjusted themselves to the regional slope and to the deepening of the Colorado Valley as the land was elevated. Many of the streams which we have called consequent are really antecedent in the western part of the quadrangle and, in their lower courses, especially as they enter the old alluvial plains of the Colorado, are controlled by the flood plains of the larger river. Such is the case with Barton Creek in South Austin and Onion Creek near its mouth, each of which, on reaching the alluvial bottom of the old river, changes its course down the valley and runs parallel with the Colorado for some distance.
Some of these streams in portions of their courses have also adapted themselves to structural features, following the joints and fissures of the Balcones fault zone. Such is the case with that portion of the stretch of the Colorado between the foot of Mount Bonnel and the city dam. Barton Creek just above the springs, Onion Creek west of Buda, near the western margin of the quadrangle, and Shoal Creek in the city of Austin show similar adaptation to fault lines.
CULTURE.
The Austin quadrangle has a comparatively dense population, but nine-tenths of its inhabitants are found on the Coastal Plain, east of the Balcones scarp, the Edwards Plateau country to the west being but sparsely populated.
The densest rural population is found in the White Rock and Taylor prairies and the Colorado bottoms, nearly the entire areas of which are devoted to the cultivation of cotton, with some minor crops. The few people inhabiting the Edwards Plateau are engaged in raising cattle and cutting cedar timber from the hills to supply the city of Austin with fuel. Occasionally farmers cultivate small areas of alluvial soil in the valleys of this district.,
There are a number of towns. Austin, the capital of the State, occupies both banks of the Colorado near the center of the quadrangle. It is a city of 30,000 inhabitants and contains many handsome public buildings, including the capitol and university. Manor, about 12 miles northeast of Austin, is a prosperous rural village of about 5000 inhabitants. McNeil, Manchaca, and Buda are small towns along the line of the International and Great Northern Railroad. The other towns on the map, such as Oak Hill, Watters, Pflugerville, Sprinkle, Bluff Springs, New Sweden, and Fiskville, are small villages with one or more stores and a few houses.
The principal railways are the International and Great Northern, running approximately north and south across the quadrangle; the Houston and Texas Central, extending from Austin eastward ; and the Austin and. Northwestern Branch of the Houston and Texas Central Railroad, running northward nearly to the northern edge of the quadrangle, when it turns westward toward Burnet.
Highways radiate from Austin in all directions. They may be classified under three heads: county roads of the first order, which lead from Austin to the neighboring county seats; lanes leading from the main highways to small communities or farms; and country roads, originally made by wood cutters, which meander through the region of the Edwards Plateau. Many of the first-class highways are macadamized in places, and commendable interest has been taken by the people of Travis County in their improvement. Three substantial bridges span the Colorado, the westernmost of which is devoted solely to railway traffic, and the others, at Austin and Montopolis, to vehicles.
It is interesting to note on the map how the courses of the principal railways and highways are governed by the geologic structure.
FOREST AND WOODLAND.
In the Austin quadrangle there is a diversified flora, which, like other features, varies with the underlying geologic formations. The highest summits of the Edwards Plateau, where patches of the Edwards limestone are preserved, are covered with a thick growth of post-oak, live-oak, and scrub-oak timber. The numerous slopes of this region were formerly covered with a dense growth of juniper, locally called cedar, which for nearly fifty years has furnished the principal domestic fuel of Austin. In the region of the Coastal Plain the Bear Creek belt is forested with post oak similar to that capping the high summits of the Edwards Plateau. The Del Rio soils of the Manchaca belt sustain a growth of mesquite trees, while the outcrops of the Buda formation are generally covered with live oak. The principal arborescent growth of the Eagle Ford formation consists of hackberry trees. The White Rock belt presents an open, park-like aspect, with here and there a clump or "motte" of gigantic live oaks, while upon the rocky slopes, as in South Austin, a thick growth of juniper may also occur.
The Manor belt is usually devoid of arborescent growth, except occasionally a mesquite bush. The Taylor Prairie is occupied by a peculiar flora of chaparral, consisting of tall mesquite bushes between which are large clusters of the Mexican nopal, a species of cactus belonging to the genus Opuntia. The western margin of the upland Atlantic timber belt, consisting of a growth of post-oak and black-jack trees, mantles the Littig country. In the bottoms of the Colorado and alluvial valleys of the other creeks grow stately pecans, as well as other large trees.
GENERAL GEOLOGY.
The geology of this quadrangle is comparatively simple. It is an area of evenly bedded rocks which, in common with all the strata of the Regional Coastward Slope, have been uplifted without serious deformation except faulting. It presents typical examples of the geologic structure of the eastern margin of the Edwards Plateau and the interior margin of the Coastal Plain, as seen between the Brazos and the Rio Grande and as shown in the Nueces and Uvalde folios (folios 42 and 64).
Classification.—The rocks found within this quadrangle are of three kinds — sedimentary, surficial, and igneous. The sedimentary rocks occupy the greater portion of the area and the surficial rocks are of considerable extent, whereas the igneous rocks are of only occasional occurrence. The sedimentary rocks were deposited in the sea and subsequently elevated so as to form a land surface. They now consist of evenly bedded horizontal limestones with occasional beds of clay, and constitute the foundation of the country. The surficial rocks consist of material derived from the marine sedimentary rocks and the igneous rocks, brought down from the northwest by streams and deposited as a veneer over the uplands, as terraces along the streams, and in occasional temporary lake basins. They are fluviatile or lacustrine sediments. The igneous rocks consists of solidified molten rock in fissures and necks and associated lava and volcanic ash.
SEDIMENTARY ROCKS.
CRETACEOUS PERIOD.
All of the marine sedimentary rocks of this quadrangle except the Lytton formation belong to the Cretaceous system. These rocks also are most important from the standpoint of economic geology, since they yield the richest soils, furnish the best material for road and building purposes,