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UNITED STATES DEPARTMENT OF THE INTERIOR
PHILIP B. KING
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CONTENTS
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ILLUSTRATIONS
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GEOLOGY OF THE MARATHON REGION, TEXAS
By PHILIP B. KING
ABSTRACT
This report describes the geology of the Marathon region, in trans-Pecos Texas. The Marathon region lies on the edge of the Mexican Highlands province, where that province merges into the Great Plains on the east. Structurally, the region is a broad dome of Cretaceous rocks, from whose central part the Cretaceous cover has been stripped away, leaving an area of low country in the center, the Marathon Basin. Here strongly folded Paleozoic rocks are exposed. The Monument Spring and Marathon quadrangles, described in detail in this report, extend across the basin area.
The Paleozoic rocks exposed in the basin and in the Glass Mountains, which flank it on the northwest, have a thickness of 21,000 feet. The greater part of them were laid down in a subsiding area, the Llanoria geosyncline. The oldest rocks are Upper Cambrian sandstones and shales, whose base is not exposed. Overlying them are 2,000 feet of Ordovician rocks, composed of shaly limestone and shale, with some beds of chert, whose chief fossils are graptolites. The Ordovician is overlain by the Caballos novaculite, possibly of Devonian age, which reaches 600 feet in thickness. This white siliceous rock is the chief ridge maker in the Marathon Basin.
The Caballos novaculite is overlain by a great series of clastic rocks of Pennsylvanian age, as much as 1200 feet thick in the southeastern part of the area but much thinner in the northwest. Two of the lower formations are a mass of arkosic sandstone and shale and are separated by a widespread thinner limestone formation. The two formations contain few fossils other than land plants. The upper of the two contains a remarkable layer of mudstone, in which are embedded large blocks of older rocks. The blocks are believed to have been derived from the erosion of advancing thrust sheets and to have marked the fist strong uplift in the region; they may have been transported to their present positions either by glaciers or by mud streams. The uppermost Pennsylvanian formation consists of conglomerate and sandstone derived from the erosion of rising folds and contains abundant upper Pennsylvanian marine fossils.
The strong deformation to which the Paleozoic rocks of the Marathon Basin have been subjected apparently culminated E after the deposition of this uppermost formation of Pennsylvanian age. The Permian rocks of the Glass Mountains, to the northwest, rest, at least in places, with great angular unconformity on the disturbed older beds. The structural features seen in the basin consist of close folds, trending northeast and overturned to the northwest which are broken by numerous thrust faults. The faulting culminated on the northwest in the nearly flat-lying Dugout Creek overthrust, with a known displacement of more than 6 miles. Farther southeast are other great thrusts, also with miles of displacement, some of which are folded and therefore older than the frontal fault. The folds of the Marathon region are a part of a system of structural features formed from the rocks of the Llanoria geosyncline, which extends northeastward in sinuous courses to the Ouachita Mountains of Oklahoma and Arkansas. Northwest of the geosyncline and folds of the Marathon region, during Paleozoic time, there was a foreland area, which was gently folded at the same time as the movements at Marathon. Southeast of them was a region underlain by pre-Cambrian crystalline rocks. Both these areas are now mostly concealed by Cretaceous and Tertiary rocks.
The Permian rocks of the Glass Mountains, 5,000 feet or more thick, consist of limestones, siliceous shales, clay shales, and sandstones, which interfinger in a most complex manner. The most striking stratigraphic features of the series as exposed in the mountains are limestone reefs, constructed in large part by limesecreting organisms. The reefs apparently had a marked influence on the development of the other lithologic facies. The Permian rocks contain marine fossils, in places very abundantly. Most of the faunas are similar to the Guadalupian fauna originally described by Girty from northern trans-Pecos Texas. The Permian rocks are tilted to the northwest, away from the Marathon Basin, and are apparently in greater part younger than the folds in the basin.
The Cretaceous rocks that surround the Marathon Basin have a maximum thickness of about 1,200 feet and are mostly limestones. They were laid down on the eroded edges of the folded or tilted Paleozoic rocks, whose surface had been reduced to a peneplain during Triassic and Jurassic time. Over the Cretaceous west of the Marathon region lie lavas and tuffs of early Tertiary age. Within the region small masses of igneous rock, in part of alkalic composition, have intruded the Paleozoic and Cretaceous rocks.
The Cretaceous rocks dip gently away from the Marathon dome on its north, east, and south sides. On the west side they are sharply buckled and locally overthrust toward the west. The structural features on the west side of the Marathon dome are in part older and in part younger than the early Tertiary lavas. All the rocks of the dome are broken by normal faults that are younger than post-Cretaceous folds and probably of later Tertiary age.
No rocks younger than the Tertiary igneous rocks exist in the vicinity of the Marathon region except gravel deposits that cover part of the lowlands. These were deposited on various surfaces of erosion. The oldest stands several hundred feet above the present streams, and the gravel on it is probably of Pleistocene age.
The rocks of the Marathon region contain relatively few materials of economic value. Locally there are some metallic minerals, chiefly near the igneous intrusions. The hard siliceous novaculites of the Marathon Basin may be of use for whetstones or road metal. The jointed bedrock of the basin and its cover of gravel contain a supply of underground water. The area does not seem to be favorable for the accumulation of oil or gas.
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INTRODUCTION
Location-This report deals with the geology of the Marathon region, which lies in the northern part of Brewster County, in western Texas. Particular attention is paid to the geologic features exposed in the Monument Spring and Marathon quadrangles, which extend across the central part of the region and cover an area more than 30 miles east and west by 20 miles north and south. In order to complete the description of the Marathon region, some of the stratigraphic and structural features to the north and south of the two quadrangles are also noted.
The region is crossed from east to west by the main line of the Sunset Route of the Southern Pacific Railroad, upon which, about halfway across the area, is the village of Marathon, the only settlement. (See fig. 1.)
Previous work-The Marathon region was mentioned in 1890 by Von Streeruwitz, who noted northeastward trending ridges south of Marathon composed of "quartz and quartzite, strongly metamorphosed limestone, and semifused siliceous conglomerations." He mistakenly correlated these with the Cretaceous rocks cropping out to the east and west, which he thought had here been "fused and thrown up by protrusive volcanic rocks."
A good description of the geologic features of the area was given by Hill in 1900. After noting the character "
Von Streeruwitz, W. H., Report on the geology and mineral resources of transPecos Texas: Texas Geol. Survey 2d Ann. Rept., for 1890, p. 686, 1891.
Hill, R. T., Physical geography of the Texas region: U. S. Geol. Survey Top. Atlas, folio 3, p. 4, 1900.
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of the Comanche or Glass Mountains, he described the Caballos Ridges, to the south. These were said to be "low ridges * * * rising from the floor of the Marathon plain * * * composed of the degraded vertical edges of Paleozoic limestone, shales, and cherts * * * trending northeast and southwest. The cherts are often white in color and form the backbone of long ridges." On the south and east he found the region to be bordered by scarps and cuestas of "subhorizontal Cretaceous limestone unconformably resting on the subvertical edges of the Paleozoic rocks." He concluded that "the Caballos and Glass Mountains are exposures of ancient post-Paleozoic structure of Appalachian type and age, which have been revealed by the erosion of the Cretaceous sediments that probably once embedded them."
Observations made by Udden in 1905 in the course of a journey to the Chisos country amplified the results of Hill. Udden noted the discovery of Ordovician and Carboniferous fossils in the Marathon region. The comprehensive work in the region by Baker and Bowman in 1915, as a part of their exploration of the southern front ranges of trans-Pecos Texas, revealed the broad outlines of the physiography, stratigraphy, and structure. Rocks of Cambrian age, Ordovician strata with fossils at four horizons, a probable Devonian formation, and four thick Pennsylvanian formations were recognized.
During the same period Udden and Böse studied the Glass Mountains, to the north of the Marathon Basin, and described the upper part of the Paleozoic section exposed there, which includes a great thickness of Permian strata.
The writer began work in the area in 1925. During this and two succeeding summers, associated with R. E. King, he studied the Glass Mountains and the adjoining part of the Marathon Basin for the Texas Bureau of Economic Geology.
Field work:-The investigation that has provided the results for the present report was an extension of the earlier studies with R. E. King into an area adjoining on the south. The report is based on 8 months' field work in the region in 1929 and 1930 under the auspices of the United States Geological Survey. The season of 1929 was devoted to a reconnaissance of the entire Marathon Basin. In 1930 the Monument Spring and Marathon quadrangles were mapped in detail. Further observations were made during short visits in 1931.
At the beginning of the present investigation only the broader features of the structure and stratigraphy of the region were known. Because of the extreme complexity and small scale of the folds, the wide areas of valley fill, and the confusion that had arisen on some of the stratigraphic problems, many of the minor features of the region were poorly understood. The geologic mapping was therefore done with considerable care. Fortunately, excellent topographic maps were available for most of the area and could be used as a base for plotting geologic observations. The mapping was done partly by an elaborate system of pacing traverses, and partly by recording the observations on enlargements of the topographic sheets. Stratigraphic sections were measured mostly by Brunton compass and by pacing. Where there were exceptionally good exposures, however, a tape measure was used.
Acknowledgments-A small part of the field observations on which this report is based were taken from notes made by the writer when he was connected with the Texas Bureau of Economic Geology and from data furnished by E. H. Sellards, C. L. Baker, and others. Mr. Baker has also made available many recent fossil collections from the older rocks of the region, for study by paleontologists of the Geological Survey, and has joined the writer on several field conferences that made it possible to correlate the present work with that of Baker and Bowman in 1917. Much information on the fossil plants of the region has been afforded by collections sent to the Geological Survey by Sidney Powers. The report has also been materially improved by visits to the field with various members of the staff of the Survey, including David White, G. H. Girty, Edwin Kirk, G. R. Mansfield, and H. D. Miser.
After this report had been written the writer had the privilege of examining some excellent aerial mosaic maps of part of the Marathon Basin made by the Edgar Tobin Aerial Surveys, of San Antonio, Tex. Through the courtesy of this organization, he has been permitted to reproduce two of the single photographs from which the maps were made. (See pls. 17, 18.)
GEOGRAPHY
PHYSICAL FEATURES OF TRANS-PECOS TEXAS
The Marathon region is in trans-Pecos Texas, the westward-projecting part of the State that lies along the Rio Grande west of the Pecos River (fig. 1). Geographically, this arid and mountainous region is "
Udden, J. A., A sketch of the geology of the Chisos country, Brewster County, Tex.: Texas Univ. Bull. 93, pp. 18-21, 76-78, 1907.
Baker, C. L., and Bowman, W. F., Geologic exploration of the southeastern Front Range of trans-Pecos Texas: Texas Univ. Bull. 1753, pp. 67-172, 1917.
Udden, J. A., Notes on the geology of the Glass Mountains: Texas Univ. Bull. 1753, pp. 5-59, 1917.
Böse, Emil, The Permo-Carboniferous ammonoids of the Glass Mountains, West Texas, and their stratigraphical significance: Texas Univ. Bull. 1752, 1917.
King, P. B., The geology of the Glass Mountains, part 1, Descriptive geology: Texas Univ. Bull. 3038, 1931. Ping, R. E., The geology of the Glass Mountains, part 2, Faunal summary and correlation of the Permian formations, with description of the Brachiopoda: Texas Univ. Bull. 3042, 1931.
For a description and classification of the physical features of the trans-Pecos region, based on somewhat different criteria, see Carter, W. T., and others, Soil survey (reconnaissance) of the trans-Pecos area, Texas: U. S. Dept. Agr., Bur. Chemistry and Soils, ser. 1928, no. 35, pp. 1-7, 1928.
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more closely related to Mexico and New Mexico than it is to the rest of Texas. It is a region of rugged sierras, of high plateaus and broad cuestas, and of gently sloping intermontane plains. The mountains have no timber except in sheltered valleys and on the higher summits.
In the clear air of the desert the mountain masses loom with sharp outlines and clear detail from a distance of many miles, and the plains that surround them are deceptively foreshortened. More than half of the region is a lowland. These intermontane areas are either bolsons (structural depressions filled by mountain waste) or destructional plains that slope upward as pediments toward the mountain masses from which they have been carved.
Ephemeral streams, which are dry gravel beds most of the year, discharge from the mountains and flow across the plains. Some of these drain into bolsons with no outlet to the sea, such as the Salt Basin, in the northwestern part of trans-Pecos Texas (fig. 1). Most of the drainage channels, however, lead to the two master streams of the area, the Rio Grande and its major tributary, the Pecos River. Their waters flow to the Gulf of Mexico. The Rio Grande is noteworthy more for its persistence through long stretches of desert land than for its breadth or volume. In its southeastward course across the area the river traverses a succession of desert basins and passes from one to the next through separating mountain barriers in which it has cut narrow and imposing canyons.
The mountains, plains, and plateaus of trans-Pecos Texas have been formed by interaction between various crustal movements of post-Mesozoic age and by the forces of erosion working upon the disturbed crust. The forms thus produced are of varying character, and the area may be divided into several geomorphic and structural provinces.
Basin and Range province-North of the Texas & Pacific Railway the mountain areas are broad and in part plateaulike, with one side presenting a steep escarpment and the other forming a gentle back slope descending from the crest. Between them are intermontane plains 5 to 15 miles across, whose margins rise as bajada slopes toward the mountains. These mountains are composed of rocks which have been very little folded but which have been broken in the later part of Cenozoic time into numerous fault blocks (pl. 22). Movement along the faults has served to outline the form of the mountain areas, and this form has been modified but slightly by subsequent erosion. The intermontane plains are mostly depressed areas filled by waste carried down from the mountains.
The mountains and desert plains of this part of trans-Pecos Texas resemble those in the adjacent part of central New Mexico, to the north, near the Rio Grande. They are also similar to those in the typical basin and range country farther west, and this part of transPecos Texas is included in the Basin and Range province.
Mexican Highlands province-South of the Texas & Pacific Railway block mountains of basin and range type are well developed in only a few areas. The mountains and plains are not caused directly by the uplift or depression of blocks of the earth's crust, but mostly by the differential erosion of bedrocks of varied character. The sedimentary rocks and the lava flows have been tilted, flexed, and in places strongly folded by crustal movements older than the block faulting of the basin and range province. In many places there are also masses of intrusive igneous rock. The nonresistant rocks of this region have been worn down into valleys and plains, and such harder rocks as limestones, thick lava flows, and igneous intrusions have been left as ridges, plateaus, and peaks. This area is the northern edge of a region of rugged highlands, whose greatest extent is in Mexico, south of the Rio Grande. It is here termed the "Mexican Highlands province."
The western part of the Mexican Highlands province in trans-Pecos Texas, comprising the Quitman and Eagle Mountains (fig. 1), consists of narrow parallel ridges and mountain chains of resistant, steeply tilted limestone and sandstone, between which are longitudinal lowlands carved from less resistant strata. In places the lowlands are covered to a moderate depth by later Cenozoic lake beds and alluvial deposits, but on the whole they seem to have been formed by erosion rather than by downfaulting or downwarping of the earth's crust. Similar parallel mountain ranges and intermontane lowlands are present southwest of the Quitman and Eagle Mountains, on the Mexican side of the Rio Grande, where they extend from the vicinity of El Paso southeastward past the great bend of the river and on into the interior of the State of Chihuahua (pl. 22, fig. 1). The two mountain ranges in Texas and the similar ranges to the southwest and south of them form the north end of the Sierra Madre Oriental of Mexico.
The eastern part of the Mexican Highlands province, comprising the eastern border ranges, is of greater diversity. Toward the north are the Davis Mountains (fig. 1), a high plateau broken up by canyons and in its more eroded parts separated into mesas, ridges, and isolated peaks. The Davis Mountains are carved from flat-lying or gently flexed lava flows. South of the Davis Mountains are various irregular mountain groups, some of them consisting of sharp peaks, and others of plateaulike blocks or narrow ridges. Between the mountains are lowland areas, some of which are smooth, gently sloping plains, whereas others have been greatly dissected and in part form picturesque
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badlands. The most conspicuous mountain group in the area, the Chisos Mountains, is a group of sharp peaks that stand in the center of a dissected lowland and are composed of masses of intrusive igneous rock and remnants of lava flows.
Southeast of the Davis Mountains and east of the Chisos Mountains are the narrow ridge of the Santiago Mountains and the high broken mountain mass of the Sierra del Carmen. These trend southeast and extend beyond the Rio Grande into Mexico. Northward the Santiago Mountains die out near the line of the Southern Pacific Railroad. The Santiago and Carmen Mountains are composed of folded, resistant limestones. East of the folds of these mountains are several domical uplifts (pl. 22). One of these, expressed topographically as the Serrania del Burro, lies wholly in Mexico, with its northern edges reaching up to the Rio Grande. It is a high dissected plateau, for the limestone cover of the dome is complete over its crest. Farther northwest, on the Texas side of the Rio Grande, is the Marathon dome. Here the limestone cover has been stripped from an extensive area on the dome's crest, and a lowland, the Marathon Basin, has been excavated from the nonresistant underlying beds.
Great Plains province-East of the Marathon Basin are escarpments of limestone which form the west edge of an extensive plateau area. The plateau summits descend gently eastward from the flanks of the Marathon region and the Serrania del Burro. The 50- to 75-mile belt between the Marathon region and the Pecos River on the east consists wholly of such plateau country, which has been carved into a maze of canyons and low tablelands. The plateau region is the western edge of the Edwards Plateau section of the Great Plains province, which extends far eastward into central Texas.
CLIMATE
Trans-Pecos Texas has an arid or semiarid climate. The average annual rainfall at Marathon and nearby stations is about 17 inches. However, this figure is the average of greatly varying observations of many years, and the amount of rainfall is erratic in both extent and time. Some spots may receive half a dozen rains within a year, whereas others may remain nearly rainless for several years. The yearly rainfall at Fort Stockton, not far north of Marathon, has been as slight as 4 inches and as great as 34 inches, although its average is 15 inches.
One-half or more of the year's rainfall comes during the summer, when most of it is of torrential character. The precipitation during any one of such rains may amount to several inches. Now and then during this time there may be one or more weeks of continuous rain, when the mountains are cloaked in clouds. The entire rainfall of a year may be produced by only a few storms. During the winter some snow falls in the mountains, but as this is the dry part of the year, the amount of such precipitation is not great.
Temperatures at Marathon range from 110° in the summer to below zero in the winter, but ordinarily the variation is not so large. In the summer the diurnal temperature range is as much as 50°. Winds are strongest in the spring, when violent gales, without rain, may persist for a week or more. Violent wind storms of short duration at times accompany the summer thunder-showers.
VEGETATION
The Marathon region and surrounding parts of trans-Pecos Texas have a vegetation adapted to the semiarid climate. The smooth plains of the Marathon Basin are grass-grown, but in the low places, where ground water is nearest to the surface, there are expanses of creosote bushes (Covillea) and dense thickets of mesquite (Prosopis juliflora) and catclaw (Acacia greggi). Low rocky ledges in the plains and terraces of limestone gravel that fringe the mountains support clumps of sotol (Basylirion wheeleri), lechuguilla (Agave lecheguilla), and other yuccas. Prickly pear or nopal (Opuntia) and ocotillo (Fouquieria splendens) grow on the low foothills. Higher in the mountains a sparse growth of juniper and pinon spreads over the exposed surfaces and summits and gathers in groves on the northern shaded slopes. Small clusters of live oak and manzanita (Arctostaphylos pungens) grow in protected valleys. Near water holes and stretches of flowing water in the stream channels is a lush growth of reeds and alders, shaded by cottonwood trees. The giant cacti that characterize the Sonoran Desert farther west are lacking in trans-Pecos Texas, but otherwise there is much similarity in the vegetation of the two regions.
The region is most attractive in the spring when numerous small plants come into blossom, covering the hillsides with a mat of brightly colored flowers. After the summer rains also, the brown and barren hills turn green as the vegetation comes to new life. Some of the plants, by reason of desert adaptation, show an immediate and wonderful rejuvenation after these unexpected downpours. The leafy clumps of resurrection plants (Selaginella pringlei ?), matted over many of the limestone surfaces, are dry and brown most of the year, but unfold and turn green within an hour after a rain.
"For a useful discussion of the vegetation of this part of Texas see Bray, W. L., The vegetation of the sotol country in Texas: Texas Univ. Bull. 60, 1905. A concise summary is also given in Carter, W. T., and others, op. cit., pp. 7-11.
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EROSIONAL AGENCIES
Agencies observed in the Marathon region.-The sedimentary rocks of the Marathon region, especially the greatly deformed strata of Paleozoic age, have been prepared for weathering by previous jointing. Where they are least fractured and have the fewest bedding planes they stand as bold cliffs and hogbacks. The dominant rocks of the region, which are limestones of various sorts, weather chiefly by solution, despite the low humidity. Solution widens joints and fractures, makes channels, pits, and shallow depressions on the exposed surfaces, and undermines ledges. In places, some of the granular limestones of Pennsylvanian age and crystalline dolomites of Permian age show a well- developed exfoliation of undetermined origin. The older Paleozoic cherts and novaculites are not affected greatly by either solution or exfoliation, but in most places they break down readily along closely spaced joints. Rock breakage by diurnal temperature change does not appear to be an important agent of weathering in any of the rocks of the region.
The fractured blocks of limestone and chert are loosened from their parent ledges by frost action. Gravity and rain wash help carry them down the steep slopes below the outcrops. Many rock masses may also be broken from the cliffs by lightning, for scars apparently produced by its impact may be seen on the faces of the steeper bluffs at many places in the area.
Because of the dry climate, there is but a thin cover of vegetation and, in comparison with humid regions, a small amount of rock decay. The soils on the hillsides and mountain slopes are therefore thin and are full of angular rock fragments. Rock ledges are abundant on the slopes, except in the shaly formations, and even here gullies only a few feet deep lay bare the underlying strata. Talus is generally lacking. In the plains the surficial material forms a thicker cover over the bedrock, but most of this has been washed in from the surrounding hillsides.
The surfaces of all the mountains, hills, and plains are covered with a network of watercourses, ranging from small gullies to broad, gravel-covered creek channels. This strongly suggests that the dominant erosional agent of the region is running water. In the semiarid climate, however, the work of water is spasmodic, and the drainage channels are dry most of the year. After rains the water runs rapidly down the mountain slopes, discharging into rocky gorges in the mountains or directly onto the plains. There is so little vegetation and soil that not much rain water is absorbed where it falls. The drainage channels leading away from the storm area become rushing turbid rivers, with the flood waters at times advancing down the hitherto dry channel like a wall. Sometimes the writer has heard these torrents emit a rumbling sound, doubtless from the impact of boulders against each other while in movement. Nearly all the erosion accomplished by the streams of the region takes place during the flood periods. Banks are deeply undercut at the stream bends, depressions are hollowed out in the channels, and gravel bars are shifted downstream. The undercutting is a phase of lateral corrasion, which, according to Blackwelder, is "geologically * * * rapid, apparently more so than most other processes in the desert."
On the level plains the flood waters may spread far beyond the insignificant swales on the surface and flow down the slope as a mass of interlacing rivulets, or even as sheet floods a few inches to a few feet deep and several miles in width. In the path of sheet floods the writer has seen on the steeper slopes closely spaced shallow gullies and on the gentler slopes small heaps of sticks, rubbish, and fine mud. Erosion and deposition of this sort, accomplished by sheet floods, appears to be of minor consequence, and in the Marathon region at least sheet floods are not the important agent of erosion that McGee suggested.
The flood waters eventually disappear into the gravel channels of the streams or in the alluvium of the plains. Very little of the run-off reaches the Pecos River or the Rio Grande by surface flow. However, there is much continuous underflow within the gravel beds of the channels. In the larger creeks there are stretches of permanently flowing water where the underflow is raised to the surface by sills of bedrock.
During years of normal climate the wind is not an important agent in the erosion of the Marathon region. The most striking wind storms are the great gusts that precede summer thundershowers. These carry great quantities of dust into the air and sometimes even across the lower mountain ridges, but they are local in extent and erratic in direction. In dry years wind storms may occupy several weeks of the spring and may carry much suspended matter into the air. During the exceptionally dry winter and spring of 1933-34 such dust storms were more prominent than usual in trans-Pecos Texas. Many storms were observed by "
Udden, J. A., Etched potholes: Texas Univ. Bull. 2509, 1925.
Blackwelder, Eliot, Exfoliation as a phase of rock weathering: Jour. Geology, vol. 33, pp. 793-806, 1925; Insolation hypothesis of rock weathering: Am. Jour. Sci., 5th ser., vol. 26, pp 97-113, 1933.
Blackwelder, Eliot, Talus slopes in the Basin Range province [abstract]: Geol. Soc. America Proc., 1934, p. 317, 1934.
The same criterion has been used by Bryan, Kirk, Wind erosion near Lees Ferry, Ariz.: Am. Jour. Sci., 5th ser., vol. 6, pp. 303-305, 1923.
Shuler, E. W., A rise down canyon [Davis Mountains]: Sci. Monthly, vol. 31, pp. 129-133, 1930.
Blackwelder, Eliot, Desert plains: Jour. Geology, vol. 39, p. 138, 1931.
McGee, W J, Sheetflood erosion: Geol. Soc. America Bull., vol. 8, pp. 87-112, 1897.
Baker, C. L., and Bowman, W. F., Geologic exploration of the southeastern front range of trans-Pecos Texas: Texas Univ. Bull. 1753, p. 163, 1917.
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the writer during this period in the Guadalupe Mountains, northwest of the Marathon region. Most of then were accompanied by sharp changes in temperature some were associated with strong winds, but in other: the movement of air currents did not exceed 10 mile,. an hour. In all of them there was a persistent movement of air in some one direction, so that transportation of dust from one region to another apparently tool place on a grand scale.
Such dust storms are exceptional under present climatic conditions, but they suggest that a relatively slight decrease in annual rainfall might permit muck fine material to be carried from the region by deflation and that this process may have been active during dry times in the past.
The only features that can be definitely attributed to wind work are the "charcos" found in low places on the plains, generally in areas of fine-grained alluvium. They are steep-banked circular or oval depressions, 10 to 25 feet in diameter, and 2 or 3 feet deep. They are probably caused by cattle in search of water trampling a wet place after a rain. The vegetation is thus destroyed, leaving mud exposed, and during dry seasons this is carried away as dust by the wind. More conspicuous features ascribable to wind work, such as sand-blasted rocks and sand dunes, are entirely lacking.
Comparison of erosional agencies in and and humid regions -In regions of arid climate, as a general rule, mechanical weathering dominates over chemical weathering, and on the steeper slopes rill wash is more effective than soil creep. Because of the lack of creep, steep mountain slopes tend to endure in the unconsumed areas of such regions until well along in the cycle of erosion, whereas in regions of humid climate they have at this stage changed to subdued forms. In arid regions the streams of both mountains and plains areas are intermittent rather than permanent, but because of the lack of vegetation the subdivision of the more steeply sloping areas into watercourses is much more minute than in humid regions. When the streams flow the material carried by them is larger in amount and coarser in texture than that of streams in humid regions. The profile of such streams, even at grade, is therefore relatively steep.
The erosional agencies of arid regions, being unlike those of humid regions, produce unlike land forms. Davis notes that "the rocky and boulder-clad slopes of maturely dissected mountains in arid regions, together with the barren pediments below them", contrast strongly with "the soil-cloaked and forested slopes of maturely dissected mountains in humid regions, together with the fertile valley floors below them." He believes, however, that "their unlikeness is rather a matter of degree than of kind" and that "the unlike features are really homologous."
According to Davis, a very baffling problem is concerned with the relative rates of erosion and degradation in humid and arid regions. It seems as if humid stream erosion must * * * be more rapid than arid stream erosion in the early stages of an erosion cycle; also that, in a much later stage, degradation may be more rapid on the bare slopes of an arid region than on the plant-covered slopes of humid regions.
MARATHON REGION
GENERAL FEATURES
The Marathon Basin, on the crest of the Marathon dome, is 30 miles wide and 40 miles long and consists of plains, hilly lowlands, and low mountain ridges, carved from folded Paleozoic strata. The basin is surrounded by limestone escarpments, which stand higher than any of the ridges in the basin. On the east, south, and west sides the limestones of the escarpments are of Cretaceous age and are mostly gently tilted away from the uplift. On the north the Paleozoic rocks beneath the Cretaceous contain a resistant mass of limestone and form the broad cuesta- like upland of the Glass Mountains.
The Cretaceous rocks now found on the escarpments bordering the Marathon Basin at one time extended entirely over the crest of the Marathon dome. They have been stripped off the higher parts of the dome by recession of their cliffs and by the excavation of the weak underlying Paleozoic beds to form the Marathon Basin. These processes are still going on.
The northern part of the Marathon region slopes northward and northeastward toward the Pecos River, but the greater part slopes southward and is drained by Maravillas, San Francisco, and smaller creeks, which flow into the Rio Grande (fig. 9, B). The maximum relief in the Monument Spring and Marathon quadrangles is 2,700 feet. The lowest point, 3,450 feet above sea level, is on San Francisco Creek where it leaves the southeast corner of the Marathon quadrangle, and the highest summit is an unnamed peak in the Del Norte Mountains, 6,151 feet high, in the northwestern part of the Monument Spring quadrangle. Horse Mountain, the summit of one of the ridges of Paleozoic rock, is the highest peak in the Marathon Basin. Its crest, 5,010 feet high, is lower than the summits of any of the limestone escarpments on the rim. Most of the ridges in the basin are not more "
Blackwelder, Eliot, Yardangs: Geol. Soc. America Bull., vol. 45, pp. 164-165,1934.
This and other possible origins of charcos are discussed by Kirk Bryan (The Papago country, Arizona: 1". S. Geol. Survey Water-Supply Paper 499, pp 121-123, 1925).
This subject has been treated at some length by W. M. Davis (Rock floors in arid and humid climates: Jour. Geology, vol. 38, pp. 146-149, 1930). It is also discussed in less technical form in his Physiographic contrasts, east and west: Sci.
Davis, W. M., Rock floors in arid and in humid climates: Jour. Geology, vol. 38, p. 145, 1930.
Idem, p. 158.
8
than 700 feet above their surroundings, but the escarpments that encircle the basin rise 1,000 to 1,500 feet above the floor.
The physical features of the Marathon region have been formed by the differential erosion of resistant and nonresistant rocks by streams. Among the more resistant rocks are limestones, which, because of the semiarid climate, stand as hogbacks, steep-sided plateaus, and high mountains. The region as a whole has reached a mature stage of erosion. There are, however, wider areas of sloping plains and much steeper and more rugged unconsumed areas than would be present in similar areas at the same stage of erosion in a humid climate.
Superb views of the Marathon Basin are to be had from the high escarpments on its east and west sides (fig. 2). The panorama is particularly impressive from Housetop Mountain (fig. 2, A), a projecting tongue of the Cretaceous plateau on the east edge of the basin, whose western face rises as a bold cliff 1,500 feet above the basin floor. From this eminence, in the clear air of the desert, the whole basin and its surroundings appear spread out like a map.
On the sky line to the west and south rise the mountains of the Mexican highlands, which lie beyond the Marathon Basin. To the southwest are the rugged peaks of the Chisos Mountains, to the south the domelike mass of the Sierra del Carmen, gashed by the canyon of the Rio Grande. Farther east the Serrania del Burro and other ranges stretch far away into Mexico until they are lost in the bright haze of the horizon.
Between the observer and the mountains of the horizon are ridges and plateaus of lesser order. To the southeast are Cretaceous tablelands, sloping toward the east, intricately carved into canyons, whose sides are banded by limestone ledges, as straight as if drawn by a rule. Westward the tablelands rise and project in long promontories into the Marathon Basin. The limestone ledges at the ends of the promontories are broken into disconnected tables and conical buttes, perched on reddish rounded slopes and hillocks of Paleozoic rock. Here and there ledges are discernible in the lower beds, but these run at a steeper angle than those of the Cretaceous limestones.
In the middle distance, between the tablelands and the observer, are the hills and plains of the Marathon Basin. The flats, streaked in places by white gravel deposits, are covered by a lacy network of drainage channels, with fringes of dark vegetation. Between the flats are bare rocky ridges and miniature mountains, each of which assumes a color and form determined by the nature of the rock from which it has been carved. Near at hand are reddish hills and ragged ledges of sandstone and narrow hogbacks of limestone. Farther away, in the center of the basin, is a broad cluster of hills, streaked by white ledges of novaculite. From this distance, most of the hills have no evident plan or arrangement, and they are apparently turned and twisted in greatest confusion. Here and there, however, the eye can distinguish sharp ridges and chains of knobs in the white rock, and in places a spine of vertical strata projects above the rest.
Such a distant view, in a region of great complexity, can only reveal the outlines of the geography and geology and serve to arouse the imagination of the observer. If he should wish to untangle the geologic history of the land, he must descend into the plains, in order to analyze its many features.
ESCARPMENTS BORDERING THE MARATHON BASIN
Escarpments and plateaus on the east and south sides- The Cretaceous limestone escarpments on the east and south sides of the basin stand 500 to 1,500 feet above the basin floor. These are parts of* the high western dissected margin of the Edwards Plateau. East of the basin the plateau surface inclines gently eastward on the stripped bedding planes of resistant layers (pl. 1, B, and fig. 3, A). To the south the inclination of the strata is steeper, and here two prominent parallel limestone cuestas, called the Maravillas scarp by Hill, face northward toward the basin (pl. 14, B, and fig. 3, B).
The rocks of the plateau consist of an alternation of resistant limestone layers with weaker beds of marl. Near the basin the cap rock of most of the escarpments is the Edwards limestone (pl. 13, D). The resistant beds of the Edwards and other limestones resemble "huge stone walls of so ancient a date that they have crumbled into ruins. The less resistant beds form less steep slopes covered with debris from the overlying more resistant layers, and the whole escarpment or canyon wall gives a buttress affect like that of ruined Gothic architecture."
Drainage in the plateau country adjacent to the Marathon region is prevailingly consequent; the streams follow the slope of the Cretaceous surface and radiate from the Marathon dome (fig. 9, B). Several of the consequent streams head in the Marathon dome and flow eastward or southward into the plateau country. These have broken the escarpments at the edge of the Marathon Basin into separate segments and in places have reduced the segments to narrow promontories. The streams in the plateau have carved innumerable canyons, which have broken the originally continuous plateau surface into small areas of tableland.
In one of the canyons east of the Marathon Basin there is evidence of a relatively recent drainage change. "
Hill, R. T., Physical geography of the Texas region: U. S. Geol. Survey Top. Atlas, folio 3, p. 4, 1900.
Baker. C. L., and Bowman, W. F., Geologic exploration of the southeastern front range of trans-Pecos Texas: Texas Univ. Bull. 1753, p. 133, 1917.
9
10
A broad valley, followed by the Southern Pacific Railroad, branches from Dry Canyon about 20 miles east of the edge of the Marathon Basin (fig. 9, B, and preliminary topographic map of the Longfellow quadrangle). The upper part of this valley, near the Marathon Basin, drains, not into Dry Canyon, but into Maxon Creek, which leaves the broad valley and flows southward through a narrow gorge into San Francisco Creek. It would seem that Maxon Creek, cutting
Most of the upland surfaces of the plateau country stand at accordant levels. These are not ancient uplifted peneplains, but plains that follow the upper surface of some resistant stratum of limestone, from which the overlying softer beds have been removed by erosion. The effect of the resistant, flat-lying beds on the topography is so great that it is difficult to prove the existence of any former levels of erosion higher than the present one in the plateau. In the country southeast of the Marathon Basin, however, are some remnants of old gravel deposits on the divides between the present streams. These have been mapped by N. H. Darton in the region between the lower course of San Francisco Creek and Dryden, 50 miles to the east (fig. 4).
One such deposit was examined by the writer on the highway between Dryden and Sanderson. It consists of well-rounded cobbles, 2 feet in maximum diameter, of chert, novaculite, and sandstone from the Maravillas, Caballos, and Tesnus formations of the Marathon Basin. There are also some cobbles of Cretaceous limestone and of volcanic rock like that now seen only northwest of the Marathon Basin. The eastward trend of the remnant gravel areas diverges at a wide angle from the courses of the present streams and the slope of the country, and none of the streams near the eastern gravel areas now head in the Marathon Basin, from which most of the material was derived. The gravel may have been deposited by one stream or several streams, but in any event it suggests a very different topography at the time of deposition from that of the present.
Escarpments on the north side-On the northern border of the Marathon Basin are the Glass Mountains, an asymmetric cuestalike upland trending northeast, carved from northwestward tilted limestones, sandstones, and shales of Permian age. The southward-facing front of the range is dissected into cuestas capped by resistant limestone beds, which are separated by lowlands or slopes carved from sandy and shaly beds. In
Blackwelder, Elliot, Desert plains: Jour, Geology, vol. 39, pp.124-135, 1931.
11
the northeastern part of the mountains (as shown on the map of the Hess Canyon quadrangle), the face of the escarpment that borders the Marathon Basin is remarkably straight and steep and simulates a fault scarp.
An examination of the structure and stratigraphy proves that there are no faults here. * * * The strike of the strata in the scarps is parallel with the direction of the scarps [and] the dip of the strata is * * * in the opposite direction from that toward which the escarpments face.
The crests and back slopes of the Glass Mountains are composed of massive dolomite. The back slope differs from that of a true cuesta in that it is not controlled by the dip of the beds that underlie it but by the stripped surface of the ancient peneplain on which the overlying Cretaceous strata were deposited. This surface also declines northwestward, but at a slighter angle than the Permian dolomites. From it, in the mountain area, most of the Cretaceous rocks have been carried away by erosion.
Most of the streams that drain the north and northwest slopes of the Glass Mountains are consequent streams that follow low places in the folded surface of the Cretaceous rocks. Most of them do not show a close relation to the numerous northwestwardtrending normal faults that disturb the rocks of the mountains; they cross them at right or oblique angles, and although most of them flow from upthrown to downthrown blocks, some flow from the downthrown to the upthrown. There is, however, one conspicuous exception. Hess Canyon, a straight, narrow gorge in the northeastern part of the mountains, follows the northwest trend of the faults and is structurally a narrow graben. "When the long, narrow wedge of Hess Canyon sank to form a deep graben, the previous consequents crossing its site could no longer maintain their flow and were diverted into a new consequent course along the bottom."
Escarpments on the west side-The escarpments on the western margin of the Marathon Basin are known as the Del Norte Mountains in the north, and the Santiago Mountains in the south (pl. 23). The two ranges are separated near the boundary between the Monument Spring and Santiago Peak quadrangles by Del Norte Gap, but structurally they are a continuous chain. The limestones that compose them are in some places as flat as the strata on the east margin of the basin, but in others they are considerably folded and faulted. The mountains are cut by several gaps. In the north is Doubtful Canyon (pl. 24), followed by a tributary of Maravillas Creek, which flows eastward across the mountains into the Marathon Basin. Next to the south is Del Norte Gap (pl. 13, B, and map of Santiago Peak quadrangle), which lies on a divide and is not now followed by any stream. South of the Marathon Basin the Santiago Mountains are also cut by the wind gap of Persimmon Gap and the water gap of Dog Canyon (both in the Bone Spring quadrangle).
The Del Norte Mountains are an upland, several miles broad and about 20 miles long, formed of limestones gently tilted toward the west. Eastward, they face the Marathon Basin in a steep, fairly straight escarpment, indented by few valleys. At the foot of the escarpment is a fault that has raised nonresistant Paleozoic beds on the east against Cretaceous limestones on the west (fig. 5, A). The escarpment has been caused by the wearing away of the nonresistant up-faulted beds, leaving the resistant down-faulted beds relatively undissected (fig. 5, B). It is thus an obsequent fault-line scarp.
The Santiago Mountains are about 40 miles in length and extend for some distance south of the southwest corner of the Marathon Basin. (See maps of Santiago Peak and Bone Spring quadrangles.) For most of their length they are a steep-sided ridge scarcely 2 miles across, carved from a narrow belt of vertical limestones (fig. 5, D). To the north, high mesas of flat-lying limestone, the Cochran Mountains (pl. 23, sec. Q-Q'-Q"; pl. 21; and map of Santiago Peak quadrangle) lie between them and the Marathon Basin. The Santiago Mountains owe their height partly to a large normal fault that has downthrown the Cretaceous beds on the east (fig. 5, C), but erosion has in most places leveled off the up-faulted beds for several miles west of its trace.
The escarpments of the Del Norte, Cochran, and Santiago Mountains are flanked on the east by pediments, or rock-cut plains, which slope down to Maravillas Creek on the east. The plains are covered by a thin layer of limestone gravel washed down from the mountains. In the Del Norte and Cochran Mountains, where the resistant limestone beds lie flat, recession of the cliffs has taken place by sapping of the nonresistant Paleozoic shales beneath. Considerable recession has apparently taken place in fairly recent time, when Maravillas Creek and its tributaries trenched the gravel of the pediments. As a result, the mountains north and south of Del Norte Gap are flanked by a lowland carved from shale, beyond which, at a distance of about a mile, are cuestalike remnants of gravel, sloping to the east and with their steepest "
Baker, C. L., and Bowman, W. F., Geologic exploration of the southeastern front range of trans-Pecos Texas: Texas Univ. Bull. 1753, p. 160, 1917.
King, P. B., Geology of the Glass Mountains, part 1, Descriptive geology: Texas Univ. Bull. 3038, p. 22, 1931.
Idem, p. 29.
Blackwelder, Eliot, The recognition of fault scarps: Jour. Geology, vol. 38, pp. 305-306, 1928.
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inclination near the mountains. The cuestalike areas of gravel probably extended up to the bases of the escarpments when they stood farther forward than now.
Relation of escarpments bordering the Marathon Basin to the later tectonic movements.-The writer believes that most of the physical features to be seen in the escarpments bordering the Marathon Basin have been formed by the stripping of the Cretaceous cover from the crest of the Marathon dome. He therefore considers it probable that they have resulted from the erosion of rocks of different composition and structure and that they are not caused directly by doming, folding, or faulting.
A somewhat different interpretation, however, has been made by Baker, who believes that "from the standpoint of physiographic development the doming of the Cretaceous rocks rimming the Marathon Basin probably occurred long subsequent to the Laramide movements, and very possibly as late as the Lafayette", an interpretation which implies that the last folding was sufficiently recent to have had a direct influence on the aspect of the present land forms. The surface rocks of the Del Norte and Santiago Mountains are thought by him to be "the surface rocks at the time the latest deformation began", and the streams in the water gaps of Doubtful and Dog Canyons are interpreted as "almost certainly antecedent."
In order that the reader may understand the problem, it seems desirable that the Tertiary structural history of the Marathon region as worked out by the writer and as more fully described on later pages be summarized at this place. Since the withdrawal of the seas, at the end of Cretaceous time, the Marathon region has remained as a land area. After the Cretaceous period two groups of rocks were laid down on parts of the surface in trans-Pecos Texas-the lava flows and associated sediments of the Davis Mountains country and the bolson and lacustrine deposits of the basin and range country. No remnants of these rocks are now found within the Marathon region, and it seems improbable that either of them were ever laid across the region in any great thickness. After the emergence of the Marathon region from the Cretaceous sea, it was subjected to three periods of movement. The movements in the first two periods, one of which was older and the other younger than the lavas to the west, caused the broad doming of the Marathon region and produced local sharp folding and thrust faulting in the Del Norte and Santiago Mountains on the west. The last movement, considerably later than the other two, broke the rocks of the region into fault blocks; its effects are particularly marked in the Glass, Del Norte, and Santiago Mountains.
It seems probable that the Marathon region has been more or less actively eroded during the whole time since the last doming and that nearly all the present surface features have resulted from that erosion. The steepness of many of the escarpments, particularly on the west side of the basin, appears to be a normal feature of the desert landscape, even in a region of as "
Compare the description of similar features along the base of the Book Cliffs by W. S. Glock (Premonitory planations in western Colorado: Pan-Am. Geologist, vol. 57, pp. 32-33,1931).
Baker, C. L., Date of the major diastrophism and other problems of the Marathon Basin, trans-Pecos Texas: Am. Assoc. Petroleum Geologists Bull., vol. 12, p. 1115,1928.
Baker, C. L., and Bowman, W. F., Geologic exploration of the southeastern front range of trans-Pecos Texas: Texas Univ. Bull. 1753, p. 168, 1917.
Idem, p. 148.
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mature dissection as this one. In places, it is true, the ridges follow the structure of the rocks within the mountains, but these ridges are composed of resistant Cretaceous limestone. That they have been produced by the stripping of nonresistant beds from their surface seems probable, and a thick sequence of such rocks, of later Cretaceous age, is exposed on the west flanks of the Del Norte and Santiago Mountains.
Moreover, these two mountain ranges, at least in the Marathon region, are not simple anticlines, but are the overturned and much faulted western limb of the Marathon dome (fig. 5 and pl. 21). The Marathon Basin, to the east of them, is structurally much higher than these mountains and has been reduced to its present low altitude by the erosion of the weak rocks that underlie it. In one of the places where the structurally greater altitude of the basin area has been caused by faulting the down-faulted rocks now rise above the up-faulted rocks in an obsequent fault-line scarp. As the region east of the mountains has a greater structural height than its surroundings, it seems rather improbable that such streams as that in Doubtful Canyon, which flow eastward across the mountains and into the basin, could have existed before the doming and maintained their courses in the face of such adverse conditions. On the contrary, it seems probable that the gaps in which these streams flow might at one time have been occupied by streams that drained westward down the structural surface of the dome, and that such streams were afterward captured by subsequent streams actively cutting in the weak rocks of the basin (fig. 9, A and B).
The normal faults of the Glass, Del Norte, and Santiago Mountains are considerably younger than the last time of doming and apparently had a much more direct influence on the topography. Thus in the Glass Mountains most of the streams seem to follow the structural lines produced by the folds but not the faults, as if they had come into existence before the time of faulting. Moreover, in at least one place, Hess Canyon, the faulting seems to have formed a new consequent stream, which cuts across the older drainage lines.
RIDGES OF THE MARATHON BASIN
The strongly folded and faulted Paleozoic rocks of the Marathon Basin have been revealed by the stripping of the cover of Cretaceous limestones from the crest of the Marathon dome. They are a fragment of the denuded roots of a widely extended mountain system, formed in the later part of Paleozoic time. The folds strike northeast, at right angles to the trend of the post-Cretaceous uplifts, and the extensions of the folds on each side are concealed by the cover of younger rocks.
Two rock formations, more resistant than the rest, stand as ridges in the Marathon Basin. The lower of these stratigraphically is the Caballos novaculite, at the top of the pre-Carboniferous succession; the upper is the Dimple limestone, lying within the Carboniferous. As the resistant beds are in all places vertical or steeply tilted, the ridges are narrow and are breached at many places by gaps or sags.
In their setting amidst the grander features of trans-Pecos Texas they must be regarded as mountains in miniature, resurrected of late to a mere shadow of their one-time glory (compare pl. 24, D and F) by the fortuitous circumstance of being denuded of the mantle accumulated on them by the sea, which had formerly entirely buried their ancient summits.
Novaculite ridges-The novaculite rises in white ridges of bare rock, mostly monoclinal, supported on the inner side by beds of chert and limestone (pls. 1, A; 7, B; fig. 2, B). Soft shales lie on both sides of the resistant beds, and their nonresistance to erosion accentuates the sharpness of the ridges. Novaculite hogbacks enclose the excavated cores of two broad anticlinoria in the western part of the basin (pl. 23). Between the anticlinoria, shorter but no less conspicuous novaculite ridges are carved from lower anticlines and thrust blocks. On the flanks of the anticlinoria the hogbacks run nearly straight and unbroken, save for water gaps of superimposed streams. Where the older rocks pitch beneath the surface on the northeast and southwest ends of the anticlinoria, the novaculite hogbacks pass into convoluted zigzag ridges, which wind across the axes of the plunging folds. The Warwick and Lightning Hills, 8 miles east of Marathon, are a maze of such winding hogbacks (pl. 19, C).
The lesser folds, between the anticlinoria and to the south of them, present all stages of degradation, from broad-backed mountains well expressing the doubly plunging anticlinal structure to mere chains of knobs projecting above the plain. The crest of Horse Mountain is still sheeted over by novaculite (fig. 6, A), as this stratum is thicker here than elsewhere. Some other mountains, stripped of this resistant member, still show their anticlinal form in the cherts and limestones beneath. Many more, like East Bourland Mountain (fig. 6, B, and pl. 6, A, B), are partly breached by axial anticlinal valleys that penetrate weak shales in the core. One of them, the largest of the Woods Hollow Mountains (fig. 6, C), has a wide depression down its center excavated from the shales, from which projects a low ridge of the next resistant member below. This axial depression is drained by a water gap scarcely 100 feet wide in the encircling novaculite ridge.
Dimple limestone ridges-The limestone ridge maker does not rise as high as the novaculite, and its ridge "
Baker, C. L., and Bowman, W. F., op. cit., pp. 162-163.
14
crests are breached more widely by superimposed streams. Its hogbacks surround synclinal areas and are most extensive in the eastern and northeastern parts of the basin (pl. 23). Those near Haymond, in the eastern part of the basin, extend in great curves around broad synclinal plains. One of these is shown, just below Horse Mountain, in figure 2, A. In the western part of the basin the limestone ridges are less extensive. The most prominent one, West Bourland Mountain, rises out of a synclinal lowland between novaculite ridges. It is a remnant patch of limestone 2 miles long, presenting steep faces outward on all sides but with a synclinal valley hollowed out in the center (pl. 6, C). In the southern part of the basin the Carboniferous sandstones also stand at considerable height and are carved into rugged tracts of broken ridges and ledges. These areas are locally known by such descriptive titles as Hells Half Acre and Devils Backbone.
Even summit levels on the ridges-Many of the hogbacks, of both the limestone and the novaculite, are conspicuously even-crested (fig. 8). Near the center of the basin most of the summits stand at about 4,500 feet, but toward the south, east, and west they gradually decline in height. Two or three ridges, such as Horse Mountain and Simpson Springs Mountain, rise conspicuously above this general level.
Unlike the even summits on the Permian dolomites in the Glass Mountains, the even summit levels in this area probably do not represent the resurrected base of the Cretaceous, for in the surrounding escarpments the base of the overlying Cretaceous beds lies at a higher altitude. There is a possibility that the even summits may represent the last remnants of one or more former high-level surfaces of erosion, now preserved only on the harder rocks of the region; if this is true, too little now remains to tell much about their character.
There is a stronger possibility that the regularity of the summit levels was produced during the last or next to the last cycle of erosion by backward cutting of the ridge slopes from the rock floors on each side. The adjacent rock floors slope in a similar direction to the crests of the ridges and at about the same angle. If the hard rocks were approximately uniformly jointed everywhere, they would tend to produce ridges having nearly the same angle of slope at all places. As the hard rocks are of small but nearly constant thickness, the ridge crests might therefore have a nearly constant height above the rock floors.
LOWLANDS OF THE MARATHON BASIN
Rock floors, their nature and origin-Under the influence of the semiarid climate erosional agencies have worn down the nonresistant rocks of the Marathon region into rock floors of wide extent. These closely resemble the worn-down surfaces which observers of desert land forms have variously termed "pediments", "graded plains", "suballuvial benches" and "subaerial platforms", "rock floors", and "planes of lateral corrasion." The term "pediment", which is most widely used for these features, is not entirely appropriate for the Marathon region, because it implies a sloping rock-cut plain fringing a mountain base, whereas the surfaces in the Marathon region may be unrelated to any mountain area. For them, Davis' term "rock floor" seems preferable, but the term "pediment" will be used for rock floors of steeper inclination near the mountain areas.
"King, P. B., The geology of the Glass Mountains, part 1, Descriptive geology: Texas Univ. Bull. 3038, p. 22, 1931.
McGee, W J, Sheetflood erosion: Geol. Soc. America Bull., vol. 8, pp. 92, 110, 1897. Bryan, Kirk, Erosion and sedimentation in the Papago country, Arizona: U. S. Geol. Survey Bull. 730, p. 54, 1922.
Udden, J. A., Sketch of the geology of the Chisos country: Texas Univ. Bull. 93, pp. 10-14,1907.
Lawson, A. C., The epigene profiles of the desert: -California Univ., Dept. Geology, Bull., vol. 9, p. 34, 1915.
Davis, W. M., Granitic domes of the Mohave Desert: San Diego Soc. Nat. History Trans., vol. 7, p. 223,1933. The term was also used without special definition in a paper published by Davis in 1930.
Johnson, D. W., Planes of lateral corrasion: Science, new ser., vol. 73, pp. 174-177, 1931.
15
Rock floors and pediments have in the past been widely mistaken for surfaces of deposition, such as bolson plains and bajadas, but as Blackwelder has pointed out, "the pediment, and not the bajada, is the normal and inevitable form developed in arid regions under stable conditions. It is not exceptional * * * but is dominant, widespread, and characteristic." According to this author ; pediments may be distinguished from bajadas by their low and uniform gradient, by their rambling and braided rather than outward-forking streams, and by the absence of a convex fan form opposite canyon mouths. Some of these criteria have recently been questioned by Johnson, who believes that rock-cut and constructional surfaces in arid regions may be more nearly alike than has been supposed. He brings forth theoretical and field evidence which suggests that "bedrock surfaces near canyon mouths * * * [may] possess the form of alluvial fans."
The manner in which rock floors may be produced has been variously interpreted. Davis has given a useful summary of the theories that were suggested before 1930, many of which need not be repeated here. The first carefully worked out sequence of events was presented by Lawson, following a briefer statement by Paige." Lawson's conclusions have been more or less closely followed by Bryan, and to a certain extent by Davis. The theory is summarized as follows:
A steep-sloping mountain front is worn back at a constant declivity by the ordinary subaerial processes of mountain recession as controlled by arid weathering and washing. Below the mountain front the worn-down surface of the rock floor-the pediment-is at first narrow, rather steep, and covered with a graded embankment of alluvium. But the embankment gradually rises * * * because its lower end reaches the rising detrital floor of an infilling intermont basin; and as it rises it overlaps farther and farther on the growing pediment, which, as it broadens, becomes less steep and almost bare.
In this theory chief emphasis is placed on the recession of the mountain slopes by weathering, and the pediment is considered to be "a slope of transportation * * * determined by the grade necessary to transport debris away from the mountains."
The problem is approached from a different viewpoint by Johnson and Blackwelder, who attribute the chief work of pediment cutting to the lateral corrasion of streams flowing across it. Johnson calls attention to an early deduction by Gilbert that downward wear of streams ceases when the load equals the capacity for transportation. Lateral corrasion then becomes relatively and actually of importance and carves an even surface covered by a thin deposit of alluvium. By cutting laterally into each other's valleys and consuming all remnants of the intervening divides, neighboring streams cooperate to carve a single plain of broad extent.
As noted above, however, Johnson and Blackwelder are not in agreement as to the forms produced. Johnson believes that rock fans, similar to alluvial fans, must be produced by lateral corrasion because the "inclined stream [is] relatively * * * fixed in position at the point of issuance from the canyon mouth and shifts more and more widely below that point.." He follows Paige in ascribing the steepness of the mountain front behind the pediments, not to weathering, but to the undercutting of streams swinging laterally on the surface of rock fans.
Most students of desert erosion believe, with Blackwelder, that the rock floor or pediment "is the desertinhabiting species of the genus peneplain * * * and the higher gradient which distinguishes it is conditioned by aridity." Johnson, however, suggests that this surface is "developed rapidly without any necessary relation to base level and may normally be trenched by streams without any change in the attitude or altitude of the areas affected."
Comparison of rock floors previously described with those in the Marathon Basin-The rock floors of the Marathon Basin show certain differences from the typical examples farther west that have been described. These differences are more in degree than in kind and are dependent on the local geology, the rainfall, and the disposition of the drainage.
At Marathon the rock floors are carved from steeply tilted sedimentary rocks of varying resistance, rather than from massive rocks such as granites. Most of the rock floors of the region are therefore cut on belts of weak rock. The sedimentary rocks are not as subject to spalling and granular decay as the granites in the pediment areas of southern Arizona and the Mojave Desert. Their weathered fragments are therefore of different size and shape, and this influences the form of the graded slopes that must be carved to transport them.
The base level of the rock floors is controlled by numerous streams which have access to the master "
Blackwelder, Eliot, Desert plains: Jour. Geology, vol. 39, p. 138, 1931.
Idem, pp. 136-137.
Johnson, D. W., Rock fans in arid regions: Am. Jour. Sci., 5th ser., vol. 23, pp. 389-420, 1932.
Johnson, D. W., Planes of lateral corrasion: Science, new ser., vol. 73, p. 175, 1931.
Davis, W. M., Rock floors in arid and humid climates: Jour. Geology, vol. 13, pp. 14-19, 1930.
Lawson, A. C., The epigene profiles of the desert: California Univ., Dept. Geology, Bull., vol. 9, pp. 23-48, 1915.
Paige, Sidney, Rock-cut surfaces in the desert ranges: Jour. Geology, vol. 20, pp. 442-450, 1912.
Davis, W. M., Rock floors in arid and in humid climates: Jour. Geology, vol. 38, p. 15, 1931.
Bryan, Kirk, Erosion and sedimentation in the Papago country, Arizona: U. S. Geol. Survey Bull. 730, p. 57, 1922.
Johnson, D. W., Planes of lateral corrasion: Science, new ser., vol. 73, p. 174, 1931.
Gilbert, G. K., Geology of the Henry Mountains, pp. 126-133, U. S. Geol. and Geog. Survey Rocky Mtn. Region, 1877.
Johnson, D. W., Rock fans in arid regions: Am. Jour. Sci., 5th ser., vol. 23, p. 392, 1932.
Paige, Sidney, op. cit., pp. 449-450.
Blackwelder, Eliot, Desert plains: Jour. Geology, vol. 39, p. 138, 1931.
Johnson, D. W., Planes of lateral corrasion: Science, new ser., vol. 73, p. 177, 1931.
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drainageways of the Rio Grande and the Pecos, rather than by an interior basin, whose surface is rising slowly by aggradation. This condition favors the development of rock floors of wide., extent, as suggested by Bryan and Davis, and the alluvial apron assumed by Lawson and Paige either does not exist or is thin.
The rock floors do not encircle mountain areas but lie in a basin between the highlands. As the resistant rocks of the basin form a relatively small part of the whole, the ridges of unconsumed material are relatively narrow and are penetrated in all directions by arms of the rock floors. The rock floors of the Marathon region thus resemble in many of their features the intramontane canyons and headwater basins in southern Arizona described by Bryan.
Rock floors of the Marathon Basin-Stratigraphically, there are three groups of weak rocks in the basin. The lowest group, consisting of pre-Carboniferous limestone and shale, lies at the surface in the two anticlinoria in the western part of the basin. The upper two groups, consisting of sandstone and shale of Carboniferous age, lie respectively below and above the ridge-making Dimple limestone and form broad expanses of plain surrounding the anticlinorial areas. Erosion in the Marathon Basin has proceeded with greatest ease along these belts of weak rock, thus favoring the development of subsequent streams, at the expense of consequent streams superimposed on the Paleozoic surface from the former Cretaceous cover. Rock floors have been cut back laterally from the superimposed or subsequent master streams, as far as the limiting belts of hard, ridge-making rocks on both sides.
Along the escarpments bordering the Marathon Basin are pediments with gradients of 200 or 300 feet to the mile, but these flatten outward and have a concave upward profile. Within the basin itself the rock floors have gradients of 75 feet or less to the mile. The rock floors have a general inclination southward toward the Rio Grande, but in detail they are a complex group of sloping surfaces, each drained by an axial stream tributary to one of the master drainage channels. The rock floors in the different drainage basins have an unlike form and gradient because of the varying character of the bedrock, amount of run-off, and relation to each master stream. Some are also controlled by local base levels, determined by sills of hard rock in the stream beds." Though the rock floors of each drainage area are accordant at their lower ends with those along the master stream, they may be discordant at their heads and margins with the floors drained by other streams. The coalescing floors of adjacent drainage areas are thus not likely to meet at the same level, and the junction in some places is marked by a low dissected escarpment that faces the lower rock floor." The discordance is most marked where the coalescing floors drain into far separated master streams. Thus the heads of rock floors drained by tributaries of the Pecos in the northeastern part of the Marathon region stand above W B Flat on the south (pl. 23 and map of Longfellow quadrangle), at the top of an escarpment 500 feet high.
In the Marathon region, so far as the writer's observations go, there are no well-developed rock fans of the type described by Johnson. It is possible, however, that some of the fanlike areas at the bases of the higher mountains, which the writer has interpreted as due to deposition on the pediments, may be underlain by fans carved from bedrock.
Erosion and deposition on the rock floors-In most places the rock floors of the Marathon Basin are either covered by a thin layer of gravel or dissected to a moderate depth.
Undissected rock floors covered by gravel are most extensive in the north half of the basin (pl. 23). The alluvial deposits here may be of some antiquity, for cobbles of Cretaceous and Dimple limestone occur in some of them where no outcrops of those formations now remain in the drainage area. They may also have been the source of the elephant bone reported to have been found near Marathon in 1930.
Most of the rock floors in the area are covered by 10 to 20 feet of gravel, which effectively masks the bedrock over wide areas, although valleys that incise the surface reveal the small thickness of the deposit. At some places the cover is thicker. Water wells in the northern part of the basin, near the Glass Mountains, penetrate 100 feet or more of gravel, and some of the streams that discharge from these mountains have deposited low alluvial fans on the rock floor." Alluvial fans have been built also along the northeast base of the Del Norte Mountains by Antelope Creek (fig. 7) and the stream in the large canyon 2 miles south of Altuda. Another large fan exists south of the Marathon Basin, in the Hood Spring quadrangle. The outer edges of the two fans in the Del Norte Mountains have been built across the courses of other streams and have ponded some of them (fig. 7), thereby interrupting the normal cycle of down cutting. Clearly these two fans cannot be "
Bryan, Kirk, op. cit. (Bull. 730), p. 56.
Davis, W. M., Rock floors in arid and in humid climates: Jour. Geology, vol. 38, p. 18, 1931.
Bryan, Kirk, op. cit., pp. 47-48.
Baker, C. L., and Bowman, W. F., Geologic exploration of the southeastern front range of trans-Peces Texas: Texas Univ. Bull. 1753, p. 164, 1917.
Davis, W. M., Granitic domes of the Mohave Desert: San Diego Soc. Nat. History Trans., vol. 7, pp. 237-239, 1933. King, P. B., Geology of the Glass Mountains, part 1; Texas Univ. Bull. 3038, p. 27, 1930.
King, P. B., op. cit. (Texas Univ. Bull. 3038), p. 20.
Blackwelder, Eliot, Desert plains: Jour. Geology, vol. 39, p. 139, 1931.
King, P. B., op. cit., fig. 9 C.
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of the rock-cut type described by Johnson. The outer edges of the fan in the Hood Spring quadrangle are dissected by tributaries of Maravillas Creek, suggesting that the building of this fan antedated the dissection of the rock floors described below.
Deposition of alluvium on the surface of the rock floors indicates that the balance of conditions which permitted their erosion has been modified, either by a change in the base level of the master streams or by a decrease in the amount of rainfall. The widespread occurrence of the deposit in all drainage basins suggests that it was caused by a change in climate toward aridity.
Dissection of the rock floors and pediments and of their alluvial cover is of several sorts. At many places along the bases of the mountains the drainage channels "cut deep, steep-walled trenches with few lateral tributaries in the heterogeneous materials of the debris fans, and lower down spread out in broad, almost imperceptible channels in the * * * materials beyond the foot of the debris slopes." Similar features have been described by