DESCRIPTION OF THE AUSTIN QUADRANGLE.
By Robert T. Hill and T. Wayland Vaughan.GEOGRAPHY.
Geographic position and relations.—The Austin quadrangle embraces one-quarter of a square degree of the earth's surface, and contains 1029.84 square miles. It is bounded by parallels 30° and 30° 30' N., and meridians 97° 30' and 98° W. The adjacent quadrangles that have been topographically mapped are the Bastrop on the east, Georgetown on the north, and Blanco on the west. To the northeast is the Taylor quadrangle, to the northwest the Burnet, and to the southeast the Flatonia. The Austin quadrangle lies southeast of the center of the State, at the interior border
TOPOGRAPHY.
GENERAL FEATURES.
In general the topography of the quadrangle is varied and picturesque, consisting of a diversified landscape of rugged hills, rolling plains, and level areas, broken by frequent streams and presenting a pleasing alternation of timber and prairie. The highest altitudes of the quadrangle are found along the western margin, the greatest upland altitude being 1200 feet, at the extreme northwest corner, and the least about 600 feet, along the eastern margin. The valley depressions as measured along the Colorado vary from 600 feet at the northwestern edge of the quadrangle to 375 feet where the Colorado leaves the eastern border.
RELATION BETWEEN FORMATION AND RELIEF.
Nowhere is there a more intimate relation between topography and stratigraphy than in the
TWO TYPES OF TOPOGRAPHY.
Notwithstanding the partial destruction of original surfaces by erosion, causing the present diversified relief, it is evident that the general configuration of the area is that of a greatly dissected plain, which, leading from the Cordilleran region to the sea, has been termed the Regional Coastward Slope. This broader feature is sharply divisible, within the area of the quadrangle, into two types of country — a higher district in the western third which is a part of the physiographic province of the Texas region known as the Edwards Plateau, and a lower district to the east which \, physiographically belongs to the interior margin of the great Atlantic Coastal Plain. (For a detailed description of the physiography of the Texas region see Topographic folio No. 3, U. S. Geological Survey, 1900.)
EDWARDS PLATEAU.
Balcones scarp.—A noteworthy topographic feature of the quadrangle is a high escarpment along the border between the Edwards Plateau and the Rio Grande Plain. It runs northeast and southwest from the vicinity of McNeil to near Driftwood post-office (Blanco quadrangle), passing by the eastern foot of Mount Bonnel and Oatmanville, and separates the two major provinces mentioned. This escarpment is called the Balcones, and in its extension south-westward to the Rio Grande is a structural feature known as a fault scarp. It rises from 100 to 300 feet above the lower country of the Coastal Plain, and properly belongs to the Edwards Plateau. Its front is not a vertical cliff, as one might infer from a too literal interpretation of the word scarp, but rather an indented line of sloping hills leading up from a lower plain to a plateau summit, as seen along the eastern front of the ridge northwest of Austin.
The country west of the Balcones scarp line, which is locally known as "the mountains," consists of bold hills, many rising 500 feet above the drainage valleys, and some of them so symmetrical in contour and stratification that they appear as if they had been turned in a lathe. The summits of the highest of these hills, which are usually flat topped and composed of a single geologic formation— the Edwards limestone — have a nearly constant topographic level. These hills are the remnants of a vast dissected plateau or cut plain, and stretch across Texas from the Brazos to the Rio Grande. In the Austin quadrangle the remnants of this plateau stand at altitudes of from 1100 feet along the western border to about 975 feet at the edge of the escarpment, having a slope of about 20 feet to the mile as measured along a westward line from the Lone Tree triangulation station. In places, such as the area termed the Jollyville Plateau on the map, considerable remnants of the old plateau exist. These are covered with a dense thicket of live-oak and post-oak brush and timber. The slopes of the hills are terraced or benched by the outcrops of the subhorizontal strata. Streams, especially the Colorado, have cut deep valleys into this plateau. (See fig. 2.) These valleys, immediately adjacent to their courses, are bordered by steep bluffs of banded yellow or white limestones of great beauty, crowned above by terraced stratification slopes leading to the summits. The terraces of these hills are often covered with a growth of juniper and Texas laurel (Sophord), while the high summits are usually covered with a dense growth of scrub oak and live oak. That portion of the Edwards Plateau which is north of the Colorado has been termed the Lampasas Cut Plain.
COASTAL PLAIN.
The area of that part of the quadrangle which is situated to the east of the Balcones scarp, although presenting within itself several diverse features, constitutes a portion of the great physiographic feature known as the Atlantic Coastal Plain. This portion of the plain represents its older, higher, and more eroded interior margin, and in general, except along the southeastern margin and western border, is largely a region of undulating upland prairie, mostly of the type which is known in Texas as rolling prairie land and which has been called the Black Prairie. The highest points of this plain rarely rise over 750 feet above the sea. This feature is subdivisible, in the Austin quadrangle, into several distinct northeast-southwest belts of country, which may be enumerated, in the order of their sequence from west to east, as follows: Bear Creek country, Manchaca belt, White Rock country, Taylor Prairie, Littig Prairie, and Lytton Springs country. Besides the above there are exceptional minor features within the area of the Coastal Plain which will be described as alluvial plains and terraces and volcanic hills.
Bear Creek country. —Immediately adjacent to the foot of the Balcones fault and extending approximately eastward toward the International and Great Northern Railway is a narrow plain of brush-covered country which ranges in altitude from 700 to 900 feet. This country corresponds with the outcrop of the geologic formations elsewhere described as the Edwards and Georgetown limestones, shown on the map east of the Balcones fault. Owing to the stony character of the surface, the shallowness of the soil, and the thick growth of brush, the region has been locally termed "hard scrabble." This plain is cut in places by steep creek gorges, while its surface is usually broken by projecting limestone ledges and covered with a thick growth of post oak and live oak. In some places the surface is covered with vast numbers of oxidized residual flints, which are suggestive of alluvial deposits, but which are the residua of the flint-bearing layers of the underlying Edwards limestone. The hard scrabble belt as a whole is a downfallen fault block. While not well adapted to agriculture this portion of the country is utilized for grazing purposes, and contains an occasional small area of tillable land
Manchaca belt. —Immediately east of the Bear Creek belt is another narrow strip of exceptional country, underlain by the Del Rio, Buda, and Eagle Ford formations. (See Historical Geology sheet.) This consists of alternations of wooded hills and prairies. Along its eastern margin are spots of forest, growing upon the Eagle Ford and Buda formations, the former bearing mostly hackberry and the latter live oak, as seen in the Sixth Ward of the city of Austin and along the eastern bank of Shoal Creek. West of these areas are small spots of prairie land, with black soil, underlain by the greenish- yellow subsoils of the Del Rio clay and covered with a thin growth of mesquite bushes. The surfaces of these small areas are also marked by hogwallows. The various features of this belt are complicated, owing to the excessive faulting along their area of occurence.North of the Colorado, the belt narrows to less than half a mile in width, or even is absent in places. A unique feature of the Manchaca belt are the cliffs of Buda limestone seen along the creek valleys.
White rock country. —A broad belt of country extending north and south through the center of the quadrangle, marked by the outcrop of the Austin chalk, may be known as the White Rock country. The formation is especially well exposed in Fiskville, Austin, South Austin, Manchaca Springs, and Mountain City. This belt is a district of gently rolling, hilly land like the English downs, and is characterized by glaring white outcrops of the Austin chalk on the slopes and in the bluffs of the creeks. Graceful clumps of live oak and sometimes groves of juniper are found. The residual soil of this belt, which is usually thin, is black and calcareous.
Taylor Prairie. —Immediately east of the White Rock belt is the main district of the Black Prairie lands of the country underlain by the Taylor and Webberville formations. This is an area of rolling prairie land, consisting of a deep mantle of black waxy soil which is exceedingly fertile. The western portion is called the Taylor Prairie. Manor and Creedmoor are situated on its eastern edge; Pflugerville and Manchaca Springs are near its western edge.
Littig Prairie.— In the extreme eastern part of the quadrangle, to the east of Manor and Creedmoor, and extending to Texas Hill, there is an area marked by a rather sterile, sandy, clay soil and covered by a growth of chaparral, large opuntias, and mesquite. This country, known as the Littig Prairie, is underlain by the Webberville formation, so far as can be determined, thinly veneered by Pleistocene surficial deposits.
Lytton Springs country .—The area covered by this division lies in the southeast corner of the quadrangle and extends from one and one-half miles northwest of Lytton Springs to near Elysium. This area consists of rather low, gently rounded hills having an altitude of from 400 to 650 feet above the sea. The soil is sandy and subject to rapid erosion. The timber consists mostly of post oak and other trees, such as characterize the southern extension of the East Texas timber belt, of which it is a part.
Alluvial plains and terraces. —Extending along the margins of the Colorado and the larger creeks are wide valleys of alluvial bottom land, subject to overflow; above these there is usually a series of more or less connected terraces, now standing above the line of overflow; and still higher, upon some of the divides, there are areas of gravelly soil. All of these deposits are composed of alluvial material which has been brought down by the streams.
The Colorado, being the oldest and most deeply incised stream, has more extensive bottom lands and more numerous higher terraces. The latter are