Geological Survey Bulletin 845 Guidebook of the Western United States: Part F. Southern Pacific Lines

SHEET No. 11 (click in image for an enlargement in a new window)

The train passes about 2 miles north of Uvalde, with which the station is connected by a paved boulevard. Uvalde is the name of a Spanish officer who with 20 soldiers defeated many times that number of Comanches. The region is mostly a smooth alluvial plain in which rise many small ridges and knobs of Cretaceous strata or the igneous masses that penetrate them. There are also higher terraces of considerable extent capped by gravel, which mark an earlier stage of the topographic development of the region. Not far north is the edge of the Edwards Plateau,⁵⁷ a highland consisting of a wide area of the hard Georgetown and Edwards limestones lying nearly horizontal and deeply trenched by many rivers and creeks.

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⁵⁷The Edwards Plateau constitutes a large part of central Texas and near Big Spring is overlapped by the High Plains without much topographic break. The late Tertiary deposits of the High Plains were spread on a plain of the limestones of the Edwards Plateau but did not cover the portion south of latitude 31°. The Edwards Plateau extends West to the Marathon uplift and east to the vicinity of San Antonio and Austin. In larger part it is capped by 100 feet or more of Georgetown limestone (Washita group), the Edwards limestone appearing in the slopes of the valleys and on some of the areas from which the younger limestone has been removed.

The sharp rise at the eastern and southern edge of the Edwards Plateau, usually referred to as the Balcones escarpment, owes its prominence to the presence of the hard limestone, in places uplifted by faults but generally rising rapidly on the dip. Parts of the plateau are so deeply trenched as to present many isolated mesas and buttes. (Mesa (may'sah), a Spanish word meaning table, is applied to a flat-topped butte or hill; one that has a sloping tabular surface is called a cuesta (kways'tah).)

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Not far north of Uvalde and at intervals to the east and southeast are exposures of Eagle Ford limestone, Buda limestone, and the yellow Del Rio clay. Along or not far from the railroad to the south are small outcrops of Austin chalk, and from 10 to 20 miles southwest and south are exposures of sandstone and clay of the Indio formation (Tertiary), in places underlain by Midway formation. Some of the general relations are shown in the cross section on sheet 11. The stratigraphic succession is given in the following table:

Formations of central Texas
[Vaughan and others]

DIAGRAMMATIC SECTION ACROSS UVALDE COUNTY ABOUT 8 MILES WEST OF UVALDE (SAYRE). Quaternary gravels and sands not shown.

There is considerable agriculture about Uvalde, but the climate approaches the semiarid, with an annual rainfall of approximately 25 inches. Although there is usually a short rainy season in the spring or early summer and another in the autumn, most of the streams are dry for the greater part of the year. Extensive pastures sustain many cattle, sheep, and goats. In places there is an extensive growth of "prickly pear," or nopal (no-pahl'), which is used rather extensively for forage after the thorns are singed off. A large amount of honey is produced in this region, aided greatly by the presence of various plants such as mesquite and huajillo (wah-hee'yo), which yield much nectar for the bees. There is a notable spring on the Leona River a mile below Uvalde, which is locally estimated to furnish about 7,000,000 gallons a day, but the volume varies somewhat with the seasons. The water is used for irrigation.

In the region between Uvalde and Del Rio portions of the Rio Grande Plain, which is a western extension of the Gulf Coastal Plain, reach the Southern Pacific line in places notably about Spofford and westward. The lower lands are mostly level and the valleys are shallow. Smooth surfaces prevail about Spofford and westward to Del Rio. The Edwards Plateau lies some distance north, beyond hills and sloping ridges of Austin chalk and Eagle Ford and other limestones, and many buttes or knobs of hard, igneous rocks project prominently above the general surface.

From Uvalde Station past Hacienda siding (ah-see-ane'da), nearly to the Nueces River, the railroad is on a smooth plain of alluvium. There is a notable exposure of the unconformable contact of the Eagle Ford slabby limestone on the massive Buda limestone on the bank of the Nueces River 4 miles northwest of Hacienda siding, as shown in Plate 10, B. The Nueces River is in a shallow valley also floored by alluvium, with rocky banks at intervals. In dry weather it carries little water, but farther south it is a large watercourse, entering the Gulf of Mexico at Corpus Christi. In the days when Texas was a part of the Republic of Mexico, as a department of Coahuila, the Nueces River was its southern boundary, between it and Tamaulipas and Coahuila, both of which straddled the Rio Grande; on the west Texas adjoined the States of Chihuahua (che-wah'wa) and Nuevo Mejico as far as the Red River. The available geographic data of those times were so few, however, that the contemporary maps were greatly distorted as to locations and distances. Under the Republic the wide area between the Nueces and the Rio Grande, long in dispute between Texas and Mexico, was a no man's land, roamed over by Comanche and other Indians and by many outlaws.

PLATE 10. A (top), QUARRY IN POROUS ANACACHO LIMESTONE IMPREGNATED WITH ASPHALT, SOUTHEAST OF CLINE, TEX. B (bottom), EAGLE FORD SLABBY LIMESTONE LYING UNCONFORMABLY ON MASSIVE BUD LIMESTONE ON BANK OF NUECES RIVER, 4 MILES NORTHWEST OF HACIENDA SIDING, TEX. (Stephenson.)

West of the Nueces River the railroad ascends into a region of low buttes and ridges of Austin chalk with scattered small outcrops. Trees are rare. Lewis Hill to the north and Obi Hill to the south are igneous, intrusive masses (basalt), and larger bodies of these rocks constitute buttes farther south. The chalk is covered by loam and sand (alluvium) near Cline and for some distance west, but it is visible in adjacent slopes. Eight miles southeast of Cline are extensive quarries in the Anacacho limestone, which there carries a large proportion of asphalt. (See pl. 10, A.) This material is used for paving, and asphalt melted out of the rock is used for paint and other purposes. It is brought to Cline by a branch railroad.

Southwest of Cline rises the prominent Anacacho Mountain, which is formed by the hard Anacacho limestone dipping south, as shown in Figure 7. In its eastern part here, as in the region farther east, the Anacacho limestone lies on Austin Chalk, but in its western part a sheet of Upson clay intervenes between them. North of the mountain are long slopes of Austin chalk succeeded by slabby buff Eagle Ford limestone, which thickens considerably to the west. About 12 miles north of Odlaw siding is Turkey Mountain, a plug of intrusive igneous rock in part capping a mound of Eagle Ford, Buda, and Del Rio beds. The railroad is on Austin chalk from Odlaw west for several miles. At Chacon Creek (cha-cone'), southwest of Anacacho siding, Anacacho Mountain ends through a rapid thinning of the limestone or through change of its upper member into softer rocks of the San Miguel formation and also of its lower member into Upson clay.

FIGURE 7.—Sketch section from Turkey Mountain south through Anacacho Mountain, Tex.

From Spofford a branch of the Southern Pacific runs to Eagle Pass (old Fort Duncan), an important town on the Rio Grande, 34 miles south. The railroad here is on the broad Rio Grande Plain, developed on the upper part of the Austin chalk, overlain by a thick deposit of sand and gravel with much caliche, a covering that extends to and beyond Amanda siding. The chalk is revealed on Pinto Creek near Pinto siding, in a quarry 3 miles beyond Pinto, and also in the arroyos to the south, which cut deeply to reach the Rio Grande. Ten miles north of Spofford, just south of Brackettville, on State highway 3, is Fort Clark, which was established in 1852 to protect travelers on the old trail. The fort was named from Maj. John B. Clark, United States Army. In it have been stationed General Gorgas when he was a second lieutenant, General Bullis, General Shafter, and General Pershing. In 1931 there were 386 soldiers and 44 officers there. The fort is near the Las Moras Springs (Spanish, moras, blackberries), which ordinarily have an average flow of about 22,000,000 gallons a day. (Meinzer.) Still farther north is Las Moras Mountain, due to an intrusion of igneous rock which has been forced up in molten condition through the Cretaceous strata. Other conspicuous igneous masses near by are Elm Mountain, 10 miles northwest of Pavo siding, and Pinto Mountain, 10 miles northwest of Brackettville.

In the region between Spofford and Del Rio the formations traversed are as follows:

Although the general regional dip of the strata is to the east and southeast, they are flexed by an anticline with about 150 feet of uplift the axis of which trends west by north. It extends from Sycamore Creek, 2 miles south of Amanda siding, to the San Felipe Valley, a few miles north of Del Rio, the axis passing about half a mile south of Johnstone siding.⁵⁸

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⁵⁸A 2,800-foot boring 3 miles east of Amanda siding is thought to have reached the base of the Edwards limestone at a depth of about 2,000 feet. A boring 6 miles north of Del Rio started in Del Rio clay and penetrated shale and gray limestone of supposed Pennsylvanian age below 2,175 feet. This indicates a thickness of about 2,235 feet for the Comanche series, which thickens to the west from the Uvalde-Brackettville region. Water-bearing beds were found in both holes (Stephenson).

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