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

SHEET No. 10 (click in image for an enlargement in a new window)

From Pearson siding northwestward for 10 miles the railroad is mostly in a valley in clays with limy layers, of the basal group (Midway) of Tertiary age. The continuity of the strata is broken by several faults, mostly trending east-northeast, which bring up the underlying Escondido formation obscurely exposed at intervals.⁵³

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⁵³In the region between San Antonio and Del Rio, although some of the Upper Cretaceous formations continue unchanged in character and relations, others show decided change. One of the most notable features is the development of the Anacacho limestone, 300 feet or more thick, replacing the Taylor shale, and merging westward into the Upson clay and the San Miguel formation. The Navarro shale also merges laterally to the west into the Escondido formation, which may be 700 feet thick in the Uvalde region. These changes are represented diagrammatically in Figure 6, which, however, necessarily has a greatly exaggerated vertical scale. The Midway, the lowest formation of the Tertiary system in this region, lies unconformably on the Escondido formation (western extension of the Navarro shale) with but little discordance of dip.

FIGURE 6.—Diagram showing changes in stratigraphy from west to east in south-central Texas, Spofford to San Antonio. (From data by L. W. Stephenson)

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Just beyond Dunlay the railroad crosses a low ridge consisting of shales with hard sandstone layers of the Escondido formation. On both sides of the pass through which the railroad goes are plateau remnants of moderate height in which the Escondido beds are overlain by the Midway group, which is capped by old terrace gravel deposited when the drainage system of the region was about 200 feet less deep than it is at present, a feature referred to on previous pages. The gravel and sand cap of the plateau, in large part filled with caliche, is well exposed on State highway 3 a short distance south of the railway. West of the divide the railroad descends into a wide alluvial plain comprising the valleys of Quihi, Hondo, Seco, and other creeks and mostly floored with gravel and sand which hide the underlying clay of the Escondido formation. This formation, however, appears in a few hills that rise out of the plain and extend along part of its southern margin. A few miles north of the railroad the underlying formations appear in succession, for there is a general rise of the strata to the north and considerable faulting, in part with uplift on the north side. In about 15 miles the Georgetown and Edwards limestones come to the surface, constituting the Edwards Plateau, which is in view far to the north as the railroad crosses the divide northwest of Dunlay. A cross section of this region is shown on sheet 10, opposite page 69.

DIAGRAMMATIC SECTION ACROSS MEDINA COUNTY NEAR LONGITUDE 99° (SAYRE). Quaternary gravels and sands not shown.

At Hondo (own'do; Spanish, deep) the railroad is on an alluvial plain out of which rise several low ridges of Escondido clay and sandstone. There are instructive exposures of the various formations in the shallow valleys of Hondo and Verde Creeks.⁵⁴

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⁵⁴On Verde Creek, 4 miles northeast of Hondo, ledges of limestone at the base of the Tertiary are exposed, with characteristic fossils, on a block dropped by a fault that crosses the region from northeast to southwest a short distance to the north. In slopes just north of this fault are exposures of Austin chalk, beyond which to the north are outcrops of the usual succession of Eagle Ford, Buda, Del Rio, Georgetown, and Edwards limestones, coming up in order on the moderate south dip, as shown in the cross section on sheet 10. (See table on p. 75.) Another outcrop of Midway limestone, similarly down-faulted, is reported near Quihi Creek, 9 miles northeast of Hondo. At King's water hole on Hondo Creek, 3 miles north by west of Hondo, are exposures of Anacacho limestone and overlying shale, as shown in Plate 11, A, and farther up the creek Austin chalk, Eagle Ford limestone, and Del Rio clay crop out. The Del Rio in this region is yellow clay carrying many Exogyra arietina, named from its shape, like a ram's horn (Latin aries, ram). The beds are considerably faulted; one fault crossing Hondo Creek about 7 miles north of Hondo between Buda limestones and Eagle Ford beds is shown in Plate 9, B.

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PLATE 9. A (top), ARMADILLOS, ABUNDANT IN CENTRAL TEXAS. B (center), FAULT IN BUDA LIMESTONE ON HONDO CREEK, 7 MILES NORTH-NORTHWEST OF HONDO, TEX. Eagle Ford beds which abutted against the fault are soft and therefore have been removed by erosion. (Stephenson.) C (bottom), COLUMNAR STRUCTURE IN BASALT IN CHATFIELD HILL, WEST OF KNIPPA, TEX. (Stephenson.)

An old Comanche Indian village was situated near Hondo, and the flint for arrowheads was obtained from pebbles, which are abundant in the neighborhood, brought by streams from the plateau of Edwards limestone to the north.

In the Hondo region and westward to Uvalde most of the land is under cultivation with varied crops, including considerable corn and cotton, and there are many cattle, sheep, and goats. Poultry raising and dairying are important industries.

D'Hanis (named after an old French settler but settled by south Germans) is on the alluvial plain, but hills of the older formations rise to the south and north.⁵⁵ To the west and south are high banks of clay of the Escondido formation, which is worked for brick and tile. Two principal terrace levels will be noted in this region, a lower one of alluvium and an upper one capped by sand and gravel (late Tertiary?). Old D'Hanis, a mile east of the station, south of the tracks, was on the early stage route from San Antonio to El Paso. Near by is a large gravel pit showing the thickness of the alluvial filling. To the south is a high hill with a cross on top, which was placed there originally as a landmark and is at present a shrine for the Mexican people of the region.

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⁵⁵Seco Creek, about 3 miles north of D'Hanis, has moderately high banks of Anacacho limestone, and a short distance farther north the Austin chalk is well exposed. The beds are considerably faulted. Farther north are foothills of Austin chalk, a zone of Buda and Del Rio outcrop, and long slopes of Georgetown limestone.

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A mile west of D'Hanis the railroad crosses Seco Creek (say'co) which drains a part of the Edwards Plateau. On its banks 2 miles to the north are the ruins of Fort Lincoln, 1849-1852, once garrisoned with 141 men to keep the Comanche Indians and outlaws in check.

Beyond Seco Creek there is a long ascent on a slope of clay (Escondido) to the summit of the wide, high plateau which separates the valleys of Seco Creek and the Sabinal River (sah-bee-nahl'). This plateau, nearly 200 feet high, is heavily capped by sand with coarse gravel and boulders, in large part cemented by caliche. There are many small exposures of this capping, notably one in a gravel pit north of the tracks just east of Seco siding. It extends north to the foot of the rise to the Edwards Plateau, about 5 miles north, and to the west it slopes down somewhat and terminates at the edge of a steep down slope 11-1/2 miles west of D'Hanis.

Sabinal (sah-bee-nahl') is on a sand and gravel plain that borders and extends west from the Sabinal River, which is crossed by the railroad a mile west of the station. On this stream there is an almost continuous succession of exposures of the strata of Upper Cretaceous age, beginning with low cliffs of Anacacho limestone south of the railroad.⁵⁶ The name of the place is derived from the word "sabina (sas-bee'na)," the Spanish name for juniper, misapplied by the Mexicans to the cypress tree, of which there is a small group on the Sabinal River a mile west of the station.

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⁵⁶The Anacacho limestone crops out at intervals to the north for 4 miles, with a short interruption caused by a low arch and fault that reveal the underlying Austin chalk. This chalk also appears in the river banks and on adjoining slopes 4 miles north of Sabinal and above. At a point 8 miles north of Sabinal faults bring up Eagle Ford, Buda, and Del Rio beds, and finally the Georgetown and underlying limestones come to the surface.

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The Blanco River, which is crossed 4 miles beyond the Sabinal River, carries but little water except in times of freshet. Just west of it are exposures of alluvial sand and gravel containing much caliche, and the ridge near Yucca is one of the numerous remnants of an old gravel-capped high terrace in this general region.

A mile southwest of Knippa is a prominent knoll known as Chatfield Hill, caused by a mass of hard diabase which has been intruded in the Cretaceous strata. It is similar to many other igneous masses that are more or less prominent topographic features in the surrounding region and for a long distance west. These igneous rocks have come in a molten condition through cracks from a deep-seated source and either formed irregular conical masses or spread out in "sills" or layers between the sedimentary strata. They lift the overlying beds and in many places flex or break them irregularly. The mass near Knippa is extensively quarried for road metal just south of the railroad. A notable feature seen especially in the upper part of the quarries is the columnar structure of the rock, such as is developed in many intruded igneous masses (notably in the Palisades of the Hudson opposite New York City). This structure is developed by shrinkage in cooling, both in intrusive masses and in many lava flows. A portion of one of the quarries is shown in Plate 9, C. Just north of Chatfield Hill the railroad crosses two main branches of the Frio River (free'o), which comes from the Edwards Plateau. The size of the bridges at this place indicates that provision is made for the passage of a great body of water in time of freshet. A short distance farther west are conspicuous limestone hills a mile or so north of the railroad which consist of an upfaulted block of Georgetown limestone. In the adjoining foothills are extensive outcrops of Eagle Ford and Buda limestones and Del Rio clay, the Eagle Ford beds being exposed for some distance along the railroad near Ange siding. Big Mountain, a mile northwest of Ange, is made up of Georgetown limestone. The Cretaceous strata in this vicinity are penetrated by igneous masses, some of which, on account of their hardness, crop out prominently. (Turn to sheet 11.)