37

Near India ranch, about 26 miles, in a straight line, below Eagle Pass, the lithologic character of the rocks changes to a coarsely crystalline sandstone of a yellowish or brownish color. The best exposures of this sandstone are seen near Moro ranch well, at Chupadero ranch, and 124 miles south of Uvalde on the road to Batesville. The grains are small quartz crystals, which are often cemented together by iron oxide.

In the arroyo immediately east of India ranch is a clay containing calcareous concretions. This clay lies below the sandstone to be next described.

From India ranch to San Ambrosia Creek the road to Laredo passes over numerous exposures of ripple-marked brown sandstone, which disintegrates rapidly and forms very poor roads. Between San Ambrosia and San Lorenzo creeks, as there is no covering of more recent deposits, loose sands, derived front the disintegration of the sandstone, constitute the surface. The best exposures of this sandstone are at Chupadero ranch. Here it contains numerous fantastically shaped concretions, and peculiar sandstone pillars are formed by erosion and weathering. When unweathered the sandstone is gray, but upon disintegration it forms coarse, loose, brown crystalline sands. Its thickness, so far as ascertained, is 150 feet. (See fig. 5, p. 53.) It is underlain by clays, and is a well-defined lithologic horizon, apparently what Owen designated the Carrizo sands.

At Chupadero ranch a well sunk into this sandstone yields a permanent supply of water, which is, however, never more than a few feet in depth.

Between Chupadero Creek and San Ambrosia Creek flint gravel forms the capping of the divide.

West of San Ambrosia Creek, going down to the Rio Grande from the divide, dirty, dark-brownish or yellow clays are seen. These pass beneath the sandstone exposed around Chupadero ranch. The bluff on Rio Grande 1 mile above the mouth of San Ambrosia Creek is 40 to 50 feet high and is capped by sandstone; the base is of laminated sands and clay. Three and one-half miles above the mouth of this creek the banks of the river are composed of sandstone and thinly bedded lignitic sands. Specimens of Venericardia plainicosta Lain. were found in a piece of sandstone from this locality, and a Specimen of Ostrea crenulimarginata Gabb was picked up nearby. These would indicate that the horizon of the beds here exposed is Midwayan Eocene.

First Rept. of Progress of Texas Geol. Survey, 1889, pp. 70-73.