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The Search for Devil's Eye
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The Geology of the Area

Some Geological Notes

The geological map of Texas depicts a patchwork of rocks representing a vast range of original environments. The massive limestone of Edward’s plateau is a remnant from a former sea covering the area between 130 and 65 million years ago (the Cretaceous time period). The distinctive replications of shale, sandstone and limestone of north central Texas bear the stamp of both sea and deltas when the main axis of drainage was to the northwest during the Pennsylvanian (around 300 million years ago). Plant remains attest to swamp conditions. The rocks of that time and earlier, which are now buried beneath later sediments, hold the ingredients for vital energy resources such as coal, oil and gas. In the Gulf coastal zone, stretching from the great arcuate Balcones fault system to the modern coastline of today, the rocks again tell the story of seas, deltas, and rivers from about 65 million years ago to the present day. We can deduce these environments because geologists have carefully studied the characteristics of the rocks formed from sediments deposited during those times, and gleaned vital clues from life forms preserved as fossils. Without this knowledge the huge resource potential of Texas would be not have been realized and developed.

In 1891, Angelo Heilprin published The Eocene Mollusca of the State of Texas, based in part on the collections of Conrad and Gabb, but also drawing upon material collected by E.T. Dumble and R.A.F. Penrose during a reconnaissance of the Colorado River in 1889. Of the localities Heilprin lists, four (Smithville, Devil’s Eye, Bombshell Bluff and ‘Camp Disaster’) are located at points along the Colorado River in Bastrop County. These are the type localities for many Claiborne-age mollusks. The Claiborne group encompasses several distinct rock formations that were formed during the geologic epoch called the Eocene, about 55 million to 34 million years before the present. The Eocene is one of five epochs that comprise the Tertiary time period (65-1.8 mya).

Additional mollusks were reported by Harris and Aldrich from the same localities, and Price and Palmer described a new fauna from the mouth of Gazley Creek, near Smithville. The Dumble collections at the Texas Memorial Museum include specimens from all these Colorado River sites, as well as: Alum Creek Bluff, David Bottom, and Shipp’s Ford. Duessen reported localities along the river with distances measured from Burleson’s Ferry, the position of which itself is uncertain. All of these sites lie along the Colorado River within (or almost within) the boundaries of Bastrop County.

Many of these names are no longer in use, and their locations can only be determined from old maps, old field notes and descriptions. Conflicting information is reported in the literature regarding certain sites, with much confusion arising from an incomplete modern knowledge of the geology and history along the Colorado River. The famous Smithville site is still known, although it is now largely inaccessible for collection. The Gazley Creek exposures, though poor, remain identifiable and readily accessible by boat. The fossil-bearing outcrops at Shipp’s Ford were reported in some detail by H.B. Stenzel. Of all the localities, the position of the one called “Devil’s Eye” is the most uncertain.