Sixteen depositional sequences, commonly called cyclothems, each composed mostly of limestone(retrogradational/transgressive) and siliciclastic (progradational/regressive) subsequences, or systems tracts, record the paleogeography during Late Pennsylvanian (Virgilian Epoch) and Early Permian (Wol

Geologic map that depicts the surface geology of Shackelford, Stephens, Palo Pinto, Callahan, and Eastland Counties and parts of Jones, Taylor, Runnels, Coleman, Brown, Comanche, Erath, Parker, and Hood Counties, Texas.

The Eastern Shelf was a constructional platform developed on the margin of the sediment-starved Midland Basin during Late Pennsylvanian and Early Permian time. A mixed terrigenous-carbonate sedimentary province characterized the shelf during most of its history.

Virgil and lower Wolfcamp rocks on the Eastern Shelf in North-central Texas are composed of several intergradational depositional systems comprising 1,200 to 1,500 feet of off-lapping, predominantly terrigenous sediments.

Upper Pensylvanian and lower Permian rocks of the Eastern Shelf in North-central Texas are composed of 10 to 15 repetitive sequences including open shelf, deltaic, fluvial, and interdeltaic depositional systems.

Lower Cretaceous sands have long been important aquifers in Central, North-Central, and North Texas. In recent years these sands also have been sources of high-silica industrial or specialty-purpose sand.

Based on need for convenient, small-scale cartographic units, the basal Cretaceous rocks in Texas from Red River to Burnet County and on the Callahan Divide are herein divided into three distinctive lithologic sequences.

These are 21 Texas county maps made in cooperation with the American Association of Petroleum Geologists and Society of Economic Paleontologists and Mineralogists, issued between 1929 and 1932 and revised in 1937.