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.

Geologic map that depicts the surface geology of Lamar, Red River, Bowie, Delta, and Titus Counties and parts of Fannin, Hunt, Camp, Cass, Hopkins, Franklin, and Morris Counties, Texas. The 6-page booklet indicates geologic formations, abbreviations, and ages.

The wedge-shaped Black Gap area in southern Brewsier County, Texas, occupies approximately 4,50 square miles east of the Big Bend National Park. The southwestern boundary of the map area is the castcrn boundary of the Park; the southeastern boundary is the Rio Grande; the north boundary is drawn at lat. 29' 45' N.