Harper quadrangle is in the Edwards Plateau province south of the Llano region. The plateau surface within the quadrangle is dissected by the Pedernales River and its tributaries in the southern part, and by Threadgill and Edwards Creeks in the northern part.

The Hood Spring quadrangle, in the central part of Brewster County, Texas, contains a segment of the southeast rim of the Marathon basin. It includes a part of the complexly folded and faulted Paleozoic rocks that occur in the center of the Marathon basin and also includes Cretaceous rocks exposed in the Maravillas scarp. This scarp has three stratigraphically separate cuestas that are formed by southeastward gently dipping beds. The southwestern corner of the quadrangle contains a faulted and folded segment of the Santiago Mountain range.

Dry Branch quadrangle is in the Edwards Plateau province southwest of the Llano region. It is near the margin of the plateau, and Johnson Creek and some of its tributaries, such as Dry Branch, Falls Branch, Fessington Branch, and Welch Branch, have cut into the plateau in the southern part of the quadrangle.

The fossils described in this paper substantiate the Permian age of the Rustler formation. The Rustler fauna is largely molluscan and has a Whitehorse aspect. Four species of the Rustler fauna are similar to species from Russian and Indian faunas, suggesting possible correlations with Upper Permian rocks. Environments have had more influence on the fauna than has geologic age, giving the Rustler fauna a striking resemblance to faunas of older Permian rocks which occupied similar biotopes. The Rustler fauna is large and varied, containing 35 species of invertebrates.

The 15-minute Agua Fria quadrangle in southwestern Brewster County, Texas, is arid, sparsely vegetated, and includes diverse topographic features that result chiefly from complex structure and variation in rock resistance to erosion. The mountainous and more complicated southern part of the area has suffered much deformation by igneous intrusions and faulting. The Comanche series is represented by the Devils River limestone, Grayson marl, and Buda limestone.