Rocks exposed in the Cathedral Mountain quadrangle in the southeastern DavisMountains of Trans-Pecos Texas are assigned to the Word formation and to Capitan limestone in the Permian Guadalupe series; to the Maxon sandstone, Edwards limestone, Georgetown limestone, and Grayson (Del Rio) marl in the Cretaceous Comanche series; to the Buda limestone and Boquillas limestone in the Cretaceous Gulf series; and to the Buck Hill volcanic series in the Tertiary. Volcanic rocks up to 4600 feet thick cover most of the quadrangle.

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.