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.

Dry Branch quadrangle is in the Edwards Plateau province southwest of the Llano region.

The Hood Spring quadrangle, in the central part of Brewster County, Texas, contains a segment of the southeast rim of the Marathon basin.