Cain City quadrangle is south of the Llano region and is in the marginal portion of the Edwards Plateau where much of the plateau surface has been destroyed by erosion. Northeastward-trending lobes and outliers of the Edwards Plateau are present in the southern part. The rest of the quadrangle is in the gently undulating broad Pedernales River basin.

The Ellenburger of Texas was first defined as a marine limestone formation of Cambrian and Ordovician age (Paige, 1912), but recently it has been subdivided into several formations and the term Ellenburger given group status (Cloud and Barnes, 1948). The group forms an important unit in the geology of Texas, its known extent in both surface and subsurface covering approximately one-half of the State (fig. 1). Consequently the Ellenburger has received the attention of many geologists over a period of more than forty years.

Palo Alto Creek quadrangle is south of the Llano region and is in the marginal portion of the Edwards Plateau where much of the plateau surface has been destroyed by erosion. Along the northern edge of the quadrangle, a portion of a finger of the plateau trends eastward, and in the west-central part, another finger of the plateau frays out into a number of isolated outliers. The rest of the quadrangle is a portion of the gently undulating broad Pedernales River basin.