The surface rocks of the Edwards Plateau are of Lower Cretaceous age, and comprise the Trinity, Fredericksburg, and Washita divisions.

The Trinity division was deposited by a sea encroaching upon an upland plain, traversed by several broad south- and east-draining river valleys, and broken by ridges formed on outcropping resistant formations. Trinity deposits filled the lower areas of the floor of deposition, so that the succeeding Fredericksburg deposits were laid upon an almost level surface except for the tops of some ridges which were never submerged in Trinity time.

The regional structure, as contoured on the base of the Fredericksburg, shows a central plateau high zone with gentle coastward dip comprising most of the Edwards Plateau proper, and a peripheral belt of steepened dip on the south and east. Comparison of regional contour maps of the pre-Cretaceous surface, Trinity thickness, and the base of the Fredericksburg, shows clearly that the regional structure of the Fredericksburg is controlled by the topography of the pre-Cretaceous surface, and down-warping of the south and east margins into the Balcones fault zone.