Proterozoic exposures in the Llano Uplift of Central Texas and in the Van Horn area and other smaller areas of western Texas represent less than one percent of the area underlain by Precambrian basement.

The tectonically quiescent Proterozoic (Texas craton) and Paleozoic (Gulf Coast) basement of Texas shows large contrasts in hydrogeological behavior, from being a fresh-water aquifer, where it crops out, to forming the base of several large sedimentary basins at depths >7 km (>23,000 ft).