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 geologic units of the Llano Uplift are dominated by felsic gneiss (34 percent), schist (26 percent), and granite (39 percent) that formed during the Mesoproterozoic Grenville orogeny. These lithologies likely form a significant component of the contiguous Texas basement under Phanerozoic strata.

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). Little is known about the basement, such as its petrographic distribution and structural features.