The domed rocks of the Central Texas Uplift form a topography different
from that of the Black Hills. Erosion of a broad, uplifted dome here has
exposed a core of old granites, gneisses, and schists, as in the Black
Hills, but in the Central Texas Uplift, erosion has produced a
topographic basin, rather than high peaks and spires, on the old rocks
of the central area. A low plateau surface dissected into rounded ridges
and narrow valleys slopes gently eastward from the edge of the central
area to an escarpment at the Balcones fault zone, which determines the
eastern edge of the Great Plains here. Northwest of the central basin
the Colorado River flows in a broad lowland about 100 miles long, but
the northern edge of the uplift, forming a divide between the Brazos and
the Colorado Rivers, is a series of mesas formed of more resistant
sandstone and limestone.
The cutting action of streams, modified or controlled in part by
differences in hardness of the rock layers, has been responsible for the
landforms of the Central Texas Uplift. Weathering of the old core rocks
has softened them sufficiently to permit deeper erosion of the central
area, and solution of limestone by ground water has formed such features
as Longhorn Caverns, 11 miles southwest of Burnet, Tex.