South of the Pecos Valley section, the Pecos River continues its journey
to the Rio Grande in a steep-walled canyon cut 400 to 500 feet below the
level of a plateau surface of Cretaceous limestone from which little has
been stripped except a thin Tertiary cover of Ogallala Formation (fig.
30). To the east, the plateau has been similarly incised by the Devils
River and the West Nueces and Nueces Rivers. East of the Nueces to the
escarpment formed by the Balcones fault zone, the southern part of the
Edwards Plateau has been intricately dissected by the Frio, Sabinal,
Medina, Guadalupe, and Pedernales Rivers and their tributary systems.
San Antonio and Austin, Tex., are located on the Coastal Plain at the
edge of the Balcones fault zone.
[Illustration: _Figure 30.—Rio Grande and the flat-lying limestone
layers of the Edwards Plateau downstream from the mouth of the Pecos
River. Mexico on the left side of picture. Photograph by V. L. Freeman,
U. S. Geological Survey._]
The Pecos River, and to a lesser extent the Devils and Nueces Rivers,
particularly in their lower courses, have entrenched themselves deeply
in the plateau in remarkable meandering courses of a type that is
usually found only in broad, low-lying floodplains. These stream courses
reflect the stream environment prior to regional uplift.
Sinkholes pit the relatively undissected limestone plateau surface in
the northeast part of the Edwards Plateau, and some underground solution
cavities in the limestone are well-known caves, such as the Caverns of
Sonora, southwest of Sonora, Tex.
Oil and gas fields are widely developed in the northern part of the
Edwards Plateau, but only cattle ranches are found in the bare southern
part.
Ancient oceans deposited the limestones that now cap the Edwards
Plateau; streams planed off the surface of the flat-lying limestone
layers and entrenched themselves in steep-walled valleys; and ground
water dissolved the limestone and created the solution cavities that are
the caves and sinks of the Edwards Plateau. Water has created this
landscape.