The most spectacular example of solution of limestone by ground water is Carlsbad Caverns, N. Mex., one of the most beautiful caves in the world. This celebrated solution cavity is preserved in a national park.
The Pecos River along much of its present course flows in a vertical-walled canyon with limestone rims. The Canadian River, flowing eastward from the Sangre de Cristo Mountains, has cut a deep canyon along the northern part of the Pecos Valley section. The sharp rims of the Dakota Sandstone at the Canadian escarpment, north of the Canadian River, form the northern boundary of the Pecos Valley section.
The sharp, northeast-trending broken flexure called the Border Hills that is crossed by U. S. Highway 70-380 about 20 miles west of Roswell is a unique landform of the Pecos Valley. This markedly linear upfolded (anticlinal) structure forms a ridge more than 30 miles long and about 200 feet high.
As in the Colorado Plateau, windblown sand and silt mantle the landscape in many places, but the greatest accumulations are along the base of the Mescalero escarpment at the northeast and southeast corners of the Pecos Valley section.
East of the Pecos River, in the southeast part of the Pecos Valley, the underlying rocks have yielded much oil and potash. Oil fields are common east of Artesia and Carlsbad, and potash is mined east of Carlsbad.
The Pecos and Canadian Rivers and their tributaries have created the general outline of the landscape of the Pecos Valley, but underground solution of limestone by ground water and the collapse of roofs of these cavities have contributed much detail to the surface that characterizes the Pecos Valley today.
EDWARDS PLATEAU
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.
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.