The Geologic Story of the Great Plains / A nontechnical description of the origin and evolution of the landscape of the Great Plains

by Donald E. Trimble

PECOS VALLEY

South of the land of volcanic rocks that is the Raton section, the Pecos River has cut a broad valley from the Sangre de Cristo Mountains, in New Mexico, southward to the Rio Grande, and has removed the piedmont cover of Ogallala Formation and cut deeply into the underlying rocks. The Ogallala Formation capping the High Plains to the east forms a rimrock at the top of the sharp Mescalero escarpment, which is the eastern boundary of the Pecos Valley. (See [figure 4].) The western boundary of the Pecos Valley is the eastern base of discontinuous mountain ranges.

The great thickness of Tertiary deposits that formed on the northern Great Plains did not accumulate here, and the Pecos River has cut its valley into the older marine sedimentary rocks. The rocks underlying the surface of much of the Pecos Valley are upper Paleozoic limestones.

The soluble nature of limestone is responsible for some of the most spectacular features of the landscape in the Pecos Valley. For about 10 miles north and 50 miles south of Vaughn, N. Mex., collapsed solution caverns in upper Paleozoic limestones have produced an unusual type of topography called karst. Karst topography is typified by numerous closely spaced sinks or closed depressions, some of which are very deep holes, caused by the collapse of the roof of a cave or solution cavity into the underground void, leaving hills, spines, or hummocks at the top of the intervening walls or ribs separating the depressions.

Although the karst in the vicinity of Vaughn is perhaps the most conspicuous solution phenomenon, sinks and caves are common throughout the Pecos Valley. At Bottomless Lakes State Park east of Roswell, N. Mex., seven lakes occupy large sinkholes caused by the solution of salt and gypsum in underlying rocks.

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.