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.