Geological Survey Bulletin 1271—E Pecos National Monument New Mexico: Its Geologic Setting

The six formations as delineated and mapped on Glorieta Mesa extend over many thousands of square miles. They appear to be coextensive from the limited viewpoint at Pecos National Monument, but beyond that view they are not. Locally, throughout New Mexico and southern Colorado, several, one, or none of these formations may be present.

The Santa Rosa Sandstone was named for exposures near the city of Santa Rosa, N. Mex. The Santa Rosa crops out along the canyons of the Canadian and Pecos Rivers of northeastern and east-central New Mexico. At most places on Glorieta Mesa, the upper rocks of the formation have been eroded. However, the complete unit is present along the east flank of the Sangre de Cristo Mountains and extends at least as far north as Ocate Creek, 50 miles north of Las Vegas where it is covered by lava flows of much younger age.

Bernal Mesa east of the village of Bernal near Starvation Peak is the locality where the Bernal Formation was first named and described. It is a local unit, crops out only on Glorieta Mesa, and caps low mesas a short distance south and southeast of Starvation Peak. It also extends along the east flank of the Sangre de Cristo Mountains, at least as far north as Ocate Creek. To the south, rocks equivalent to this unit are known by another name.

The San Andres Limestone was named for exposures in the San Andres Mountains of south-central New Mexico. Of all the rock units occurring on Glorieta Mesa, the San Andres probably covers the surface over more square miles than any other unit. The San Andres Limestone crops out in large separated areas across southern New Mexico into trans-Pecos Texas. The formation has also been mapped in the Zuni Mountains and the Lucero uplift west of Belen, N. Mex. North of Glorieta Mesa the limestone thins to a point where it can no longer be distinguished as a unit.

The name for the Glorieta Sandstone was derived from exposures on Glorieta Mesa just 2-1/2 miles south of Cerro de Escobas. Because the Glorieta forms the main caprock of Glorieta Mesa, it is beautifully and conspicuously exposed for many miles; and thus the mesa very appropriately gives its name to the formation. The Glorieta Sandstone crops out in large separated areas that cover many thousands of square miles of New Mexico. It extends northward along the east flank of the Sangre de Cristo Mountains at least as far as Ocate Creek. The Glorieta Sandstone has been mapped in the Zuni Mountains of west-central New Mexico, the Lucero uplift west of Belen, and much of central New Mexico.

The Yeso Formation was named from Mesa del Yeso, 12 miles northeast of Socorro, N. Mex. A thick bed of gypsum, or yeso, crops out on the slopes of Mesa del Yeso, and therefore furnishes the name for both the geologic formation and the mesa. From Glorieta Mesa, rocks of the Yeso Formation extend southward and westward and form the surface rocks over large parts of west-central, central, and south-central New Mexico. North from Glorieta Mesa the sandstones and siltstones of the Yeso Formation grade into rocks typical of the Sangre de Cristo Formation and thus are included as part of the Sangre de Cristo Formation in northern New Mexico and southern Colorado.

Although the Sangre de Cristo Mountains occupy a large part of northern New Mexico, the Sangre de Cristo Formation was first named and described not in New Mexico but in Colorado. The name was first used in 1899 for rocks of this formation on the east slopes of the Sangre de Cristo Mountains northwest of the Spanish Peaks—130 miles north of Pecos National Monument. The formation is appropriately named, for its outcrops are confined almost entirely to the Sangre de Cristo Mountains of northeastern New Mexico and south-central Colorado.