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

The sedimentary rock layers of Glorieta Mesa slope gently south from the Sangre de Cristo Mountains. The slope is so gentle at some places that it cannot be discerned with the eye. North from the mesa and Pecos National Monument, the slopes of the sedimentary layers become steeper and steeper toward the central crystalline core of the high mountains.

Along the south end of the Sangre de Cristos, there are no large faults (fractures along which rocks have moved) or steep folds in the rocks, except near the west and east margins of the mountains—near Canoncito and Bernal. Also, there are neither volcanic lava flows on the surface nor similar rocks formed by the injection of molten rock material into the sedimentary rocks below the surface. These rocks derived from molten material are typical of the Sangre de Cristo Mountains and surrounding regions elsewhere in northern New Mexico and southern Colorado. In spite of the cataclysmic mountain building that took place just a few miles north of Glorieta Mesa and Pecos National Monument about 30-50 million years ago, there are no features to show the effects of the volcanoes and earthquakes that must have accompanied or followed the uplift of the mountains—instead, the sedimentary rock layers are just tilted gently, almost imperceptibly, away from the uplifted mountain area.