The base of the sedimentary formations is a regionally extensive erosional surface. This means that a much older mountain uplift must have been planed off by erosion to produce the flat base on which the sedimentary layers were deposited. Little is known about these Precambrian mountains, because their remains are so deeply eroded and have been overprinted by successive episodes of geologic deformation.
Another set of now-vanished mountains rose approximately 300 million years ago during the Late Pennsylvanian and Early Permian geologic periods, at the time of the super continent called Pangea. These are called the Ancestral Rocky Mountains, and were located in northern New Mexico, Colorado, and eastern Utah. Following uplift of the Ancestral Rockies, erosional processes delivered sediment to alluvial fans, deserts, river deltas, beaches and shallow seas, evidence of which can be seen in the rock layers of Glorieta Mesa. Eventually the Ancestral Rocky Mountains eroded completely and a shallow continental sea took their place during the late Cretaceous geologic period (95-65 million years ago).
The third period of mountain uplift, called the Laramide Orogeny, produced the main ranges of the Rocky Mountains, including much of the Sangre de Cristo Range, approximately 70-40 million years ago. Glorieta Mesa is a transitional feature between the Southern Rocky Mountains and the Great Plains and was uplifted during the Laramide. Interestingly, the locations of the Laramide Rocky Mountains aren’t exactly the same as the Ancestral Rocky Mountains. For example, the southern Sangre de Cristo range has basement outcrops on its east and west sides, but a lot of uplifted sedimentary rock in its middle, in the former basin beside the Ancestral Rockies. The Pecos River drains much of this central trough, which explains both the unusually large size of the river so close to its source, and its odd orientation parallel to the elongation of the mountains.
Finally, north-central New Mexico experienced yet another mountain-building event in the Neogene Period, approximately 25 to 3 million years ago. During this time the Rio Grande Rift formed as a tectonic spreading feature along a broad crustal arch. Rifting produced a north-south string of basins flanked by mountain uplifts such as the Taos and Sandia Ranges; eventually these basins became linked by the Rio Grande. The upper Pecos River valley is east of the Rio Grande Rift, but experienced some additional uplift at this time.
The rest of the story of the local landscape is erosional. The modern Pecos Canyon and Pecos River formed as a result of melting alpine glaciers high in the mountains during the Pleistocene Ice Age, 700,000 to 12,000 years ago. Erosion from the river and creeks greatly influenced the shapes of the valley and mesa you see today. As we stand beside the ruins in the middle of the Park and contemplate the panorama, we can wonder how a small river could make such a wide valley. It is easy enough to see how the river carries clay, silt, sand and gravel along its bed, but even in the largest floods it directly affects only a narrow ribbon in the valley floor. Yet the valley is continuing to grow wider and deeper, and the river’s channel is the only conduit for removing this eroded material. Most of this solid detritus moves to the river by rockfall off the cliffs of Glorieta Mesa, then by gradual creep, sheetflood and flash-flood wash in the many small tributary arroyos.