By Gary Ladd
The Colorado Plateau is characterized by a thick sequence of mostly sedimentary rock layers that have been uplifted thousands of feet by plate tectonics movements. The uplift began about 70 million years ago, thus ending almost all sedimentary rock layer creation. Normally such tectonic activity warps and buckles the original flat-laying sedimentary rock layers until they are severely tilted and twisted. In every direction around the Colorado Plateau (the Rocky Mountains to the east and north, the Basin and Range Province to the west and south) sedimentary rock layers are exactly this way—tilted and twisted. But this is never true within the Colorado Plateau. Well, almost never.
Because tilts and twists of the rock layers are relatively uncommon in the Colorado Plateau, wherever they do occur they stand out theatrically. For instance, there’s the dramatic Waterpocket Fold featured in Capitol Reef National Park and there’s Comb Ridge that crosses the San Juan River west of Mexican Hat, Utah.
Based upon their orientation and shape, folds in the rock are categorized by geologists as synclines, anticlines or monoclines. Folds that bend down then back up (like a trough) are called synclines, folds that arch upwards and back down again (like a hill) are called anticlines. And then there are monoclines, folds that are simply half of one of the other types, high on one side and low on the other (like a ramp). All three of these shapes are evidence of times when the crust of the Colorado Plateau was subjected to compressive forces from the jostling of tectonic plates. In the case of the most prominent Colorado Plateau rock folds, the compressional events occurred about 65 million years ago at about the same time that the most recent version of the Rocky Mountains were being created to the east.
It turns out that monoclines play a critical role in creating the setting that eventually led to the formation of both Grand Canyon and Glen Canyon.
Throughout the length of Glen Canyon its rock layers remain mostly flat-lying. To someone riding in a boat along the flat surface of Lake Powell, the shoreline rock layers in most areas undulate gently throughout the length of Glen Canyon. So far, so good. But just before the Colorado River (downstream of Glen Canyon Dam) exits Glen Canyon the enclosing rock layers tilt up about 2,500 feet rapidly and conspicuously. In these final few miles of Glen Canyon the Colorado River crosses the Echo Cliffs Monocline, running roughly north and south. The uplift brings older rock units to the surface and the first of these is our old friend the Kaibab Formation, Grand Canyon’s rimrock layer. The emergence of the Kaibab from river level marks the beginning of Grand Canyon. Glen Canyon has ended.
It is difficult to overemphasize the importance of the Lees Ferry area. It marks the end of Glen Canyon, the beginning of Grand Canyon, the stratigraphic location of the Great Dying extinction event in the local rock sequence, and therefore the transition from Paleozoic to Mesozoic time. If that is not enough, Lees Ferry is the place where humans long ago found a surprisingly accessible and easy crossing of the Colorado River. (It is relatively easy to cross the Colorado River here because the lowest-most rock units of the Mesozoic, the Moenkopi and the Chine Formations are mostly soft and easily eroded. Therefore no cliffs of resistant rock guard way to the edge of the river at Lees Ferry.)
About 70 miles downstream from Lees Ferry the Colorado River encounters a second massive monocline. This one also runs north and south. The Colorado River seems to enjoy the challenge of hitting older, rock units because it turns directly into the face of the East Kaibab Monocline and drills in. The result is a Grand Canyon that suddenly jumps from about 3,000 feet deep to 5,000 feet deep as the older and highly resistant rock sequence rises higher and higher to the west.
In summary, it is not only the rock layers themselves that determine the look and structure of the canyons; it’s the later folding and faulting of those rock layers, too.
