Geology of Lees Ferry, Colorado River, northern Arizona

Lee's Ferry, Arizona | The Colorado River exits Glen Canyon (bottom) and begins its entry into Grand Canyon (far right). The view is to the south in the direction the river flows and captures Lees Ferry, the historic crossing of the river from 1864 to 1929. Note the USGS gauging station and measuring cable in the lower right. Outcrops of the Triassic Chinle Formation fill the left 2/3rds of the photo with the Shinarump Conglomerate Member forming the lighter colored resistant ledge (above the gauging station) and the much softer and colorful Petrified Forest Member that caps the Shinarump. The Shinarump is a fluvial pebble conglomerate and coarse sandstone with paleoflow indicators to the northwest. The Petrified Forest Member is a shale, mudstone, and siltstone that formed also in fluvial environments. Both units contain petrified wood and a popular stop on Grand Canyon river trips visits some large specimens located near the contact of the two units near the shaded bluffs at the base of the Petrified Forest Member.

Looking a bit downstream and partially in shadow is the Triassic Moenkopi Formation, rising steadily above river level. This fine-grained siltstone, shale and sandstone is also fluvial in origin. If you look closely in the shadows at the left-bending turn where the river disappears, outcrops of the cliff-forming Permian Kaibab Limestone (or Formation) rise above the river and signal the official beginning to the Grand Canyon. The Kaibab is a limestone and chert deposit laid down in shallow marine environments. The Moenkopi-Shinarump couplet can also be seen as a slightly tilted mesa in the middle distance, with the Jurassic Moenave, Kayenta (Formations) and Navajo Sandstone exposed in the extreme upper right in the Vermilion Cliffs. In the far distance on the skyline is the uplifted Kaibab Plateau (elevation 8,000 to 9,000 feet), raised along the East Kaibab monocline during the Laramide Orogeny and containing Paleozoic formations exposed in the heart of the Grand Canyon. 

This spectacular setting shows the intimate relationship between geology and human history. If a person on foot were to follow the "easiest" topography in this area, it would naturally bring them to the Colorado River at Lees Ferry. This is because the river has carved the sheer walls of the Grand Canyon to the south and Glen Canyon to the north. The northeast tilted "ramp" that has formed on the top surface of the Kaibab Limestone would bring that traveler directly to this spot, where the Kaibab dives into the subsurface. Such an occurrence happened in November, 1776, when members of the Dominguez-Escalante party arrived here after their failed attempt to reach California from Santa Fe. They turned around at Utah Lake and wandered along the topography of northern Arizona, arriving at this spot which they named San Benito de Salsipuedes, translated as Saint Benito "Get Out If You Can". They attempted to cross the river here on foot but were thwarted and finally did find a suitable crossing 11 days later at the Crossing of the Fathers in Glen Canyon to the north. Here geology determines human existence. (Photo by Wayne Ranney)