By Gary Ladd
Throughout the middle and downstream sections of Lake Powell about a dozen rock units can be seen. All are sedimentary in origin. Of these, only about four are exceptionally prominent, and three of them are Aeolian (wind-deposited) sandstones.
The Aeolian Sandstones of Glen Canyon
- Wingate Sandstone is about 205 million years old. It is the first of three aeolian sand dune environments to occupy the future site of the Glen Canyon area in the early and middle Jurassic Period, a time of recurring extreme droughts. The Wingate is a cliff-former and is usually orange in color (and is often therefore mistaken for the Navajo Sandstone).
- Navajo Sandstone is about 190 million years old. It rests upon the Kayenta Sandstone which rests upon the slightly older Wingate Sandstone. The Navajo is undoubtedly the most widely recognized and important of Glen Canyon’s rock units because it is thick (over 1,000 feet in this area), and it often forms imposing cliffs draped with desert varnish. (It also plays important roles in other Colorado Plateau parks, forming the largest part of the walls of Zion National Park, makes major ridges in Capitol Reef National Park, is the sculpted rock of Rainbow Bridge and plays important roles in Navajo National Monument, Canyonlands and other locations.) It is believed that the quartz sand grains that compose the Navajo Sandstone probably originated in the southern Appalachian Mountains. Those mountains were already 50 million years old at the time and were actively weathering and falling apart. Some of their rock particles were being carried westward by a river system that eventually dispersed in a desert area that is now Utah. Winds distributed the sand grains over a large area of the western US including Glen Canyon to become today’s Navajo Sandstone.
- Entrada Sandstone is about 160 million years old and is separated from the underlying Navajo Sandstone by the Carmel Formation (evidence of another incursion of a sea). It appears white in the Wahweap area of Lake Powell but transitions to red in the vicinity of Padre Bay. It represents the third and last of the great wind-blown deserts of the middle Mesozoic.
And the Amazing Chine Formation
The final important rock unit of the Glen Canyon area is the Chinle Formation. In the upper reaches of Lake Powell and especially along its San Juan River arm, the Chinle Formation appears intermittently. It’s about 225 million years old and is most easily identified by its crumbling, multi-colored (gray, green, blue and brown) slopes. It was deposited in streams and, less often, in lakes. Some portions contain volcanic ash beds. It’s made of sandstones, mudstones and shales. Most interestingly, the lower levels within the Chinle Formation often contain an abundance of petrified wood. In fact, south of Glen Canyon it is the Chinle Formation that gives Petrified Forest National Park its huge collections of petrified logs.