GEOLOGY HAPPENINGS - April 2025

The iconic canyon country landscapes found throughout southern Utah and northern Arizona are dominated by red-to-orange cliffs. Reddish cliffs stand above Moab and along the Colorado River, they rim the Island in the Sky mesa of Canyonlands National Park, and they hold up Dead Horse Point. Further afield, they make up Comb Ridge near Bluff, tower above Fruita in Capitol Reef National Park, and are a hallmark of the San Rafael Swell.

Red rocks also form the Vermilion and Echo Cliffs in northern Arizona and greet river runners at Lees Ferry when they arrive to embark on their Colorado River journey through Grand Canyon. They span across the Arizona Strip and extend into Zion National Park. Throughout the southern portion of the Colorado Plateau (e.g., the canyon country that surrounds the Four Corners spot where Utah, Colorado, New Mexico, and Arizona meet), the rock layers (formations to geologists) make up these red cliffs are usually either the Wingate Sandstone or the Moenave Formation. *

Not only are the Wingate Sandstone and the Moenave Formation geologic doppelgangers (e.g., two layers that appear almost identical to one another), but they were also deposited at the same time. The two layers record two different types of ancient environments that resulted from differences in the paleogeography at the time they were being deposited. The Wingate Sandstone is eolian in origin and contains sediment transported by blowing winds that was deposited in an ancient sand dune field. The Moenave Formation is mostly fluvial in origin and was deposited largely by ancient rivers and on their floodplains.

These two ancient depositional environments were adjacent to one another, with the Moenave river systems being to the south and southwest, and the ancient sand dunes of the Wingate to the north and northeast. Hence the Moenave is found at the Echo Cliffs, Lees Ferry, Vermilion Cliffs, and at Zion in northern Arizona and southwestern Utah, and the Wingate is here near Moab, and at Comb Ridge in southern Utah, and Capitol Reef and the San Rafael Swell in south-central Utah. The two formations intertongue (e.g., small wedges of the Moenave and Wingate beds interlayer) with one another in the narrow area where the two environments met, but most places on the southern Colorado Plateau either have the Moenave or the Wingate, and not both.

Both rock layers are predominantly made of sandstone (e.g., a rock made of sand grains that have been cemented together), but the Moenave also contains other rock types, including siltstone and mudstone, that were deposited on floodplains and in lake beds. The proportion of these other rock types is more prevalent to the northwest where they are predominant around Zion and the city of St. George. The Moenave Formation in that area contains a great number of important fossils of dinosaurs and other vertebrates, especially fossil tracks.

Both the Wingate Sandstone and Moenave Formation were deposited at the very end of the Triassic Period, and into the Jurassic Period at around 200 million years ago. At the time, Utah and Arizona were near the western edge of the supercontinent Pangea that was just starting to break apart with the initial opening of the Atlantic Ocean. In this part of the continent, conditions were warm and mostly arid. The Wingate dune field covered most of what is now Utah and was fed by winds that blew from the northwest to the southeast. Near Moab, the Wingate Sandstone is around 350 feet thick.

The river systems that deposited the Moenave were ephemeral and very sandy. Like the area to the north where the Wingate was being deposited, the environment was fairly arid. The headwaters for the Moenave rivers were farther to the south, and the drainages flowed towards the northwest. In Zion National Park, the Moenave is about 275 feet thick.

Both the Wingate Sandstone and the Kayenta Formation are capped by another rock layer, the Kayenta Formation. The Kayenta is very resistant to erosion so it forms many of the canyon rims throughout the southern Colorado Plateau. About half of the heights of most of the Wingate or Moenave red cliffs are actually made of the Kayenta, with the contact (boundary) between the formations being hard to see, particularly at a distance. Overall, the Kayenta makes ledgey beds with horizontal layering versus the more massive Wingate Sandstone.

Another feature that the Wingate and the Moenave share is their color. The red and orange colors come not from the color of the sand grains themselves (which are nearly colorless), but from thin films of iron oxides that coat the grains. Iron oxide is a very strong pigment so small amounts can go a long way in tinting rocks.

* The main exception to this is in Monument Valley along the Utah and Arizona border. The red cliffs and spires there are made of another rock layer entirely: the DeChelly Sandstone, which is significantly older than the Wingate and the Moenave.

A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.
To learn more about Moab’s geology, visit the Geology Happenings archive online at https://www.moabhappenings.com/Archives/000archiveindex.htm#geology