Know Your Layers – Canyonlands Edition by Allyson Mathis
Although Canyonlands and Arches National Parks are red-rock desert wonderlands only about a 30-mile drive from one another, the two parks are geologically distinct. Much of Canyonlands is, as its name describes, a landscape consisting of branching canyons stemming from the two major rivers (the Green and the Colorado) that join in the deep canyon in the center of the park. This landscape of rock is also populated with a multitude of towers and spires, buttes and mesas, and needles and standing rocks. The hard White Rim Sandstone forms a broad bench (e.g., the “white rim”) below the Island in the Sky. The Organ Rock Formation is exposed in Monument Basin where a side drainage of the Colorado River carved an amphitheater-like area.
One of the most significant differences between the two parks is that they feature different rock layers (formations to geologists). Each rock layer has its own set of unique characteristics—color, rock type(s), thickness, type of internal layering, and fossils. Many of these characteristics have profound influences on landscapes carved through the processes of erosion that shaped the parks. In short, different rocks mean different scenery when other geologic factors are much the same, as is the case in Arches and Canyonlands.
Most of the rock layers exposed in southern Utah are actually found in both parks, but the main scenic areas of Arches are in rock layers that are not in Canyonlands. These younger rock layers (the Dewey Bridge Member and the Entrada Formation) have been removed by erosion in Canyonlands.
Canyonlands National Park is immense at more than 337,000 acres. The rivers divide the parks into distinct districts: the Island in the Sky, the Needles, and the Maze. Canyonlands’ older rock layers are present in all three districts, but the Island in the Sky contains several layers that have been completely eroded from the Needles and Maze. The name Island in the Sky is derived from the district’s tall mesa that is seemingly surrounded by sky when viewed from other parts of the park.
White Rim Sandstone and Cedar Mesa Sandstone
The White Rim and Cedar Mesa Sandstones were both formed prior to the age of the dinosaurs in sand dunes systems that were close in geographic location. But the Cedar Mesa is slightly older and was deposited southeast of where the White Rim was.
The Needles region was near the northern edge of Cedar Mesa dunefield and the White Rim dunefield did not extend east of where the Colorado River is now. Hence, the White Rim Sandstone is not found in the Needles District, and the Cedar Mesa isn’t at the Island in the Sky. (Both layers are in the rugged backcountry of the Maze). They are part of a larger package of rocks known as the Cutler Group.
Moenkopi Formation and Chinle Formation
These layers are found underneath the Island in the Sky cliffs where they are largely obscured by boulders and rocky debris that have fallen from the cliffs above them. The Moenkopi and Chinle contain many mudstone and siltstone beds that are softer than the sandstones and below and above them. Hence, they readily are eroded in areas where they are not protected by caps of harder rocks.
Wingate Sandstone & Kayenta Formation
The Wingate Sandstone and the overlying Kayenta Formation are cliff-makers. Together, they make up the Island in the Sky cliffs, as well as the Orange Cliffs west of the Maze and those of the Canyon Rims Recreation Area east of Canyonlands.
These layers were deposited during the Jurassic (when dinosaurs lived), but in different environments. The Wingate was deposited in yet another sand dune system, and the Kayenta by sandy rivers during a wetter period. It is hard to tell the two layers apart, but the Kayenta has thin ledgey beds that make up the upper part of the cliffs.
Navajo Sandstone
The Navajo Sandstone is the youngest rock layer exposed in Canyonlands and it makes one of the park’s best known scenic spots: Mesa Arch, perched on the very rim of the Island in the Sky. Also found in Arches, Capitol Reef, and Zion, and in Glen Canyon National Recreation Area, the sheer sandstone cliffs and knobs of the Navajo make the prettiest scenery in all of canyon country.
The Navajo is another eolian sandstone (e.g., deposited in ancient sand dunes). The Navajo dunefield was the largest in North America’s geologic history, covering most of Utah and parts of adjacent states about 185 million years ago.
The Cedar Mesa Sandstone in the Needles District. The red-and-white-striped Cedar Mesa Sandstone was mostly deposited in sand dunes, with the color variation resulting from more iron oxides in red bands. The Cedar Mesa contains some thin beds of fluvial (river) sediment as the Needles area was near the dunefield margin.
The cliffs of the Island in the Sky.
Mesa Arch in the Navajo Sandstone.
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
A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab.