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GEOLOGY HAPPENINGS July 2020

The Cedar Mountain Formation: Scientific Significance Instead of Scenic Splendorby Allyson Mathis
Rock layers (formations to geologists) can be important in a number of different ways. Near Moab, the scenic qualities of a rock layer seemingly reigns supreme. Many people consider the most significant rock layers to be the ones that play the most prominent roles in the iconic scenery of the canyon country landscape.
Typical Cedar Mountain Formation scenery near Moab
Typical Cedar Mountain Formation scenery near Moab
Think the Wingate Sandstone holding up the soaring cliffs above Moab and along the Colorado River, or the Entrada Sandstone in Arches National Park. These two rock layers are certainly among the ones that are the subject of the most tourist photographs, and readily come to mind when someone considers Moab’s geology.

Other rock layers have value in other ways, sometimes literally in the case of ones that have economic significance. For example, the Paradox Formation contains potash deposits that are mined south of Moab, and uranium ores were mined from the Chinle and Morrison formations.

Yet other layers are critical because of their scientific significance. The Cedar Mountain Formation is one such layer. While the Cedar Mountain Formation wasn’t scientifically named until 1944, active research into the unit and its dinosaur fossils has provided important new information on dinosaurian diversity and evolution in western North America during the early Cretaceous. Overall, the Cedar Mountain isn’t as heralded among the general public as the better-known Morrison Formation, but its dinosaur fossils are just as important as those from the Morrison, if not even more so, especially since more species of dinosaurs are known from the Cedar Mountain than from the Morrison.
The Cedar Mountain Formation east of Capitol Reef NP
The Cedar Mountain Formation east of Capitol Reef NP


Much of the Cedar Mountain is rather drab in color without much to make it stand out as far as scenery goes, especially compared to the otherwise brilliant red-to-orange rocks exposed near Moab. It is generally colored a pale olive-green to tan and buff, and consists mostly of mudstones that weather to low hills with a few ledge-forming sandstones.

By definition, the Cedar Mountain is only found on the west side of the Colorado River, with the mostly-equivalent Burro Canyon Formation found east of the river. The Cedar Mountain was deposited on a broad floodplain by rivers that flowed mostly from the southwest shifting to west with onset of mountain building in western Utah. The unit gets its name from Cedar Mountain in the San Rafael Swell where it was first described.

The Dalton Wells Quarry area. Photo courtesy of Jim Kirkland.
The Dalton Wells Quarry area. Photo courtesy of Jim Kirkland.
The Cedar Mountain sits on top of the Morrison Formation, which has colorful banded hills in its upper part that are distinctive from the more muted colors in the Cedar Mountain. The Morrison is one of the most famous rock layers in the American west because of its dinosaur fossils, including Allosaurus, Stegosaurus, and other species discovered at Dinosaur National Monument and the Cleveland-Lloyd Dinosaur Quarry (now at Jurassic National Monument). An unconformity, or period of missing time, separates the Morrison from the Cedar Mountain, with a temporal gap of approximately 25 million years, meaning the fossils in the Cedar Mountain are much younger than those from the Morrison.

The Cedar Mountain Formation is not very thick, only ranging up to 250 feet thick in southeastern Utah, but it spans a large time interval (from approximately 140 to 100 million years ago in the early Cretaceous Period), which is as great or greater than that of any of the other rock layers found near Moab. Additionally, rocks of the same age are relatively rare in western North America, increasing the Cedar Mountain’s importance in the geologic and fossil record.

The Mill Canyon Dinosaur Tracksite
The Mill Canyon Dinosaur Tracksite
Until the 1990s, the Cedar Mountain was thought to be largely devoid of dinosaur fossils, but since then many important quarries have been documented, including the Dalton Wells Quarry near Moab. The carnivorous theropod Utahraptor, the Utah state dinosaur, comes from the Cedar Mountain Formation, as does the sauropod Moabosaurus. Gastonia, a type of armored herbivorous dinosaurs and the namesake of the Moab chapter of the Utah Friends of Paleontology, are all from the lower Cedar Mountain unique to Grand County.

Overall, the Cedar Mountain contains six distinct dinosaur faunas divided by unconformities within the unit that reveal a previously unknown mass extinction when dinosaurs like Utahraptor, Moabosaurus, and Gastonia, went extinct approximately 120 Ma. During the next 20 million years North America was an island continent with endemic dinosaurs, primitive sauropods, smaller raptors, and armored dinosaurs.

The uppermost Cedar Mountain Formation is only preserved on the west side of the San Rafael Swell. Over 100 species of fish, frog, lizard, dinosaur, mammal, etc. and dinosaur nests have been recovered from this last two million years of deposition of the Cedar Mountain Formation.

The Mill Canyon Tracksite northwest of Moab with its interpretive trail is also in the Cedar Mountain. Tracks of eight different types of dinosaurs may be viewed from the boardwalk, along with tracks from birds, turtle, and crocodiles, making the Mill Canyon site one of the most diverse and significant tracksites from the early Cretaceous in the world.

Hopefully, this column promotes a wider appreciation for the Cedar Mountain Formation. While most folks may not purposely set out to spend time in the Cedar Mountain Formation just for its scenery, it is rich in scientific significance, leaving scenic splendor to other rock layers found near Moab. Special thanks to Jim Kirkland, Utah State Paleontologist, and ReBecca Hunt-Foster.

You can read more geology articles HERE

A self-described “rock nerd,” Allyson Mathis is a geologist, informal geoscience educator and science writer living in Moab. Allyson tries to appreciate rocks from both the aesthetic and scientific standpoint.

 
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