Cedar Mountain Formation

Early Cretaceous

~120 million years ago

 

Horse Ranch Mountain, in the Kolob Canyons section of Zion

NPS photo/Adrienne Fitzgerald

The Cedar Mountain Formation contains conglomerate and sandstone deposited by rivers flowing toward an inland sea.

On the very top of Horse Ranch Mountain, the highest point in Zion National Park, there is a small outcrop of a brown pebbly conglomerate. In the past these rocks were believed to be part of the Dakota Formation, and may appear in older publications as such. However, recent geological mapping by the Utah Geological Survey has determined that this rock actually belongs to the Cedar Mountain Formation, which is exposed more broadly east of the park.

 

Depiction of Western North America during the deposition of the Cedar Mountain Formation, ~120 million years ago.

Paleogeographic map courtesy of Ron Blakey, Colorado Plateau Geosystems, Inc.

These sediments were deposited in the Early Cretaceous, by rivers flowing northeast from the newly-formed mountains associated with the Sevier Orogeny (mountain-building tectonic event).

All of the sedimentary layers younger than the Cedar Mountain Formation have been eroded from Zion, but most may still be found in areas east and north of the park, where erosion has not yet cut so deeply through the sedimentary sequence. Millions of years ago, there were many thousands of feet of additional rock layers on top of what we see today—a impressive reminder of the powerful forces of erosion at work in the Colorado Plateau.

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