Geology of the Foothills

Geology of the SLC Foothills and Adjacent Wasatch Mountains

Utah is famous for its geology. People come from all over the world to see the astounding geological formations exposed in Arches, Canyonlands, Capitol Reef, Bryce, and Zion National Parks.  However, how many people know about the fascinating geology that exists throughout the state, including the foothills and mountains adjacent to Salt Lake City? While the geology in our neighborhood may be more subtle than in the National Parks, the story it tells is more complex and equally amazing.

The Salt Lake Foothills and adjacent mountains are composed primarily of sedimentary rocks deposited over a period of 550 million years. The oldest sedimentary rocks, visible on the western ridge of Grandview Peak, are Cambrian sandstones deposited between 545 and 520 million years ago (Ma). The youngest sedimentary layers are unconsolidated sandstones and conglomerates that accumulated along the shores of Lake Bonneville between 30,000 and 13,000 years ago. The summit ridge of Black Mountain includes Late Paleozoic (325-248 Ma) shallow marine limestones and sandstones. These once horizontal, fossil-rich layers were faulted and rotated during the Cretaceous Period (145-60 Ma) and now stand nearly vertical. 

The red sandstones that make the cliffs of Red Butte formed as sand dunes during the early Jurassic (206-180 Ma). These rocks are equivalent to the red and white sandstones that we see in Arches, Capitol Reef and Zion National Parks, all of which were originally sand dunes as well. In contrast, the Foothills above lower City Creek are cemented conglomerates that were deposited as mud-flows, landslides and alluvial fans along the flanks of the rising Wasatch Mountains during the Miocene (24-5.3 Ma).

Finally, look south across the Salt Lake Valley from the Foothills. The steep western flank of the Wasatch Range is located along the presently active Wasatch Fault. The Salt Lake Valley has dropped an estimated 20,000 feet relative to the summits of the Central Wasatch Range since the early Oligocene (35-30 Ma). That’s the same height as Mt. Denali in Alaska, the highest mountain in North America. 

For many geologists, the geology of the Salt Lake City area is every bit as interesting as that seen anywhere else in the Rocky Mountain region, including Utah’s famous National Parks. Next time you are out in the Foothills, look around and enjoy the rocks in our back yard.

Daniel Schelling