Geology Happenings

The Moab Fault is one of the most important geological features in southeastern Utah. Like all faults, the Moab Fault is a planar surface where segments of the Earth’s crust have moved relative to one another. Evidence of such motion can be observed by noticing how rock layers have been displaced along a fault. Here in Canyon Country, the rock layers( formations to geologists) are mostly in their original horizontal position (e.g., how they were deposited as sediments on the Earth’s surface) that can be traced laterally across the landscape, but this layering can be disrupted by faults. As a result, faults can be identified where one rock layer abruptly abuts against another and are sometimes seen associated with tilting or folding of rock layers.

Visitors to Arches National Park who pull off at the first overlook after the switchbacks learn about the Moab Fault from a roadside exhibit. Geology students examine the roadcut across from the visitor center where the rocks have been sliced and segmented by many splays (branches) of the

Moab Fault. But there is a lot more to the Moab Fault than what can be seen near the entrance to Arches. The fault can be traced about 28 miles northwest of Moab and it extends to the southeast where it is buried underneath surface sediments in Moab-Spanish Valley.

1. Moab Fault Viewpoint, Arches National Park: The Moab Fault is responsible for the fact that there are higher cliffs on the far side of the highway that aren’t present in the park. Movement along a fault is always described as relative. Here the far side has moved up relative to the near side. If it hadn’t been eroded away on the far side of the fault, the Entrada Sandstone would sit a few hundred feet on top of the Wingate Sandstone.

2. Moab-Spanish Valley View: A view to the southeast taken very near the Moab Fault at the western boundary of Arches National Park provides a great perspective of the Moab and Spanish Valleys. Although called valleys, the combined Moab-Spanish Valley do not have a river flowing down them (note the Colorado River cutting across the valley). This indicates that their origin must result from something besides normal fluvial erosion. This valley-like feature results from movement of underground salt deposits. Because it is less dense, the salt flowed up towards the surface, doming and bulging the overlying rock layers. Groundwater dissolved the salt near the surface, causing the land to sag into an elongated valley-like depression. Movement on the Moab Fault is associated with the subsurface movement of this salt, making the Moab Fault different than most other faults that result from tectonic stresses in the Earth.

3. Cotter Mine Road, north of Moab: The Cotter Mine Road is a dirt track popular with 4-wheelers. Part of the road is very close to the trace of the Moab Fault. However, the fault is not as evident here as it is further south. Still, it can be recognized by where two different rock layers are laterally adjacent to one another.

3. Bartlett Wash: One of the most dramatic places to see the Moab Fault is near Bartlett Wash where it brings the beautiful red Entrada Sandstone next to the drab Cedar Mountain Formation. Note the tilting of the Cedar Mountain Formation where it was downdropped next to the fault surface.

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