GEOLOGY HAPPENINGS - February 2026

The landscape that surrounds Moab today is a rocky place. Cliffs of shear rock, rounded rock domes, and rocky slopes make the contours of the desert as they are the building blocks of the spectacular scenery that hosts two national parks and two state parks. But the Moab area didn’t always look like this.

The rocks that make up today’s landscape also tell the stories of how the land and environment has changed across the expanses of geologic time. Most of the rocks exposed near Moab were deposited during or before the age of the dinosaurs and are largely between about 310 and 80 million years old. This interval saw not only the rise of the dinosaurs and spanned most of the time during which they lived, but also the breakup of the supercontinent Pangaea, as well as changing climates and environments.

Sedimentary rocks, like those found in southeastern Utah, are typically laid down in depositional basins; e.g., low areas where sediments transported by water or wind accumulate. Broadly speaking, basins are either marine; e.g., that are below sea level and filled with water, or continental, whereas deposition takes place on land. Of course, sediments are also deposited in transitional environments where land meets the sea; e.g., in beaches, river deltas, and tidal flats.

Sediments are deposited on land in a variety of environments: fluvial (rivers), eolian (wind deposited), and lacustrine (lake). Each of these environments, as well as the marine and transitional ones, yield sedimentary deposits with specific characteristics that can be used to infer the conditions under which they were deposited and can help geologists understand the geologic history preserved in the rock record.

The rock layers (formations to geologists) exposed in southeastern Utah record a variety of depositional environments, but more often than not, they were deposited in seas of water or seas of sand, although fluvial and transitional environments are also represented.

Worldwide and in other parts of the Southwest such as at Grand Canyon, marine sedimentary rocks are extremely common. Marine rocks are most easily identified by their fossils, including those of invertebrate organisms with hard parts like clams or snails, corals, crinoids, and others, which are readily fossilized. Different types of sedimentary rocks (shale/mudstone, siltstone, limestone, and sandstone) can be deposited in marine basins, with muds in deep waters, limestone in warm shallow water, and sand near the shoreline.

As it turns out, in southeastern Utah, marine sedimentary rocks are generally the oldest and the youngest layers exposed. These rocks, including the Honaker Trail Formation, are found in the deepest part of Cataract Canyon and in the Book Cliffs along I-70, north of Moab. The slopes of the Book Cliffs is the marine Mancos Shale.

One of the most striking characteristics of canyon country, both in terms of its geologic history and its scenery, is the abundance of eolian sandstones. In fact, relative to the rock record in the rest of the world, southeastern Utah and northern Arizona contain especially numerous eolian units. At least six different rock layers in southeastern Utah are either exclusively or mostly eolian in origin, and eolian beds are found in several other layers. The eolian rock layers, the Cedar Mesa, White Rim, Wingate, Navajo, and Entrada Sandstones, and the Moab Member of the Curtis Formation, are the principal scenery makers throughout canyon country. They hold up canyon walls, make picturesque rock domes, host natural arches, and form mesas, buttes, and the rock spires known as needles.

These layers were deposited in ergs, otherwise known as sand seas, that are large expanses where sand is transported by wind to form sand sheets and dunes. Ergs typically form in arid to semiarid basins where there is a good supply of sand, and today only cover a small proportion of desert regions. Many different ergs, including some especially large ones, were present in the Southwest while the rocks now exposed near Moab were being deposited.

Eolian sandstones are generally quite uniform. They are typically very fine grained, versus the coarser sand that can be transported by water, and often are light in color. They can make massive cliffs which may show internal swoopy layering (cross-bedding) that formed parallel to dune surfaces as they migrated with the blowing winds.

They can be very thick. Although the Navajo Sandstone is only a few hundred feet thick in the the Petrified Dunes in Arches National Park, that same unit ranges up to 2,000 feet thick in Zion National Park. These ancient dunes were inhabited, particularly in low areas between dunes where surface water was present, but fossils are rare, except for animal tracks. Most of the dinosaur trackways found in southeastern Utah are in eolian units, including the Moab Member.

The eolian and marine rock layers exposed throughout canyon country form a link between the geologic past and present. The fossils found in them and features like cross-bedding tell the stories of the seas of water and seas of sand that were once here. Can you imagine what those ancient landscapes looked like?

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