GEOLOGY HAPPENINGS - September 2025

One of the more striking things about Arches and Canyonlands, the two national parks close to Moab (besides of course their breath-taking beauty and overall red-rock-canyon theme), is how different they are. Each features cliffs and canyons that were formed through erosion and both are dominated by exposures of bare rock. Yet the two parks are distinct in their characteristic landforms.

One of the main reasons why the two parks each have their own unique character is that they feature different rock layers (formations to geologists). These rock layers are the building blocks of canyon country scenery. Therefore, rock units greatly impact what a landscape looks like.

A formation consists of a body of sediment that was deposited at some point in the geologic past under a specific set of environmental conditions in a particular geographic locality. A formation is composed of certain types of sediment (like sand, silt, or clay). For example, the Navajo Sandstone was deposited in an ancient sand dune field that covered much of the Four Corners region about 185 million years ago during the Jurassic Period when dinosaurs were alive. The Entrada Sandstone was deposited in a different and much smaller sand dune field that existed a bit later in time (about 159 million years ago).
Geologists spend a great deal of time studying each rock layer in detail, including its fossils and where it crops out across a geographic area, in order to understand Earth history. Fortunately for park visitors, hikers, and others out enjoying canyon country scenery, most layers have easily observable features that make them recognizable without having to undertake such scientific study. Many rock layers have characteristics that form distinctive landforms. These characteristics do relate back to their geologic history, but also can be appreciated just purely from a landscape sense.

Arches National Park contains at least 15 different rock layers. Only four make up most of the main features in the park and are responsible for most of its iconic scenery. Note that other geologic events also impact scenery. For example, the rock layers in some parts of Arches have been folded and contain sets of parallel fractures (joints). These joints have facilitated the erosion that carved the fins (elongated rock ridges) of Devils Garden and the Fiery Furnace.

Navajo Sandstone
The Navajo Sandstone makes up some of the most compelling scenery in Utah. It is the “rock star” in Zion and Capitol Reef national parks and makes up the iconic Mesa Arch in Canyonlands. In Arches, it is mostly found in the area known as the Petrified Dunes and it has more of a supporting role in the scenery. Although the Navajo Sandstone was in fact deposited in ancient sand dunes, the rounded knobs of the Petrified Dunes were formed by erosion. The Navajo Sandstone is light yellow in color and displays a type of inclined layering known as cross bedding that formed along dune surfaces.

Carmel Formation (Dewey Bridge Member)
The Dewey Bridge Member is a unique subdivision of the Carmel Formation found only in Southeastern Utah. Its most famous exposures are in the Windows Section of Arches National Park. It is dark red to orange in color and is striking because of its contorted bedded formed by soft-sediment deformation. The conditions that caused the Dewey Bridge’s beds to wrinkle while its sediments were still soft (e.g., before they were lithified) are unknown, but the wavy and uneven bedding is pervasive. Even the contact (boundary) with the overlying Entrada Sandstone shows this uneven layering.

Entrada Sandstone (Slick Rock Member)
The Entrada Sandstone has the starring role in Arches National Park. Most of the park’s arches are found in this beautiful reddish-orange layer. It also makes scenic cliffs with shear walls like those in Park Avenue. Other scenic spots like Devils Garden, Delicate Arch, and the Fiery Furnace are predominantly made of the Entrada.

While also eolian (e.g., deposited in sand dunes) in origin like the Navajo, the Entrada Sandstone is also massive, meaning that it lacks pronounced layering within it. Therefore, the Entrada is more uniform than the other layers in Arches, which helps make it a good tableau for the weathering and erosional processes that carve natural arches. The fact that the Entrada is strong enough to hold up vertical cliffs and overhanging slopes while still eroding rather readily is also key.

Curtis Formation (Moab Member)
The Moab Member is another unique subdivision of a rock layer found (you guessed it) in the Moab area. It is also another eolian in origin but is thinner than the other prominent layers in Arches at usually less than 100 feet thick. Besides being off-white and the lightest-colored rock unit in the park, it is also harder (e.g., more resistant to erosion). It forms the hard cap rock on the top of fins in places like Devils Garden and the cap on Delicate Arch.

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