GEOLOGY HAPPENINGS - September 2026

The cobbles found throughout the Colorado River corridor in southeastern Utah might not seem remarkable. After all, they are only rocks. But the cobbles, both individually and collectively, tell stories related to the origin and history of the Colorado River and of the geology of the entire American Southwest.

First, the cobbles themselves. They reveal where they are from and how they got to their resting spots along the Colorado River and on topographic benches above it.

In geology, the term cobble refers to a clast (a rock fragment) of a specific grain size. Cobbles have diameters between approximately 2.5 and 10 inches. In the grain size hierarchy, cobbles are larger than gravel but smaller than boulders.

Cobbles are usually transported by flowing water that is moving at a fairly rapid pace. Wind and slow-moving water currents do not have enough power to move particles larger than sand grains or very fine gravel. Hence, cobble deposits are usually emplaced by rivers with relatively steep gradients like the Colorado River or they may be deposited in certain coastal zones with high-energy currents.

Many cobbles become rounded into spherical or disc-like shapes during transport. Abrasion occurs as they move along the bed of the river as corners break off first. The common disc shapes result from the fact that many rocks have some sort of layering in them so that they break on discrete planes, which remain surfaces that slide along the riverbed as they move, thereby maintaining the generally flat shapes even as the sides are rounded into discs.

Although cobble deposits are sedimentary in origin as they consist of rock material deposited on the Earth’s surface, individual clasts within them may be igneous, metamorphic, or sedimentary. In fact, igneous and metamorphic clasts are quite common in Colorado River cobble deposits because these crystalline rocks are harder than sedimentary rocks like the mudstones, siltstones, or even the sandstones that make up the bedrock of canyon country. Any sedimentary cobble found along the Colorado River probably didn’t travel very far from its source area as these softer rocks would break up during transport.

Igneous rocks in cobble deposits include both volcanics and granites. Many (but not all) of the granite clasts are pink in color due to a high proportion of the mineral potassium feldspar. Metamorphic rocks such as gneiss, schist, or quartzite are also present. Most of the igneous and metamorphic cobbles were transported great distances from where they were eroded in the heart of the Rocky Mountains. Geologists study the rock types found in cobble and gravel deposits in order to understand the history a river’s history and the landscape it was eroding. In fact, the presence of clasts with distinctive rock types can help geologists constrain the timing of when a river first appeared in an area.

For example, although the Colorado River is now the major agent driving the erosion of the Colorado Plateau (e.g., the high elevation Four Corners region that contains southeastern Utah), it didn’t always exist. The pre-Colorado River Four Corners landscape once held large lakes and even other rivers that didn’t have an outlet to the sea.

The area just south of Grand Canyon in northern Arizona hosted inland lakes prior to the arrival of the Colorado River as evidenced by fine-grained lake sediments found there. The telltale presence of cobble deposits heralds the arrival of the Colorado River in that area, which has major implications for the evolution of the modern canyon landscapes throughout the Southwest.

Many of the cobble and gravel deposits found in southeastern Utah are located on benches that sit as high as a few hundred feet above the current level of the Colorado River. Such river terraces are remnants of a stream’s former floodplain that become perched above the current channel after it downcuts or incises into its valley. The highest Colorado River terraces near Moab are near the Corona Arch Trailhead and are about 300 feet above the current river elevation. These deposits record the elevation of the river approximately 200,000 years ago, and they show how much the river has downcut during that time interval.
The cobble and boulder deposits found along the Colorado River, as well as those along Pack and Mill Creeks, and even the ancient ones present in some rock layers, don’t seem like they would be anything more than mere curiosities. But together they help tell important parts of the main lessons of geology: that the Earth is always changing and its history is recorded in the rocks.

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