GEOLOGY HAPPENINGS - June 2026

One of the many pleasures of being a geologist is having the ability to read the rocks; that is to decipher their features, structures, and characteristics to explore the history of our planet. Every rock including those that fit in the palm of a hand, every rock layer (a formation to geologists) that can be traced across the landscape, and every landform whether it is a cliff, a natural arch, or a deposit of cobbles along a river, has stories to tell. Some of these stories tell how the rock itself was formed, whether it was through erosion and deposition (for sedimentary rocks), or from melting followed by solidification (for igneous rocks). Other rocks reveal the history of life as told by the fossils found within them. Yet other geologic stories are of how landscapes form and are changed through time.

On a recent hike in Capitol Reef National Park, I stopped to admire the towering rock walls rising from the canyon floor where I was walking. While I was taking in the sight, my eyes focused on three features: the angled and swooping layering within the rock, the streaky surface coatings, and the holes (pockets) in the cliff face.

I realized then that the particular cliff face I was looking at told at least three different geologic stories and that I was reading the rocks without even really being conscious I was doing so. That brief moment of awareness deepened my appreciation of the landscape around me and made me thankful once again for my geologic training.

Of the three stories revealed on that cliff face, one provided information about how the rock was deposited and the other two about how the cliff developed its geologic good looks.

The swooping rock layering within the otherwise massive cliff only forms in sands deposited by wind in sand dunes. Known as cross-bedding, it forms as wind picks up sand on the windward side of a dune and transports it to the leeward side, where it is deposited in layers parallel to moving dune surfaces. Closer examination of the rock shows that it is indeed made up of very small and round sand grains of the type transported by wind.

Much of the state of Utah and parts of adjacent states was covered by an extensive expanse of sand dunes during the Jurassic Period, about 185 million years ago. Imagine a region somewhat like the sandy deserts of the Sahara, but with dinosaurs and relatives of primitive mammals inhabiting oases that held small lakes. The Navajo Sandstone is also near Moab, but the layer is not nearly as thick here indicating that southeastern Utah was near the edge of the dunefield and that Capitol Reef was closer to its heart.

I next took note of the dark colored streaks and blotches on cliff’s surface and then turned around to view the rock wall just upcanyon which had a more beautifully developed tapestry. These surface coatings in shades of back to brown and red and even to white are collectively called desert varnish. Technically, desert varnish consists of many distinct types of rock coatings, including rock varnish, heavy metal skins, and silica glaze. Desert varnish forms on stable rock surfaces as discrete surface patinas. Rock varnish, especially when it has a high manganese content, and heavy metal skins make the dark colors, and silica glaze creates the subtle light colors. Varnish preferentially forms where water flows during cliff faces during storms, yielding a streaky appearance. Unlike cross-bedding, desert varnish is not an inherent characteristic of the rock itself as it is distinct from it. These surface coatings can form in any environment, but they are prevalent in canyon country because of the arid environment and abundance of stable rock surfaces like in these canyon walls.

Finally, I zoomed in on the abundant cavities or holes in the cliff surface. The ones in front of me were only a few inches in diameter, but others I saw on this hike ranged from the size that a child could climb into to smaller than the diameter of a dime. Geologists term these pockets tafoni and they are a product of weathering wherein cavities are formed in rock outcrops. More specifically, these pockets form through honeycomb weathering, which is the physical disintegration of rock surfaces by the repeated dissolution, precipitation, and expansion of natural salts and similar compounds on the surface due to wetting and drying cycles. Once a pocket begins to form, it generally preferentially enlarges because the depression forms a subtle microclimate and protects the surface salts from washing away during rainstorms. Tafoni formation may follow the rock layering where slight differences in the proportion of cementing agents create softer horizons more susceptible to pocket development.

These three stories in this one cliff are just small vignettes within the larger geologic narratives present in the rocks and landscapes near Moab. Canyon county is not just a great place to hike, adventure, and sightsee, but also a tremendous places to learn about the history of the Earth.

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