Desert Mountain—Utah's latest GeoSight

"now a quiet, remote monument to that violent geological time." (Smith 2025)

Desert Mountain's peaks and ridges lower right quarter of photo; white mark is high point (Google Earth).

Desert Mountain is small mountain—an isolated cluster of low peaks, ridges and knobs in Utah's West Desert. The first geologist to write about it thought "Desert Hills" more appropriate (Loughlin 1920). But its story is huge—complicated and filled with drama. And being geological, it's long.

We could start 300 million years ago, when collisions on both sides of a young North America were deforming it far inland, for example in today's western Utah. Or we could start 300 million years before that, when western Utah was covered in shallow water of the great Paleozoic Sea. Or we could go back yet another 300 million years to the creation of that sea, when the supercontinent Rodinia was coming apart. But we won't. Instead we'll start three months ago, on a hot spring day.

Shortly before I left home, the spring issue of Survey Notes showed up in my mailbox. Inside was a GeoSight—a new one, and in the general area of my travels. Of course I would go there! Of the many resources offered by the Utah Geological Survey (UGS), my favorite is GeoSights. I visited my first, the Honeycombs, in 2012. Awed by the rocks and their story, I've been geotripping to GeoSights ever since.

Sunset on the Honeycombs, 2012.

My visit to Desert Mountain was about 80 miles round trip from Delta. I took UT Hwy 6 north to the Jericho Callao Road, then drove west. Pavement soon gave way to gravel, a bit rough in places but generally good. The road crossed open juniper woodlands and sparse dry grasslands, with expansive playas to the south. After 22.5 miles, with Desert Mountain visible nearby, I stayed left at a junction and was soon at its base.

Approaching Desert Mountain from the north. Kelly Hewitt photo via Google Earth.

Rocks abound, trees not so much; pale band close to road is a fence, covered in tumbleweeds.

The road continued along the base of a steep slope with granite outcrops, then climbed a short distance to Desert Mountain Pass where there was no shade to be had. I parked and reread Jackson Smith's GeoSight article inside the van, cooled by light breezes wafting through opened windows and doors.

Desert Mountain was born c. 30 to 40 million years ago, during the Great Ignimbrite Flareup which ravaged much of Nevada and western Utah. For 15 million years large volcanoes, supervolcanoes and complexes of supervolcanoes (1) produced on the order of 5.5 million km3 of volcanic material—great clouds of ash that blocked the sun, huge volumes of rock fragments hurled hundreds of miles, and searing pyroclastic flows that destroyed everything in their path. For comparison, the 1980 Mount St. Helens eruption produced only one km3 of material (source).

Geologic map shows rock units discussed here. Blue B's mark shoreline of glacial Lake Bonneville, black lines are faults, labeled arrows are mine (Smith 2025).

Today's rock outcrops suggest that the life of the Desert Mountain volcano included three stages. In the first, viscous rhyolite oozing from vents formed thick deposits of lava. Rhyolite outcrops are the remains of this relatively peaceful eruption (not part of my tour).

But while the lava oozed ever so slowly, trouble was brewing below. Gas was accumulating in the viscous magma, increasing in pressure until it literally exploded. Massive amounts of rock fragments and ash were sent flying. These pyroclastic deposits are said to be common east of the mountain, a project for a cooler day.

The eruption largely emptied the magma chamber, causing the roof of the volcano to collapse. The result was a caldera—a very large bowl-shaped depression. When the roof collapsed it broke up into a mishmash of preexisting rocks and erupted material, forming today's volcanic breccia. I may have seen it at Desert Mountain Pass, adjacent to the beautiful pale granite outcrops.

A close look at the volcanic breccia of Desert Mountain (UGS).

Desert Mountain Pass. Is that volcanic breccia behind the granite? I thought so at the time.

From the pass I drove south, using the geologic map to figure out what I was seeing. Occasionally I spotted large outcrops of much darker rock. This is granodiorite, an older intrusion predating the Desert Mountain volcano (age unknown).

Utah Juniper on pale granite; dark granodiorite in distance.

The star of the show, hands down, was the beautiful pale granite, the youngest and most extensive of Desert Mountain's rock outcrops. It was not present during the cataclysmic eruption, arriving later in the volcano's life. In the third stage, remaining magma rose but didn't reach the surface. Instead it cooled deep enough to form visibly crystalline rock—an exceptionally pale granite sometimes called leucogranite (2).

That may have been the final eruption but obviously there's more to the story, for the granite no longer is fully buried. Exhumation started about 17 million years ago, when the part of North America between the Wasatch Mountains and the Sierra Nevada (today's Basin and Range Province) began to stretch east to west. This extension deformed and fragmented older landscapes, including Desert Mountain. The caldera was uplifted, tilted and fractured, allowing erosion to slowly expose and sculpt the lovely granite.

Desert Mountain granite. Sonny Wilson photo, via Google Earth (cropped).

Granite on the west side of Desert Mountain. gjagiels photo, via Summit Post.

Spectacular outcrop at south end of Desert Mountain. Hmmm ... what are those black and white bands?

There's one more chapter in the Desert Mountain story. It's incomplete, difficult to properly place in the overall timeline, and has rock classification issues. But there's also fun to had.

Sometime after the granite was intruded—perhaps while the magma was cooling or later during extension (or both)—molten material filled fractures forming dikes. Whitish aplite dikes formed first, followed by dark andesitic dikes (3). How do we know the order? By their cross-cutting relationships! These are fun to find and worthy of attention for the story they tell. At Desert Mountain, the white dikes cross the pale granite and are therefore younger. The dark dikes cross both the white dikes and the granite and are therefore the youngest of the three.

 Wonderful display of cross-cutting relationships; white arrows mark less conspicuous aplite.

I considered camping at the base of this outcrop but it was much too hot for me. On the drive to Delta, I stopped and took one more photo of the beautiful pale granite. Then I continued south.

See the dike?

Notes

(1) Desert Mountain is part of the Thomas-Keg-Desert mountains caldera complex (DeCourten 2003). The Honeycombs, mentioned early in the post, may be related.

(2) The pale granite at Desert Mountain was called leucogranite early on (e.g., Kattelman 1968). Now "leucogranite" is increasingly used for a pale granite formed in collisional tectonic settings, for example in the Himalayas (E. H. Christiansen, personal communication). For more, see Miller's excellent Perspective (2024). He explains that though collisional is by far the most common tectonic setting for leucogranite formation, it can form in others, including extensional, if certain conditions are met (e.g., composition low in aluminum). In any case, "leucogranite" is used in Jackson's GeoSights article; Christiansen prefers "granite". [Suggestion to UGS: In GeoSights articles, cite a few sources for additional information.]

(3) The dark dikes also are controversial. According to Jackson, the rock "apparently" is very dark lamprophyre—a catch-all term for various peculiar ultramafic rocks not amenable to the usual classifications (source). Christiansen and colleagues prefer andesite.

Sources (in addition to links in post)

I'm grateful to Eric Christiansen, Professor Emeritus at Brigham Young University, for answering my questions about igneous rocks at Desert Mountain, and for his appreciation of cross-cutting relationships.

Brigham Young University. 2013. Supervolcano in Utah: massive ancient volcano discovered by BYU geologists. YouTube.

DeCourten, FL. 2003. The Broken Land; adventures in Great Basin geology. U. Utah Press.

Loughlin, G. F., 1920, Desert Mountain, in B. S. Butler, and others, Ore deposits of Utah: U. S. Geol. Survey Prof. Paper III, 444-445. PDF

Miller, CF. 2024. Granites, leucogranites, Himalayan leucogranites ... Elements 20(6):359–364. doi: https://doi.org/10.2138/gselements.20.6.359

Rees, DC, Erickson, MP, Whelan, JA. 1973. Geology and diatremes of Desert Mountain, Utah. Utah Geological & Mineralogical Survey Special Studies 42. PDF

Smith, J. 2025. Geosights: Desert Mountain, Juab County, Utah. Utah Geological Survey, Survey Notes.

The Monthly Fern??—Prairie Spikemoss

Selaginella densa—moss, fern, fern ally, or none of the above? Coin is 19 mm across.

This month the South Dakota fern series features another oddity—a spikemoss, genus Selaginella. It's even more unusual than last month's Water Clover, for while water clovers are ferns, spikemosses are not, at least not anymore. So where in the greater scheme of plant classification do they belong?

The Prairie or Dense Spikemoss, Selaginella densa, is the more common of South Dakota's two spikemosses. It occurs in the Black Hills and scattered across the west half of the state. If you live in or have wandered across the Great Plains or Rocky Mountains, you may have seen it, for it grows on a wide range of sites—prairies, alpine meadows, dry rocky slopes, rock crevices, sandstone, quartzite or granite rock, and dry gravelly, clayey or sandy soil (Flora North America). Or maybe you overlooked it, as I used to do. After all, it looks very much like a moss (1).

Spikemosses were first classified—given a name and assigned to a plant group—by the great Swedish botanist Carl Linnaeus, founder of today's system of naming organisms. In his Species plantarum (1754) he placed them in the CRYPTOGAMIA MUSCI section—the mosses. That was a big mistake, but at the time it was a reasonable decision. Like mosses, Selaginella produces spores (2). But unlike mosses, it has vascular tissue—plumbing for transporting water and nutrients.

In the late 1890s another Swedish botanist was studying spikemosses, while preparing a Catalogue of the Flora of Montana and the Yellowstone National Park. Per Axel Rydberg had emigrated to the United States in 1882, hoping for a career as a mining engineer. But after a serious injury in an iron mine in Michigan, he moved to eastern Nebraska to teach mathematics. He also studied botany at the University of Nebraska—the beginning of a "lifelong devotion to plant studies" in the Great Plains and Rocky Mountains (source).

In 1895 and 1896, Rydberg was sent to Montana by the US Department of Agriculture to collect grasses and forage plants. The next summer he returned, with the first field expedition of the New York Botanical Garden. He made about 1800 collections representing 800 species—20,000 specimens in all (replicates were collected for exchange or sale to other institutions).

Rydberg's Catalogue included a large foldout map showing localities mentioned in the text. He noted that the eastern half of the state was "practically unexplored botanically." (BHL)

Going through his collections that winter, Rydberg saw that the flora of Montana was poorly known, even with his additions. "It was therefore considered advisable to extend the work and study all the material from the state that was accessible." He examined specimens from 16 institutions and private individuals, ranging from the Lewis & Clark collection (1803–1804) to the Montana Ladies' [Columbian] World's Fair Set (1893). By the time the Catalogue was published in 1900, Rydberg had added 776 species to the flora of the Rocky Mountain region, including 163 novelties—species new to science (Rydberg would become known as a notorious splitter).

Among the novelties was a low densely-tufted plant with very short stems covered in bristle-tipped leaves 3–5 mm long. Fertile stems were taller, to c. 4 cm, with spore-bearing leaves (sporophylls) neatly arranged in four ranks, forming terminal strobili (aka cones).

Prairie Spikemoss forms dense mats in this soil crust. Matt Lavin photo.

Selaginella densa's 4-angled strobili rise above very short sterile stems that look like clusters of bristle-tipped leaves. cinthyadasilva photo.

Rydberg knew the plant was a spikemoss, but the dense "moss-like" form was not something he had seen before. After careful study of seven specimens, he concluded it was a new species, calling it Selaginella densa. The holotype (basis for formal description) was a specimen collected in 1889 by Valery Havard, a French-born American military physician, explorer and botanist.

Havard identified his specimen (NYBG) as S. rupestris, which is widespread in the east half of the US.

In his Catalogue Rydberg followed the accepted classification of the day. He included Selaginella densa in the Pteridophytes—spore-bearing vascular plants, mainly ferns. He put it near the end of the section, with horsetails, clubmosses and other oddballs. These were the Fern Allies. Like ferns they bore spores, had vascular tissue, and reproduced via two distinct independent life stages. But otherwise they were decidedly unfernlike, and very different from each other. Just look below!

Horsetails and scouring rushes, genus Equisteum, have jointed stems with cylindrical sheaths tipped with teeth. These are thought to be highly modified leaves. Spores are born in terminal cones.

Unbranched species of Equisetum are called scouring rushes. Andre Zharkikh photo.

Whisk ferns, genus Psilotum, have linear shoots that fork in the upper half. Minuscule scale-like leaves subtend globose spore containers 2–3 mm across.

Sideways view of a whisk fern. Mary Keim photo.

Quillworts, genus Isoetes, are aquatic, with grass-like clusters of linear leaves. Spores are born in sac-like structures in enlarged leaf bases.

Bolander's Quillworts in a lake in the Wasatch Mountains, Utah. Andrey Zharkikh photo.

Clubmosses, family Lycopodiacee, are a more diverse group, with 7 genera and 27 species in North America. Some are suggestive of spikemosses; in fact spikemosses were put in the genus Lycopodium by Linnaeus.

These clubmosses, all formerly genus Lycopodium, are now 4 separate genera; from Ferns and Evergreens of New England, 1895 (BHL).

The Fern Allies group came into use in the early 1800s, as a catchall for diverse, puzzling, somewhat fernlike plants. But after about a century botanical experts began to object. Some Allies appeared to be more closely related to ferns, others not so much. Then less than a century later, the Allies got caught up in a revolution. Biologists were switching to a phylogenetic approach to classification. In a nutshell (a very tiny one), they now hope to classify organisms based on evolutionary relationships, i.e., so that all members of a group share a common ancestor. The Fern Allies do not, so they were reclassified (3).

The commonly accepted classification splits the Allies into two groups that diverged long ago, early in the evolution of vascular plants. One includes ferns, horsetails, whisk ferns and seed plants. The other group is much smaller, a collection of relatively primitive plants: quillworts, clubmosses and spikemosses. These are lycophytes (answer to question at top of post). For a longer summary, see The Ferns and their Allies at Cliffnotes. For a deep discussion, start with Pteridophyte taxonomy on Wikipedia.

Fern and lycophyte classification from the Pteridophyte Phylogeny Group. Black labels added, not sure how that guy in the corner snuck in.

Notes

(1) Not all spikemosses are as humble and mosslike as ours. Selaginella is a large genus with c. 800 species, mainly of the tropics and subtropics. In hospitable habitat, spikemosses can be quite showy—some are iridescent!

Selaginella uncinata, Blue Spikemoss, is native to moist shady sites in southern Chile and is widely cultivated; leaves are 3–4 mm long (Flora North America). GKA Dickson photo.

(2) Actually Linnaeus couldn't decide whether "fern dust" was pollen or seeds. The concept of spores would come later.

(3) It's been really hard to give up Fern Allies! It's such a handy label for those diverse kinda-fernlike species. Not surprisingly, the name hasn't gone away. Sometimes it appears under an alias, for example "Fern Relatives" in Ferns of Northeastern and Central North America (2005). More often it pops up in casual conversation, or is used by older botanists who haven't bothered to learn the new scheme. After resorting to "Fern Allies" in a message to pteridologist Robbin Moran, I committed to learning it (Robbin is much too kind to disapprove directly, but he did refer to "lycophytes" in his reply).

Sources (in addition to links in post)

Cobb, B, Farnsworth, E, Lowe, C. 2005. Ferns of Northeastern and Central North America. 2nd ed. Peterson Field Guide Series.

Linné, Cv. 1754. Species plantarum v2. BHL

Moran, Robbin. 2004. A Natural History of Ferns. Timber Press.

Rydberg, PA. 1900. Catalogue of the Flora of Montana and the Yellowstone National Park. Memoirs of the New York Botanical Garden. BHL