those birds unknown, that left only footprints

Big toes! Huge bird!!

Along the rim of Bull Canyon, on the north slope of the La Sal Mountains in southeast Utah, my field assistant and I followed footprints in sand. Three-toed two-footed creatures had passed this way when the sand was wet, before it turned to rock. They were common then, traveling in packs.

Similar 3-pronged impressions occur 2000 miles to the east, in the Connecticut River Valley in Massachusetts. Some are famous—among the earliest to be studied and published. They were found in 1835 by Dexter Marsh, who was laying a flagstone sidewalk. He showed the slabs to the owner of the property who gave them to a physician who then gave them to state geologist Edward Hitchcock.

"They consist of two slabs, about forty inches square, originally united face to face; but on separation, presenting four most distinct depressions on one of them, with four correspondent projections on the other; precisely resembling the impressions of the feet of a large bird in mud." (Hitchcock 1836, italics mine)

Sandstone slabs, each 36.5" x 34"; depressions (molds) on left, projections (casts) on right (source, click on "Fossil Slabs Found by Dexter Marsh").

Hitchcock was understandably excited. Very few bird fossils had been found anywhere, and geologists had decided that because most birds were lightweight creatures of the air, they were unlikely to be submerged and preserved on the bottom of lakes, oceans and such. "Even when they chance to perish in the water, they float so long upon the surface, as to be most certainly discovered, and devoured by rapacious animals."

As it turned out, such marks were fairly common in the area. Hitchcock studied slabs from five quarries, concluding that the impressions must have been made by birds:

1. These impressions are evidently the tracks of a biped animal. For I have not been able to find an instance, where more than a single row of impressions exists.

2. They could not have been made by any other known biped, except birds. On this point, I am happy to have the opinion of more than one distinguished zoologist.

3. They correspond very well with the tracks of birds.

Some of Hitchcock's drawings of modern-day bird tracks, from his 1836 publication (source).

However, many of the tracks Hitchcock studied were too large to have been made by the birds we know—to 18 inches long and 13 inches wide, and separated by 6-foot strides. Therefore these tracks must have been made by large birds now extinct. Eminent geologists of the day agreed with Hitchcock (2). But by the end of the century they had been "proven" wrong. These tracks were not avian, they were reptilian (Dean 1969).

Science marches on of course, and we now know that in a sense, Hitchcock and his colleagues were correct. The creatures that left footprints in the Connecticut River Valley and southeast Utah were indeed birds. But they also were dinosaurs, specifically theropods (study the images below before discussing this at cocktail parties).

Dinosaur classification (3). While all birds are dinosaurs, not all dinosaurs are birds; similarly, birds are a subset of theropods (from Zureks).

Evolution of birds from a dinosaur ancestor; Manti-La Sal National Forest, Bull Canyon Tracksite.

You might be wondering why dinosaurs traveled the rim of Bull Canyon. Well ... actually they didn't. There was no Bull Canyon 157 million years ago. Instead this was a broad coastal plain, where large bipeds could cruise along at 2.5–3.5 mph (Hunt-Foster 2016).

The Bull Canyon Tracksite includes at least 50 well-preserved large theropod tracks, to 18 x 14 inches in size. But aside from footprints little is known about these creatures, for no bones have been found. So rather than naming a species, paleontologists named their tracks: Megalosauripus. These are ichnofossils—"a fossil record of biological activity by lifeforms but not the preserved remains of the organism itself." They're also called trace fossils, the term I learned.

 Theropods passed this way, in a pack perhaps.

They were big! (40-pound dog for scale).

Megalosauripus is a theropod track, not the theropod itself.

The wet sand where theropods once walked is now sandstone, part of the Moab Member of the Curtis Formation, dating from 157 million years ago (Late Jurassic). The setting was dynamic—changing sea level, oscillating shoreline, occasional sand dunes—making classification and dating of rock units difficult (Mathis 2021). But no matter. Whatever geologists decide to call the rock, its theropod tracks go on and on and on. They occur across Arches National Park, east to the Bull Canyon area and the Colorado–Utah state line, and perhaps as far south as Blanding. This is the Moab Megatracksite, also known as the Dinosaur Freeway. A conservative estimate of its size is 700 square miles; as of 2016, c. 3000 tracks had been reported from 30 sites (Hunt-Foster 2016).

Dinosaur Stomping Grounds, aka Jurassic Dancefloor, with at least 2000 theropod tracks (Sierra Club).

Our visit to the Bull Canyon Tracksite last fall was but a brief introduction. Many more opportunities to commune with large extinct birds await. Fortunately many of the sites are on public land, and there's a handy guide available (Hunt-Foster 2016). We shall return!

Dreaming of giant birds after a day in the field.

Notes

(1) The title of this post comes from a poem by Henry Wadsworth Longfellow—To the Driving Cloud. He referred to fossil bird tracks in several poems (Dean 1969).

(2) Edward Hitchcock was a cleric and amateur geologist. In his time, so-called amateurs made major contributions, his study of fossil birds being a good example. Charles Lyell confirmed Hitchcock's findings, and included them in lectures and later editions of Principles of Geology. Louis Agassiz and others also spread the word (more in Dean 1969).

(3) Are you wondering, as I did, why birds are NOT included in the seemingly eponymous Ornithischia (lower right in diagram)? That group includes dinosaurs with hips that superficially resemble those of birds. Maybe the name predates the realization that birds evolved from a theropod.

Sources (in addition to links in post)

Dean, DR. 1969. Hitchcock's Dinosaur Tracks. American Quarterly 21:639–644. https://www.jstor.org/stable/2711940

Hitchcock, E. 1836. Ornithichnology—description of the footmarks of birds, (Ornithichnites) on New Red Sandstone in Massachusetts. American Journal of Science and Arts, XXIX:307-40. Internet Archive.

Hunt-Foster, RK, et al. 2016. Tracking Dinosaurs in BLM Canyon Country, Utah. Utah Geological Association, Geology of the Intermountain West, Vol. 3. PDF

Mathis, A. 2021. Moab, Goblin Valley, and the Curtis Formation. Moab Happenings Archive.

A Restless Region on the Colorado Plateau

Castle Valley, 20 miles east of Moab, UT; Round Mountain rises from the valley floor on the right.

The Colorado Plateau is a thick block of crust in the Four Corners area of the American Southwest, an immense stack of sedimentary rock. It's remarkably stable, remaining a region of geological calm even when severe deformation was underway right next to it—uplift and faulting of the Rocky Mountains to the east, and stretching and breaking of the Basin and Range Province to the west. This is why the Plateau's sedimentary rocks are largely horizontal, like the deposits they once were (source).

This is not to say the landscapes are boring. In fact they're spectacular—sweeping vistas with colorful roughhewn features. For the last 10 million years the Plateau has been rising, invigorating streams and accelerating erosion. The result is a seemingly endless collection of buttes, arches, rimrock, fantastical spires, deep winding canyons, and more.

The Colorado Plateau covers c. 130,000 sq mi; note complex topography in adjacent areas (source unknown).

Valley of the Gods near Bluff, UT; vertical and horizontal erosional features are common on the Colorado Plateau.

Entrenched meander of the San Juan River, cut through horizontal strata; Goosenecks State Park.

Looking down Castle Valley. Is this a standard Colorado Plateau landscape? Bryant Olsen photo.

Castle Valley appears to be dominated by vertical and horizontal features, as is typical on the Colorado Plateau. Is it another example of recent uplift and erosion? Only partly. Its story is much more complicated—repeated flooding, prolonged deposition, unusual deformation, weird intrusions, and finally ... collapse.

About three hundred million years ago, not far east of today's Castle Valley, the great Uncompahgre Range was rising. At the same time the Paradox Basin was subsiding along the range's base, and filling with sediments eroded off the mountains. Critical to the Castle Valley story, seas repeatedly flooded the Basin (1). Then whenever sea level dropped, the saltwater left behind evaporated and deposited evaporites, including lots of salt (halite). At least 29 such cycles took place over a period of at least 8 million years. The result was extensive deposits of evaporites at least 4000 ft thick—the Paradox Formation (2).

Ancestral Rocky Mountains c. 300 million years ago; darker blobs are major ranges; Castle Valley location approximate (modified from Soreghan et al. 2009).

Being evanescent creatures, it's difficult for us to think of geologic structures as ephemeral. But the rock record clearly shows that they are. Even mountains have lifetimes. The great Uncompahgre Range is now gone, razed by erosion. The Paradox Basin also disappeared, filled to overflowing with sediments and then deeply buried. But in a sense both are still with us. Uncompahgre sediments are widely displayed in colorful rocks across the Colorado Plateau. And the Paradox Basin maintains a ghostly presence—dramatic, but difficult to explain.

After deposition of the last Paradox evaporites, the region was inundated by tropical seas—source of the impressive layers of limestone, sandstone, siltstone and shale that line Plateau drainages. Then about 200 million years ago, when the supercontinent Pangaea started to come apart, there was a shift to terrestrial deposits—dune sand, volcanic ash, and sediments from rivers, lakes, and inland seas. Under this immense "lithic layer cake" lay the Paradox salt, deeply interred but not dead (source).

Salt is a strange kind of sedimentary rock. Sediments such as sand and mud can be compressed to form dense rocks, but salt remains nearly unchanged under pressure. It's weaker and less dense than the rock around it, but also plastic—with properties of both solids and liquids. It can flow to escape from its "stressful surroundings", deforming any rocks in its way (source). Deformation can occur at the scale of landscapes, and the old Paradox Basin has many fine examples: fins and arches of the Fiery Furnace, closely-spaced meanders on the Colorado River, and at least seven parallel northwest-trending valleys, including Castle Valley.

Dotted elongate blobs are parallel northwest-trending valleys. Blue circle marks intriguing overlap of Castle and Spanish Valleys with La Sal Mountains (3). Modified from Doelling 1985.

Castle Valley begins at the base of the northern La Sal Mountains and extends northwest about 12 miles. It's a broad valley, to 2 miles wide. The southwest wall is capped by a nearly continuous outcrop of the erosion-resistant Wingate Sandstone, known as Porcupine Rim. On the other side of the valley, the wall is less continuous but equally dramatic—carved into mesas, buttes, rimrock and spires.

Porcupine Rim—Jurassic Wingate Sandstone caps southwest wall of Castle Valley.

Northeast side of Castle Valley; strata tilting away from valley center visible at arrow.

Geologists find Castle Valley intriguing. The floor is much broader than would be expected for its little streams, and quite flat. Rock layers on both sides of the valley tilt down away from the valley center (more easily seen on the northeast side, photo above). Most exciting is what lies beneath the surface. Wells drilled in the center of the valley revealed a long steep-sided bed of salt to 1000 feet thick! Castle Valley must be a salt anticline, an elongate convex uplifted fold cored by salt. On this geologists agree. But as to how it formed and what happened to it ... that's another matter.

Castle Valley (Google Earth). But where's the anticline?! Valley walls hint at what happened.

The diagram below shows a common explanation for salt anticlines. In the top panel, flowing salt accumulates to form a convex fold, pushing up overlying rock layers. Castle Valley salt is thought to have flowed and formed an anticline 300 to 200 million years ago (Ornduff 2006, Trudgill 2011).

Salt anticline in cross section; at the time of the top panel, Castle "Valley" would have been a long ridge.

Now the Castle Valley anticline is mostly gone. The second and third panels in the diagram show a possible demise, but first, a major disturbance very close by needs to be considered—uplift of the La Sal Mountains about 28–25 million years ago (3).

The La Sals are not a mountain range but rather clustered peaks. They're similar to volcanoes except that magma never reached the surface. Pioneering geologist AC Peale called them "eruptive mountains of a peculiar type ... igneous and yet non-volcanic". Recent studies indicate that magma stopped just 1–3 miles below the surface, making them shallow intrusions, specifically laccoliths.

La Sal Mountains rise 8000+ feet above the Colorado Plateau—a major disturbance! (source)

La Sal high country: La Sal Peak (right) is intruded trachyte; Castle Mountain (left) is still capped with sedimentary rock (Ross 1998).

Now we're faced with another question. If magma never reached the surface, why are the La Sal "intrusions" visible? Instead of being 1–3 miles below the surface, their tops stand over a mile above the Plateau. The likely answer is the recent uplift and erosion of the Colorado Plateau mentioned at the beginning of this post.

Starting about ten million years ago, both the Colorado Plateau and the Basin and Range Province (to the west) have been rising. But while the latter was stretched and faulted, forming its eponymous basins and ranges, the Plateau remained a single block. Eventually it rose about kilometer higher than the Basin and Range. Why? That's a puzzle not yet solved (source). In any case, streams were steepened and invigorated, and erosion sped up enough to reveal the La Sal laccoliths.

Recent uplift and erosion probably explain the demise of the Castle Valley anticline as well. Erosion and/or faulting of overlying rock would have exposed the salt to water. Being salt, it of course dissolved. When enough was removed, rock layers at the crest fractured and collapsed, creating a breached anticline. But others think differently. Regional extension may have been the cause, perhaps related to ongoing extension in the Basin and Range Province. Or as Naqi et al. (2016) safely concluded, "formation of the salt valleys might be attributed to multiple factors (i.e., extensional forces, salt dissolution, and internal salt flow) rather than a single mechanism."

A salt anticline's demise may start with salt dissolution, followed by collapse of rock layers at the crest.

Breached anticline; dashed line shows former continuity across crest (Grabau 1920, A Textbook of Geology).

Let's visit!

Castle Valley is a great destination for geotrippers. Enough remains of the anticline to see and appreciate what happened. The tilted rock layers of the flanks are now the valley walls. Imagine them reaching higher and arching across the broad floor. Consider the depth of the valley below the now-imaginary crest and think about how much salt and rock must have been removed! Then look toward the head of the valley, at the dark hill rising from the floor. That's Round Mountain—a little relative of the La Sal intrusions. It was exposed when the anticline was breached and deeply eroded.

Castle Creek Road (paved) runs the length of the valley, providing easy access. Spires, buttes and mesas on the northeast side can be reached from several parking areas. Round Mountain is a short distance south of Castle Valley Road via a rough eroded 2-track; I parked just off the paved road and walked. Tour the southwest side via the Porcupine Rim trail—highly recommended, though maybe not on weekends and holidays.

Castleton Tower is a short hike from Castle Valley Road.

La Sals on left, Round Mountain on right, rabbitbrush in foreground.

En route to Round Mountain, Porcupine Rim beyond.

Looking up Castle Valley from Porcupine Rim, La Sal Mountains on horizon; redrockrubi.

Refreshing shade, courtesy uplift and erosion of the Colorado Plateau.

Notes in addition to links in post

(1) Cyclicity of Paradox deposition is well documented, but the cause is debated. Glaciation seems to be most popular, specifically sea level change with alternating glacial and interglacial periods. Other possibilities include rise of the Uncompahgre Range and climate change. Trudgill (2011) concluded that glaciation-driven changes in sea level was the main cause; tectonics and/or climate change may have made lesser contributions.

(2) In my reading, I found a range of estimates for Paradox deposition: 29 or 33 cycles over 8 to 15 million years, producing evaporites 4000, 6000 or 8000 ft thick (Ornduff et al. 2006, Trudgill 2011, USGS).

(3) Geologists have long wondered whether the La Sal Mountain intrusions and salt anticlines such as Castle Valley are related. Thomas Harrison, who surveyed the Paradox Basin area in 1926, discussed the possibility in his report (1927):

"It is interesting to know that igneous intrusive rocks of very considerable importance are closely associated with the saline anticlines. The laccolithic La Salle Mountains occupy an area on and between two parallel anticlines ... A small isolated igneous stock [Round Mountain] surrounded by gypsum occurs in the Castle Valley salt [anticline]."

Harrison noted that salt anticlines were thought to be associated with lines of weakness dating from Precambrian time. Their uplift was followed by subsidence, and massive accumulation of sediments. Perhaps this "heavy load" generated heat that "liquefied rocks within the zone of fracture, resulting in the [magma] which formed the laccolith, and the Castle Valley stock" [Round Mountain].

Today's geologists may chuckle at the idea of a "heavy load" of sediments melting igneous rock below. In contrast, "lines of weakness dating from Precambrian time" are taken seriously. Many northwest-trending faults cut basement rocks in the Paradox Basin area. Ross (1998) concluded that "the locations of the La Sal Mountains intrusive centers along the trend of subsurface faults ... suggest that the faults were avenues of weakness for the ascent of magma in the upper crust. This is especially true for the northern and southern clusters of peaks [which coincide with salt anticlines]".

Sources

Doelling, HH. 1985. Geology of Arches National Park. Utah Geological Survey, to accompany Map 74. PDF

Harrison, TS. 1927. Colorado–Utah Salt Domes. Am. Assoc. Petroleum Geologists 11:111–133.

Ornduff, RL, Wieder, RW, Futey, DG. 2006. Geology Underfoot in Southern Utah. Mountain Press Publishing. For salt anticlines see Vignette 29, A Sea of Fins; for La Sal Mountains see Vignette 32, Intruders in a Sedimentary Domain.

Ross, ML. 1998. Geology of the Tertiary intrusive centers of the La Sal Mountains, Utah; influence of preexisting structural features on emplacement and morphology, in Laccolith complexes of southeastern Utah; time of emplacement and tectonic setting. USGS Bull. 2158: 61-83. PDF

Snyder, NP. 1996. Recharge area and water quality of the valley-fill aquifer; Castle Valley, Grand County, Utah. Report of Investigation 229. Utah Geological Survey. PDF

Soreghan, GS, et al. 2009. Hot fan or cold outwash? Hypothesized proglacial deposition in the upper Paleozoic Cutler Formation, western tropical Pangea. J. Sed. Res. 79:495-522.

Trudgill, BD. 2011. Evolution of salt structures in the northern Paradox Basin: controls on evaporite deposition, salt wall growth and supra-salt stratigraphic architecture. Basin Research 23:208–238. https://onlinelibrary.wiley.com/doi/10.1111/j.1365-2117.2010.00478.x

US Geologic Survey. Mid 2000s. Geologic Provinces of the United States: Colorado Plateau Province. Internet Archive WayBackMachine.

Peculiar Eruptive Mountains on the Colorado Plateau

The La Sal Mountains rise 8000+ feet above the Colorado Plateau (source).

In 1875, two geologists employed by the US government were studying mountains in southeast Utah. They worked 90 miles apart, each one in an isolated cluster of peaks rising above the mostly horizontal Colorado Plateau. They found the same strange type of structure and the same kinds of igneous rocks, and in their reports published two years later, they reached the same conclusions.

Albert Charles Peale was an employee of the War Department, specifically the Geological and Geographical Survey of the Territories led by Ferdinand Vandeveer Hayden. His party—one geologist, two topographers, two packers, and a cook—was surveying the Grand River District in western Colorado and eastern Utah (1). They spent a week in the Sierra la Sal, "which afforded magnificent opportunities for work" and then headed south. But hostile locals ("Indian trouble") brought field work to a sudden end. In their hasty exit, "all [rock] specimens had to be abandoned."

Grove Karl Gilbert was an employee of the Department of the Interior, specifically the Geographical and Geological Survey of the Rocky Mountain Region led by John Wesley Powell. On his descents of the Colorado River, Powell had seen an unmapped cluster of peaks to the west, which he named the Henry Mountains (2). They looked volcanic—domed, with dark lava-like rock on the top. Volcanology was a young science then, and geologists were debating whether volcanos were elevated craters or built from accumulated lava. So Powell sent Gilbert to the Henrys to "determine the facts" (Hunt 1988). He and his party stayed two months, more than enough time to answer the volcano question.

Peale worked in the La Sals, Gilbert in the Henrys; map based on data from National Atlas, labels added.

That winter Peale wrote up his findings, but two years would pass before Geological Report on the Grand River District was published (3). He estimated they had surveyed 6000 square miles of which "the greater part ... is plateau in character, the Sierra la Sal being the only mountain group." It was an isolated cluster of about 30 peaks arranged in three "eruptive centers". Peale was emphatic about origins: "there can be no doubt of the eruptive character of the mountains... porphyritic trachyte has been pushed up through the sedimentary layers which now dip away from the mountains" (Peale 1877a).

Peale called the La Sals "eruptive mountains of a peculiar type ... igneous and yet non-volcanic" (1877b). They were non-volcanic because lava didn't reach the surface. But neither were they plutons emplaced deep underground. Instead, magma had stopped somewhere in between, deforming the overlying rocks. The intruded rock was exposed much later by erosion. There was no name for this type of structure, so he described it in detail, pointed out its peculiarity, and left it at that.

Sections across Sierra la Sal showing tilted sedimentary strata on intruded trachyte (Peale 1877a, cropped).

From the Sierra la Sal, Peale studied the Henry Mountains off to the west. He knew John Wesley Powell (Gilbert's boss) thought they were volcanic—"the summits of these mountains mark in reality the level of former valleys down which the volcanic material flowed" (Powell 1875, quoted by Peale). But even from ninety miles away Peale could see that was incorrect. "l am inclined to class the Henry Mountains with the Sierra la Sal and Abajo [Mountains], as their outline is similar ..."

From his vantage point in the heart of the Henrys, Gilbert "agreed" with Peale (unknowingly). The peaks were neither elevated craters nor accumulated lava nor even volcanic. In fact they were a novel type of structure, as he warned his readers:

"If the structure of the mountains be as novel to the reader as it was to the writer, and if it be as strongly opposed to his preconception of the manner in which igneous mountains are constituted, he may well question the conclusions in regard to it while they are unsustained by proof. I can only beg him to suspend his judgment until the whole case shall have been presented." (Gilbert 1877)

Gilbert gave the novel structure a name—laccolite—thereby making the Henry Mountains the type locality for laccoliths (today's term). He distinguished them from volcanic eruptions, where lava reaches the surface and accumulates. "The lava of the Henry Mountains behaved differently ... it stopped at a lower horizon, insinuated itself between two strata, and opened for itself a chamber by lifting all the superior beds."

Gilbert's sections across the familiar Mountain of Eruption (volcano) and the novel Laccolite.

Like Peale, Gilbert had to wait two years for publication of his findings. He finished his monograph the winter after his second season in the Henrys. "It was at once put in type, and in anticipation of a speedy issue the current year [1877] was marked on the imprint..." But the many illustrations caused delays. Geology of the Henry Mountains was finally bound and distributed in 1879.

By that time a wealth of information about igneous mountains had accumulated, prompting Gilbert to prepare a second edition (1880). It differed from the first mainly in the addition of an Appendix: Recently Published Descriptions Of Intrusive Phenomena Comparable With Those Of The Henry Mountains. At the end of the section about Peale's findings in the La Sals, Gilbert concluded, "All of these features are paralleled in the Henry Mountains and they leave no reasonable doubt that the structures are identical."

I visited the Henry Mountains in 2012, accompanied by the spirit of Grove Karl Gilbert. I camped at Starr Springs as he had, and hiked to the spectacular south face of Mount Hillers, "revetted by walls of Vermilion and Gray Cliff sandstone" as he explained.

South face of Mount Hillers—steeply tilted sandstone on flanks, intruded trachyte on crest (Jack Share).

Vermillion sandstone "tilted almost to the vertical".

Since then, I've been keen to visit more of the peculiar eruptive mountains on the Colorado Plateau. Last September I finally did, spending a week in the Sierra la Sal.

La Sals upper right; snow highlights 3 clusters of peaks. Upheaval Dome upper left. (Google Earth)

Peale's three eruptive centers live on, though they're now called intrusive centers ("eruptive" means volcanic). But these are special intrusions—emplaced at depths intermediate between volcanos (surface) and plutons (deep). They now have their own descriptor—hypabyssal, aka subvolcanic (but still laccoliths).

Two hypabyssal intrusion-cored peaks: Castle Mountain (left) retains a cap of sedimentary rock; La Sal Peak (right) is trachyte (Ross 1998, cropped).

The three intrusive centers of the La Sal Mountains are conveniently named northern, middle, and southern (4). All normally are accessible via the paved Loop Road, but road construction kept me in the northern one. From a very nice small primitive campground, I hiked to see what I wanted to see—the distinctive features of these peculiar eruptive mountains.

It was a short walk to Castle Valley Overlook with views of the Colorado Plateau. The Plateau doesn't look horizontal, but the rock layers are. The spectacular landforms—towers, buttes, rims. deep winding canyons—are erosional. True uplifts like the La Sal Mountains are uncommon. Gilbert called them "disturbances in a region of geological calm."

Looking northwest near Castle Rock Overlook; in the valley bottom left of center is Round Mountain, a small intrusion perhaps connected to the La Sals.

Let's head on down and see what we can see.

Among Peale's important observations were tilted sedimentary strata that "now dip away from the mountains". He concluded they were pushed up and tilted by rising magma. The hike provided good views of steeply tilted sedimentary rocks.

Nearly vertical beds of reddish sedimentary rocks below trachyte slopes of Grand View Mountain (left); high peaks visible on horizon just right of center.

With part of the Loop Road closed, the high peaks weren't easy to access. So the next day we hiked up a rough dirt road to view trachyte. It's common above the flanking sedimentary rocks, forming steep slopes and discouraging travel as Peale noted. "The only difficulty met with in the study of this interesting region is the great amount of debris that has accumulated ..."

Some kind of outcrop (could be rhyolite) beyond steep slope of "debris".

Trachyte with a dark xenolith—country rock broken off and carried up by magma. 

Fall colors on trachyte.

Mount Peale, a large laccolith and high point of the La Sals (Suffusion of Yellow).

Peale was hesitant to identify the igneous rock of the La Sals, without specimens to give to petrologists for "critical examination". But it looked very much like rock he had seen in similar intrusions in Colorado. So he assigned it to a general category—porphyritic trachyte. It seems trachyte was the accepted name for shallow intrusive rocks low in silica in Peale's time (see Appendix in Gilbert 1880). Now it may be trachyte or diorite, depending in part on whom you ask (5). Being very much a 19th century naturalist at heart, I will stick with trachyte.

On the other hand, everyone agrees the rock is porphyry—visible crystals (phenocrysts) in a fine-grained matrix of trachyte. This is a very cool rock, with an interesting history. As the magma rose it gradually lost heat, eventually dropping to a temperature where hornblende and plagioclase formed crystals. This changed the composition of the remaining molten magma, and when it stopped c. 6–10 km below the surface, it rapidly crystallized to form the trachyte matrix (Ornduff et al. 2006; see also Fractional crystallization).

Porphyritic trachyte mementos from Henry Mountains (left) and La Sal Mountains.

As I drove away from the La Sals, I thought a lot about the pioneering geologists of the American West. Like me, they were inspired by geology and the beauty of the landscapes, but their geotripping was very different. Travel (route-finding required) and camping were much more challenging. And where specifically should they go? (no guidebook). However they had the promise of discovery, which surely made up for all the hardships!

AC Peale and two unidentified men, probably during the
Geological & Geographical Survey of the Territories (Smithsonian Archives).

Notes

(1) The Grand River was the section of the Colorado above the confluence with the Green. Its name was changed in 1921.

(2) Powell named the cluster of peaks for Joseph Henry of the Smithsonian Institution, who helped secure funding for Powell's exploration of the Colorado River.

(3) Publication of Peale's report was delayed through no fault of his own. As his boss, FV Hayden explained, it was caused by "the great increase of labor incident to the International Exposition at Philadelphia", labor that would have gone toward preparation of reports. In general, the regular Reports of the Geological and Geographical Survey of the Territories were inadequate for sharing discoveries. In 1874, a new publication—Bulletins— was created to "publish without delay ... new or specially interesting matter". Peale had an article in Bulletin No. 3, about the peculiar eruptive mountains of Colorado and adjacent Utah, including the La Sals (1877b).

(4) Some sources refer to the La Sal intrusive centers as composite plutons or coalesced intrusions.

(5) Ross (1998) reported that La Sal igneous rocks were 59–71% Si02 (silica), and called them trachyte based on "the Total Alkali-Silica classification of LeBas and others". Wilson and others (2016, based on reports from 1953, 1959, and 1992) reported that igneous rocks of the Henry Mountains were 58–63% SiO2, and called them diorite. (Thanks to Mike for taking a stab at trachyte vs. diorite.)

Sources

Bartlett, RA. 1962. Great Surveys of the American West. Norman, OK: University of Oklahoma Press.

Fillmore, R. 2011. Geological evolution of the Colorado Plateau of eastern Utah and western Colorado. Univ. Utah Press.

Gilbert, GK. 1877. Report on the Geology of the Henry Mountains. GPO. BHL.

Gilbert, GK. 1880. Report on the Geology of the Henry Mountains. 2nd edition. GPO. Google Books PDF. Appendix p 153–161 contains added material about igneous mountains.

Gould, LM. 1927. Geology of the La Sal Mountains, Utah Papers of the Michigan Academy of Science, Arts and Letters Vol. 7: 55-106. HathiTrust

Hunt, CB. 1958. Structural and igneous geology of the La Sal Mountains, Utah. USGS Professional Paper 294-1. PDF

Ornduff, RL, Wieder, RW, Futey, DG. 2006. Geology Underfoot in Southern Utah. Mountain Press Publishing. (see Vignette 32, Intruders in a sedimentary domain)

Peale, AC. 1877a. Geological report on the Grand River District, in Hayden, FV. Ninth annual report of the United States Geological and Geographical Survey of the Territories (p. 31–101). BHL

Peale, AC. 1877b. On a peculiar type of eruptive mountains in Colorado. Art. XVIII in US Geological and Geographical Survey of the Territories Bulletin No. 3: 551–564. BHL

Powell, JW. 1875. Exploration of the Colorado River of the West. Geographical and Geological Survey of the Rocky Mountain Region (Henry Mts p. 200-203).

Ross, ML. 1998. Geology of the Tertiary intrusive centers of the La Sal Mountains, Utah; influence of preexisting structural features on emplacement and morphology in Laccolith complexes of southeastern Utah; time of emplacement and tectonic setting. USGS Bull. 2158: 61-83. PDF

Wilson et al. 2016. Deformation structures associated with the Trachyte Mesa intrusion, Henry Mountains, Utah, Implications for sill and laccolith emplacement mechanisms. J. Structural Geology 87: 30-46. free online

Upheaval Dome, the most peculiar structural feature in southeast Utah

Severely contorted innards of Upheaval Dome.

Last September I visited two anomalous features on the generally orderly Colorado Plateau. The first was a cluster of igneous peaks in a sedimentary setting, to be the subject of a later post. I'm starting with the second—a large round hole in the ground. I thought it would be simpler.

The Colorado Plateau covers c. 130,000 sq mi in the Four Corners region in the southwest USA. For the last six million years it has been slowly rising, and yet it's remarkably stable, with limited deformation (NPS). Its wonderful landscapes are largely erosional, dominated by horizontal and vertical features. No wonder Upheaval Dome stands out.

Valley of the Gods shows the horizontal/vertical nature of the Colorado Plateau.

The remarkably round structure center left is Upheaval Dome (Google Earth; annotations added).

"Upheaval Dome" may seem an inappropriate name for a hole. However the rock layers surrounding it are indeed tilted, and though there's no top, geologists agree this is a dome. But what heaved it up is another matter. Wildly different theories have been debated for at least a century.

A decapitated dome with tilted strata encircling highly contorted rock; UGS photo, annotations added.

I would have loved to read the thoughts of the first geologists to peer over the rim! Surely they were surprised. But it appears this dome wasn't described until 1927, by which time it was already known to geologists—as Christmas Canyon Dome.

In the summer of 1926, petroleum geologist Thomas Harrison surveyed the area between the Colorado and Green Rivers, known to be underlain by a thick layer of salt. Elsewhere in the world buried salt created reservoirs for petroleum. Maybe this salt did too.

In his report Harrison explained that many of the domes he examined were gentle folds deserving further exploration. However there was a dramatic exception: "one discovered by Marland [Oil Co.] geologists shows a remarkable and very unusual development. This is the Christmas Canyon Dome." Harrison described a "sharp and highly distorted crest" and a "trough which closely and completely circumvents it" but that was all (1).

How could anyone look at Upheaval Dome and limit themselves to a one sentence description?! Perhaps Harrison didn't visit it himself, relying instead on discussions with Ben Parker of Marland Oil, who supplied a map and diagram.

Ben Parker's diagram of Christmas Canyon Dome; note steepness of deformed strata.

Harrison was not the only geologist working between the Green and Colorado Rivers in 1926. Edwin McKnight of the US Geological Survey was there too—mapping topography, describing and mapping rock units, investigating geologic structures, and assessing potential for manganese, oil and gas, among other things. After finishing in early summer of the next year, he promptly prepared a preliminary report. But the final report was delayed "by the assignment of the writer to other projects." Geology of Area between Green and Colorado Rivers was finally published in 1940, by which time Christmas Canyon Dome had become Upheaval Dome (2).

McKnight devoted five pages to Upheaval Dome, "the most peculiar structural feature in southeast Utah". He described it from the center outward. An interior conical dome, circular at the base, is surrounded by a ringlike syncline (narrow valley) and, beyond that, a circular ridge about a half mile wide. "The complete diameter of the affected area is 3 miles."

From map accompanying McKnight's report; Upheaval Dome is the tightly concentric red contour lines, 100 vertical feet apart (3).

The slopes of the interior dome were steep, generally 40–60º. But they ended prematurely, and the dome's summit was gone. Instead a large hole revealed spectacularly contorted innards. Here, McKnight couldn't hide his excitement:

"The White Rim member does not occur in place but appears as huge up-ended blocks the size of a house in the highly disturbed area of jagged pinnacles at the center of the dome. Surrounding this is the Moenkopi, very much crumpled and dissected by numerous gullies. The Shinarump [now part of the Chinle] forms a jagged fringe to the Moenkopi, its huge tilted triangular blocks sticking up like the teeth of a saw." [names refer to rock layers]

Huge tilted triangular block of the Shinarump sticking up like the tooth of a saw.

Just as amazing, though not so dramatic visually, are the rock layers immediately beyond the outer rim. They're horizontal! Intense deformation had been highly localized.

From McKnight's cross-section, labeled arrows added; note horizontal layers beyond the dome, and question marks inside it.

McKnight attributed the rise of Upheaval Dome to the thick layer of buried salt below, the one that drove Harrison's search for petroleum. It was deposited 300 million years ago in the great Paradox Sea, an inland sea sometimes connected to the ocean, sometimes not. When sea level dropped sufficiently, it was sucked dry by evaporation leaving thick salt deposits. Then the sea returned. There were on the order of 29 such cycles over a period of 15 million years, producing 6000 vertical feet of salt. With burial under younger sediments, it turned to rock.

Extent of the great Paradox Sea; courtesy Jack Share.

Salt is a sedimentary rock but an odd one—plastic and able to flow. It can move underground, accumulate and ooze upward, and deform overlying rock. Given the abundance of salt in the area, McKnight thought salt uplift the likely explanation for Upheaval Dome (4).

"Because of the known occurrence of thick salt under the Upheaval Dome, the writer prefers to consider this feature a salt dome. The rock in the center of the dome is greatly broken, mashed, and squeezed, as if it had been plastically kneaded ... The massive sandstones on the axis of the peripheral syncline also appear to have been deformed plastically and do not show the breaking and shattering that would be expected had they been deformed rapidly and near the surface. ... Every indication points to slow deformation under thick cover ..." (italics added).

Uplift is only part of the story behind today's Upheaval Dome. For millions of years after it rose, younger sediments were deposited over it, eventually becoming a cover of rock something like a mile thick (UGS). Then about six million years ago the Colorado Plateau began to rise. Streams were steepened and invigorated, enabling rapid erosion (NPS). Thousands of vertical feet of rock were removed, along with the summit of Upheaval Dome.

Upheaval Dome, revealed by erosion. Photo by Doc Searls.

The "salt theory" is fascinating, but it has problems. For example no remnants of salt have been found in the area of the dome (UGS). And seismic survey and drill holes have shown Paradox salt to be 1500 ft below the surface, i.e., well below the dome (Fillmore 2011). So the salt theory was modified. Perhaps Upheaval Dome is a salt diapir—created by a blob of salt that rose and was pinched off from its source below. Subsequent erosion removed it along with overlying rock, explaining the lack of salt remnants. But if this really is a salt diapir, it's the weirdest one ever, unlike any other in the world (Ornduff et al. 2006).

Fortunately there's another way to create a decapitated dome with highly contorted rocks and markedly localized deformation. And it can be done in less than a minute instead of 20 million years.

From NPS Upheaval Dome Trail Guide, 1993.

Perhaps c. 170 million years ago a meteorite slammed into this very spot. In the first tenth of a second, it would have greatly compressed the surface, and then sent a shock wave radiating outward, excavating a giant crater. This was followed by collapse of the crater rim and rebound of the compressed core, creating a dome of deformed rock. As in the salt theory, subsequent deposition buried the rebounded dome; then erosion removed the rock cover and top of the dome, exposing the contorted rocks inside. (Today's hole is NOT the impact crater, whose remnants were removed by erosion. But the two are easily confused.)

For years evidence had been accumulating in support of meteorite impact (e.g. Kriens et al. 1999). But there was a problem. No altered rocks singularly diagnostic of meteorite impact had been found. So the salt-meteorite debate raged on.

Then in 2008, Buchner and Kenkman proclaimed that impact origin for the "Sphinx of Geology" (Upheaval Dome) had been confirmed. They examined 120 thin sections of rock from the outer edge of the ring syncline and found shocked quartz, which only forms in meteorite impacts and nuclear explosions! Actually, the "vast majority" of the quartz grains in the rocks did not exhibit shock features. But they found two that did. These tiny "smoking guns" were said to be unequivocal evidence of meteorite impact.

The world of information adapted. Wikipedia declared meteorite impact the accepted theory. The Utah Geologic Survey announced that Utah's Belly Button, once considered an "outie" is now an "innie". My favorite southern Utah geo guides—Ornduff and pals—argued persuasively against salt, noting that "the most recent studies point to the meteorite theory".

As of 2024, the National Park Service wisely remains non-committal.

Yet there's still a problem. In addition to shocked quartz, Upheaval Dome has rock layers that clearly were tilted slowly, on the order of millions of years. So another possibility must be considered—perhaps a meteorite impact caused a salt diapir! (Daly & Kattenhorn 2010; Gessaman et al. 2015).

But I'm stopping here, having dwelt long enough on how Upheaval Dome might have formed. For me, the mystery doesn't diminish its chaotic and awesome beauty. In fact, it enhances it.

What hath God wrought?

Notes

(1) Harrison concluded that at Christmas Canyon Dome "beds have been too highly buckled and faulted" to justify exploration for oil.

(2) Thanks to the Utah Geologic Survey for supplying me with papers by early geologists, and for trying to solve the mystery of "Upheaval Dome" (the name). If you know its source, please Comment below.

(3) McKnight took pains to explain the unusual contour lines of Upheaval Dome: "The general shape of the dome and surrounding syncline is depicted with fair accuracy on plate 3, but because the information on which this part of the map is based was not detailed enough for mathematical representation of such features as the exact structural depth and configuration of the syncline and the exact closure on the central dome, the structure contours within the involved area have been dotted."

(4) As further evidence of salt deformation, McKnight noted that "Upheaval Dome closely approximates the theoretical form for salt domes under certain conditions", citing Nettleton, LL. 1934. Fluid mechanics of salt domes. Am. Assoc. Petr. Geol. Bull. 18: 175-1204.

Sources

The amount of information (and speculation) available for Upheaval Dome is truly overwhelming! These are sources I found useful.

Buchner, E. & Kenkmann, T. (2008) Upheaval Dome, Utah, USA: impact origin confirmed. Geology, 36, 227–230.

Daly, RG, and Kattenhorn, SA. 2010. Deformation styles At Upheaval Dome, Utah imply both meteorite impact and subsequent salt diapirism. 41st Lunar and Planetary Science Conference. PDF

Fillmore, R. 2011. Geological Evolution of the Colorado Plateau of Eastern Utah and Western Colorado. Includes lengthy discussion of competing theories.

Geesaman, PJ, et al. 2015. New evidence for long-term, salt-related deformation at Upheaval Dome, SE Utah. Abstract and slides.

Harrison, TS. 1927. Colorado–Utah Salt Domes. Am. Assoc. Petroleum Geologists 11:111–133.

Kriens, BJ, et al. 1997. Structure and kinematics of a complex impact crater, Upheaval Dome, southeast Utah. USGS.

McKnight, TS. 1940. Geology of area between Green and Colorado rivers, Grand and San Juan Counties, Utah. USGS Bull. 908.  [Upheaval Dome p 124–128]

National Park Service (NPS). Stretching of the Basin and Range and Lifting of the Colorado Plateau. Accessed Feb 2025.

Ornduff, RL, et al. 2006. Geology Underfoot in Southern Utah. Mountain Press. Vignette 28, "At the Mystery's Core", is about Upheaval Dome.

Share, Jack. 2011 (May 29). The Enigma of Upheaval Dome: Diapiric Salt or Ground Zero.

Utah Geologic Survey. Utah's Belly Button, once considered an "outie" is now an "innie". [UD is one of  many wonderful Utah GeoSites offered online, great for planning roadtrips.]