"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).

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."

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.

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.

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.