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 (with route-finding) and camping were much more challenging. And where 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 et al. 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

The Monthly Fern: Northern Holly Fern, a spore shooter

This month's South Dakota fern is Polystichum lonchitis, Northern Holly Fern (Andre Zharkikh).

It was the first paragraph on the first page of Robbin Moran's Natural History of Ferns that hooked me—specifically the words of a thief in Shakespeare's Henry IV:

"We steal as in a castle, cock-sure; we have the receipt of fern-seed, we walk invisible."

So intriguing! Or maybe not. Maybe you doubt that fern-seed can bestow invisibility (1). But think about it ... have you ever seen fern seeds? 

In Shakespeare's time no one had seen fern seeds because they were invisible. In our time we haven't seen fern seeds because they don't exist. And for those who answered "yes" to the question above—spores are not seeds (2).

Northern Holly Fern, well-armed with clusters of sporangia, aka spore shooters (Andre Zharkikh, bar added).

Northern Holly Fern is widespread in the Northern Hemisphere, but often uncommon where it occurs. It's considered an arctic-alpine, boreal, and montane species, which may seem odd for a South Dakota plant. However it grows specifically in the Black Hills (3), which are famous for plants seemingly out of place. Northern Holly Fern is likely a relic of the last Ice Age, when western South Dakota was much cooler (but not glaciated).

Circumboreal Polystichum lonchitis (Cremastra); Black Hills added (location approximate).

Like most ferns, Polystichum lonchitis has little clusters on the underside of its leaves. These are sori (singular sorus), and deep inside are spores. Shape and position of sori are helpful in identification. Often each sorus has a little cover, the indusium, and its shape (or absence) also is useful in identification. Sori of holly ferns are round, lined up between the leaflet midrib and edges, and have peltate (umbrella-like) indusia—tiny round membranous covers on short stalks.

Northern Holly Fern's round sori with peltate indusia (Andre Zharkikh).

Now a short tour of sori diversity, providing a glimpse the level of detail needed for identification. It's handy to have a 10X magnifier.

Left: Maidenhair Fern, Adiantum pedatum, has linear sori partly covered by rolled leaf edges (false indusia). Right: Bracken, Pteridium aquilinum, has continuous sori with indusia hidden under rolled leaf edges (MWI & MWI).

Top: young Fragile Fern, Cystopteris fragilis, with round sori; indusia will wither as spores mature. Bottom: Wood Fern, Dryopteris carthusiana, with round sori covered by kidney-shaped indusia (MWI & MWI).

Common Polypody, Polypodium virginianum, has plump yellow sori with no indusia (MWI).

However we still haven't seen any spores. The sori's brown or yellow dots are not spores, they are sporangia. Inside the sporangia, finally, are the spores—in abundance!

 

Fern leaf with sori containing sporangia containing spores (and some scattered about); USDA Forest Service.

Fern spores are tiny, usually 30–50 micrometers across (1 µ = 0.001 mm), which is narrower than a human hair (source). Or think of it this way—a fern frond just 60 cm long will produce something like 7,000,000 spores! (source) As fine as dust, spores can fly in the lightest breeze, especially if the sporangium kicks them out of the house.

Holly Fern spores, scale bar = 10 µ. © Robbin Moran 2012.

In some ferns, the sporangia simply split open and let their spores fall to the ground. Last month's Sensitive Fern is an example. But in Holly Ferns, in fact in many ferns, sporangia hurl their spores. I like to call them spore shooters, as does Robbin Moran. Others call them launchers or catapults. In any case, their spores can reach speeds of 10 m/sec! (Llorens et al. 2015)

The diagram below shows a spore-shooting sporangium in action. The ring of blue and red cells is the annulus; the cells are filled with water. As the outside dries the annulus curves back, opening the sporangium and cocking the catapult or shooter. Elastic pressure increases until the annulus suddenly collapses, sending spores flying. They get quite a boost in speed, but probably just as important, they're dislodged from the interior of the sporangium. Apparently my "house" analogy above was appropriate. These clingy spores need encouragement to go out into the world.

Modified from Llorens et al. 2015.

Some readers may still be bothered by my earlier statement that spores are NOT seeds. After all, won't a spore give rise to a full-sized fern, like the pea that becomes a pea plant? Well ... yes, but not directly. A true seed has a full set of chromosomes (diploid), and can give birth to a seedling. But a spore has only half a set (haploid). Before a fernling can arise, fertilization must take place.

This leads us to the fern life cycle, which even pteridomaniacs call "the bugbear of botany students". Maybe "fern sex" would be less scary. In any case, stay tuned. I will squeeze it in amidst the beauty and intrigue of the next Monthly Fern.

Northern Holly Fern. Thanks to Andre Zharkikh, who kindly shares his many plant photos "to show other people the beauty of nature".

Notes

(1) To provide invisibility, fern seed must be collected precisely at midnight on Midsummer's Night Eve, while it's falling from the plant.

(2) If you thought spores were seeds, you're in venerable company. Carl Linnaeus, father of modern taxonomy, did also. He first thought the fine dust shed by ferns was pollen. But he then admitted he knew too little about primitive plants to conclude "whether what I see is seed, or dust of the anthers." However years later, in 1751, he announced that the dust was fern seed (Moran 2004).

(3) Flora North America and USDA Plants, sources usually considered reliable, do not include South Dakota in the known range of Polystichum lonchitis (the latter shows it as reported but without documentation). It was first documented in the Black Hills in 1977, and has been found elsewhere in the northern Hills since (see map from SEINet herbarium search, zoom in to South Dakota).

Sources, in addition to links in post

Llorens, C, et al. 2015. The fern cavitation catapult: mechanism and design principles. J. R. Soc. Interface 13: 20150930.

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Pinson, Jerald. About Ferns. American Fern Society.

Summers, A. 2005 (Dec). Spore Launchers; Ferns and fungi that explosively reproduce. Natural History.

USDA Forest Service. Ferns. Helpful information, photos, and diagrams for aspiring pteridomaniacs. And there are coloring pages! (have a range of greens ready)

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.]

The Monthly Fern: Sensitive or Bead Fern

"... the fertile ones being so unlike the sterile, that no one who is unacquainted with the plant would suppose they had anything to do with each other." Photo by peganum.

As some readers know, I'm "working" on a Web-based guide to plants of South Dakota, a wonderful retirement project. Last year, once a month, I posted about a South Dakota tree starting with Black Hills Spruce, the state tree, and finishing with Osage Orange, God's Gift to the Prairie Farmer. It was such a pleasure and I learned so much! Inspired by the experience, I'm continuing in 2025 but switching to ferns (hence my fern holiday card).

First, let's be clear what ferns are (I needed this, Botany 101 being but a faint memory). Personally I think it's easier to describe what they're not. Ferns are neither mosses nor seed plants.

Ferns are like mosses in that they reproduce via spores. But unlike mosses, they have vascular tissue—pipe-like cells for transporting water and minerals through the plant. Because of this plumbing they're classified as vascular plants—tracheophytes. While mosses stay low to be near moisture, ferns can grow tall, even tree-size.

Brush Pot Tree (Sphaeropteris lepifera), a fern reaching for the sun. Photo by AraucariaHeterophylla.

Ferns are just a small subset of plants with vascular tissue. Tracheophytes include flowering plants, conifers, ginkos, cycads, and Gnetophytes (e.g. Ephedra, Mormon Tea). Unlike ferns, these plants reproduce by seeds, making them spermatophytes. 

Evolutionary diagram for plants; ferns and "Fern Allies" in green box. Geeks can click on image to view details (modified from source).

Ferns and the former Fern Allies used to be classified as pteridophytes, which you won't find in the diagram above. Turns out they're insufficiently related to qualify as a single taxonomic group. Even so, fern enthusiasts remain pteridologists, and infatuation with ferns is still referred to as pteridomania. I may well find myself in that state if the first Monthly Fern is any indication.

Sensitive Fern, Onoclea sensibilis. By rawpixel.

The Sensitive Fern is one of the oldest fern species still around. Its fossils go back at least 55 million years, and have been found in Greenland, western United States, Canada, Japan, United Kingdom and easternmost Russia (Moran 2004). Its distribution has shrunk a bit since; it grows in the eastern and midwestern US and eastern Asia. In South Dakota it does well in the Black Hills in the western part of the state, with a few scattered occurrences to the east. It seems to prefer moist to wet sites: stream and pond margins, wetlands, wet meadows and such.

Especially interesting are the leaves. There are two kinds and they differ dramatically, as pteridologist Daniel Cady Eaton explained (1881): "The fronds [leaves] are truly dimorphous, the fertile ones being so unlike the sterile, that no one who is unacquainted with the plant would suppose they had anything to do with each other." I agree!

The sterile leaves look like fern leaves—green and several times lobed. They're the basis for "Sensitive" as Eaton explained: "The fronds wilt very soon after plucking them ... The first frost of autumn destroys the sterile fronds; and a late frost in May or June does the same."

Sterile leaf of the Sensitive Fern. Photo by jillllybean.

In stark contrast, the fertile leaves are very odd! "The fertile fronds are not very common, and a young botanist may search in vain for them for a long time. ... They are nearly black in color ... [and] divided into a double row of sub-globose bead-like segments or pinnules; the whole looking like a small and narrow but dense cluster of diminutive grapes" (Eaton again).

No wonder Bead Fern is another common name. The beads are made of tightly rolled-up leaflet lobes, protecting spores waiting to fly in spring.

Sensitive/Bead Fern's curious fertile leaf (MWI).

Fertile leaves standing after sterile have wilted; var. interrupta, Japan. Photo by Aomorikuma.

Onoclea sensibilis from Eaton 1881. Emerton, JH, & Faxon, CE, illustrators.

The plate above shows two fertile bead-bearing leaves on the left, next to a large sterile leaf. But what's that leaf lower right? That was my question for Robbin Moran, eminent pteridologist and author of The Natural History of Ferns. He writes about the science of ferns with obvious and contagious joy, so I thought he might be willing to help.

Mystery leaf up close; arrows point to examples of possible sori ("spore clusters" for now).

A close look at the mystery leaf reveals what look like spore clusters, arranged as in many other fern species. "Is this a fertile leaf early in development?" I asked. No, but I wasn't totally off track, as Robbin explained:

"The leaf in the lower right is more highly cut (1-pinnate-pinnatifid), and such leaves are often produced late in the year, usually in response to trauma from mowing or a late frost. Sometimes these leaves represent a part-sterile and part-fertile condition, being developmentally intermediate between the two extremes of the normal sterile and fertile leaves ... " (2).

Intermediate leaves "often are produced in response to frost or mowing late in the season, although they also occur naturally without disturbance." (© Robbin Moran 2021)

Note elongate indusiate sori [spore clusters with little flap-like covers] (© Robbin Moran 2021). Oval lobes are maybe 5 mm long (my ball-park estimate).

With that mystery solved, we move on to spore dispersal. In Ferns of North America (also by Eaton, 1879–80) the Sensitive Fern plate includes an opened bead filled with what look like tinier beads. These are sporangia, which contain the even tinier spores (dust-sized!).

Open Sensitive Fern bead showing lobes and reproductive structures; when rolled up, beads are 2–6 mm long. From Plate LXXII in Eaton 1879–80.

Sensitive Fern in winter. Photo by Cephas.

By fall, Sensitive Fern's sterile leaves are gone and the fertile leaves look very much like dead plants. But they're not. Sometime in late winter the lobes of the beads begin to unroll in response to decreasing humidity. After enough drying, spores simply fall to the ground, perhaps to be blown away by wind (Suissa 2022).

That's so boring! Many ferns shoot their offspring out into the world, but not the Sensitive Fern. Sorry. The excitement of spore-shooting will have to wait for a future post.

Original source not given.

Notes

(1) Dimorphic sterile/fertile leaves are not unique to Onoclea sensibilis, but its leaves "differ drastically" (Beital et al. 1981).

(2) In the past, plants with intermediate sterile-fertile leaves have been recognized as a distinct species, a variety or a taxonomic form (e.g., Onoclea obtusilobata). Beital et al. (1981) showed none of these are valid. See also Flora North America.


Sources in addition to links in post

Beital, JM, Wagner, WH Jr., Walter,KS. 1981. Unusual frond development in Sensitive Fern, Onoclea sensibilis L. American Midland Naturalist 105:396-400. https://www.jstor.org/stable/2424762

Eaton, DC. 1879–80. The ferns of North America ... v. 2. Emerton, JH, & Faxon, CE, illustrators. Onoclea sensiblis Plate XII, pages 195–200. BHL

Eaton, DC. 1881. Beautiful ferns from original water-color drawings after nature. Emerton, JH, & Faxon, CE, illustrators. Onoclea sensiblis pages 153–158 & preceding plate. BHL

Moran, RC. 2004. The Natural History of Ferns. Timber Press.

Pinson, Jerald. About Ferns. American Fern Society (accessed 9 Feb 2025).

Suissa, JS. 2022. Fern fronds that move like pine cones: humidity-driven motion of fertile leaflets governs the timing of spore dispersal in a widespread fern species. Annals of Botany 129:519-528. https://doi.org/10.1093/aob/mcab137 (open access).