White Mountain in the Black Rock Desert

Gypsum as sand.

If you've read my recent posts (1), you know that in May I visited Utah’s Black Rock Desert volcanic field. But volcanics weren’t the only attraction. Lake Bonneville was just as obvious, fascinating and demanding of attention. As recently as 15,000 years ago, this Ice Age lake covered much of western Utah. Now thick beds of sediments obscure volcanic evidence, and brilliant white playas—Bonneville’s desiccated remains—flash in the sunlight. One afternoon we visited a special playa near the south end of the old lake. It’s made of gypsum, and the rhyolite dome at the north end is nearly buried in gypsum sand.

Gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O) is a common mineral generally found in evaporite beds associated with sedimentary rocks. Where it occurs in large quantities, it’s mined (2). It’s used mainly for plaster and drywall (aka sheet rock, wallboard), as well as other less common but important applications.

Gypsum forms through evaporation, crystallizing out of increasingly concentrated solutions. Therefore it dissolves easily in water, and rarely lasts long enough to form sand. The most notable exception is the 700 sq km White Sands dunefield in New Mexico, the largest accumulation of gypsum sand in the world. It’s said to contain enough gypsum to meet global drywall demand for the next thousand years. Fortunately commercial exploitation was nipped in the bud in 1933, when President Herbert Hoover designated the dunes a national monument.

Google Earth view of White Sands National Monument, New Mexico; note scale.

Then there’s the White Mountain dunefield five miles west-northwest of Meadow, Utah. It covers only 0.1 sq km, so there’s little risk of commercial development, though a great geologist—the first to describe the area—noted:

“Perhaps no gypsum deposit in the world is so easily exploited as this; it needs merely to be shoveled into wagons and hauled away.” GK Gilbert, 1890

Google Earth view of White Mountain in the Black Rock Desert, Utah; note scale.

The White Mountain dunefield is tiny, but there’s enough of interest to justify a stop. And there’s little to interrupt ones reverie. On a warm day midweek in May, the only visitors were myself, my dog and the spirit of Grove Karl Gilbert, my favorite pioneering geologist.

GK Gilbert, “one of the greatest American geologists of the 19th century—a man who should be ranked with Lyell, Agassiz, and Smith” (National Academy of Sciences Archives photo, from Picard 2008).

From 1871 to 1879, Gilbert served as geologist on two of the Great Surveys of the American West, working for both Lt. George Wheeler and John Wesley Powell. He loved field work, in spite of (or maybe because of) the rigors of western exploration (3). He had a sharp and creative mind, and his contributions to geology are legendary, for example discovery of laccoliths, comprehensive description and mapping of Ice Age lakes, and development of some of the most basic concepts in geomorphology.

Lake Bonneville (Gilbert 1890); added circle and arrow mark Sevier Body and White Mountain.

Gilbert spent much of his field time in western Utah and adjacent Nevada, ostensibly gathering practical information such as location of irrigable lands. But he also studied and mapped playas, relic shorelines and other features of Lake Bonneville—the basis for his great 1890 monograph. Near the southern end of the Sevier Body, he discovered a playa that “differs from all the others in that its material is gypsum.”

“It was ascertained by digging in the playa that a portion of the deposit is amorphous and another portion crystalline. One phase of the precipitation results in the formation of small free crystals, which the wind sweeps from the surface of the playa and gathers in dunes. The dunes do not travel to a great distance, but are arrested by a low rhyolitic butte near by, to which they have given the name of White Mountain.”

Partial map, from Gilbert’s 1890 Lake Bonneville monograph.

The map above shows the southern part of the “Volcanic District near Fillmore, Utah” (today’s Black Rock Desert) with the playa of gypsiferous clay (GC, pale yellow) and crystalline gypsum (G, horizontal hatching). At the north end is White Mountain—rhyolite (R, red dots) and accumulations of gypsum sand (GS, vertical hatching).

Trail to top.

White sand verbena, Abronia elliptica.

Unknown travelers.

White Mountain is underpinned by a “small” rhyolite dome that erupted about 400,000 years ago. Its actual size is unknown due to overlying Lake Bonneville sediments and basalt flows. The White Mountain rhyolite may be significantly more extensive below the surface, as blocks have been found in the Tabernacle volcanic field to the west, active just 12,000 years ago (Johnsen 2010).

View from White Mountain summit: lava flow and cinder cone, with Pavant Butte behind.

Enjoying a bit of shade provided by White Mountain rhyolite.

Closer view, note layering … ??

Of course one wonders why Lake Bonneville would leave behind only a single gypsiferous playa. In his monograph, Gilbert attributed its uniqueness to nothing more than location—downstream from gypsum-containing rocks:

“It is probable that the deposit is independent of any special chemical reaction, and is due simply to the discharge by evaporation of a mineral dissolved from the rocks. The streams whose waters occasionally flood the playa rise among strata of Jurassic and Triassic age, and such strata in a neighboring mountain range are known to be highly gypsiferous.”

Grove Karl Gilbert c. 1910; USGS photo.

In my readings for this post, I learned that sometime before 1906, Gilbert met and fell in love with the great California botanist Alice Eastwood, probably on a Sierra Club outing. Plants and rocks—a wonderful match! In 1917, after a decade of "courtship," they made plans to marry the next year. Sadly, Gilbert died on May 1, 1918.

Alice Eastwood next to “rift” from the 1906 San Francisco earthquake (Marin County, GK Gilbert photo; Archives of the California Academy of Sciences, see Aldrich et al. 2006).

Notes

(1) Previous Black Rock Desert posts include Volcanoes in Utah—how can that be?!, Sunstones at Sunstone Knoll and A Caldera Lake?

(2) Flue-Gas Desulfurization (FGD) gypsum now equals natural (mined) gypsum in wallboard manufacturing. FGD gypsum production utilizes emissions captured from fossil-fuel power plants. The product is chemically identical to natural gypsum, but with higher purity (more here).

(3) Ironically, Gilbert was a sickly child, and even at 20 was frail enough to be passed over in the Civil War draft. Apparently field work changed him dramatically.

Sources

Aldrich, ML, Leviton, AE, and Elsbernd, K. 2006. Grove Karl Gilbert’s photographs as evidence in geology: Documenting the 1906 San Francisco earthquake. PDF

Gilbert, GK. 1890. Lake Bonneville. Monographs of the US Geological Survey, v. 1. Available here.

Jenkins, MC. 2018 (January 8). Grove Karl Gilbert, ‘A Captain Bold’ National Geographic blog.

Johnsen, R, et al. 2010. Subalkaline volcanism in the Black Rock Desert and Markagunt Plateau volcanic fields, in Carney, SM, et al., eds., Geology of south-central Utah. Utah Geological Association Publication 39.

Picard, MD. 2008. Grove Karl Gilbert, master of laccoliths and lakes, in Profiles of Rocky Mountain geologists, a continuing series. Rocky Mountain Geology 43:111-118.

Sepp, Siim (Sandatlas). 2015. Gypsum sand.

I can only beg ... suspend judgment until the whole case shall have been presented

“Hillers trachyte is a pale gray paste with large white crystals of feldspar and crystals large and small of hornblende.  This describes the variety at hand.”

This is one of the least eye-catching rocks in my kitchen.  No one picks it up, ponders it, asks questions.  Yet it’s special for me ... and for the history of geology.  I found it one morning on the picnic table at my campsite, on top of yellowed sheets of paper with notes and sketches.  They were not there the evening before.

I brewed a cup of coffee and sat down to read the mysterious missive.  This is what I found [notes in italics are mine]:

The Henry Mountains are not a range, and have no trend; they are simply a group of five individual mountains, separated by low passes and arranged without discernible system. [here there was an arrow to the diagram above] ... they stand on the right bank of the Colorado River of the West, and between its tributaries, the Fremont and the Escalante.

Geological exploration had shown that they were well disposed for examination, and that they promised to give the key to a type of structure which was at best obscurely known.  [emphasis added]

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 ...  I can only beg him to suspend his judgement until the whole case shall have been presented.

It is usual for igneous rocks to ascend to the surface of the earth, there issue forth and build up mountains or hills by successive eruptions [arrow to diagram below]

Mountain of Eruption

The lava of the Henry Mountains behaved differently.  Instead of rising through all the beds of the earth’s crust, it stopped at a lower horizon, insinuated itself between two strata, and opened for itself a chamber by lifting all the superior beds.  In this chamber it congealed, forming a massive body of trap.  For this body the name “laccolite” (Greek: cistern and stone) will be used.  [hmmm ... I thought the term was “laccolith”]

[arrow to sketch above] The simplest type of Henry Mt. structure is a lenticular mass of trap above which the strata are arched [this diagram looked very familiar, where had I seen it?]

or they have made these subterranean mounds at many different levels so as to produce a structure of which [arrow to sketch above] is an ideal cross section.

Contemporaneous and subsequent denudation have left the existing phenomena:

Mt. Ellsworth

Mt. Hillers

[there was one more sheet, and it was there that the author’s identity was revealed]

If these pages fail to give a correct account of the structure of the Henry Mountains the fault is mine and I have no excuse.  G.K.G.

I was stunned!!  "G.K.G." -- that's Grove Karl Gilbert, one of the greatest 19th-century geologists of western North America.  That familiar diagram is his classic cross-section through a laccolith, from his study of the geology of the Henry Mountains.  In fact, Gilbert had camped at this very spot at the southeast base of Mt. Hillers.  Had he dropped his field notes?  No, that couldn't be.  He was last here 136 years ago.  They must have been left on the table by some ghostly visitor in the night ... 

Grove Karl Gilbert in 1894; National Academy of Sciences Archives.

I reread the yellowed sheets, paying close attention to every detail.  An hour passed unnoticed and by then the day was getting warm.  But no matter.  I loaded my pack and drove around to the south side of Mt. Hillers to hike up to the base.  I wanted to see sandstone spurs, hogbacks, revet crags and trap for myself.

Modern-day view of the south base of Hillers.

--- ✿ ✿ ✿ ✿ ✿ ✿ ✿ ---

In 1869, the great explorer and geologist John Wesley Powell was traveling down the Colorado River through today’s Utah.  He spotted an isolated cluster of peaks to the west, which he called the Unknown Mountains.  They would become the last range to be named in the Lower 48 States.

The Unknown Mountains from the air.

View from Google Earth (click on image).  Lake Powell (Colorado River) in southeast corner.

Powell thought the Unknown Mountains looked like volcanos.  This was of great interest because there was a debate raging as to whether volcanos were 1) “craters of elevation” or 2) craters built from piles of debris accumulated through repeated eruption (geology was still a young science).  Even from a distance Powell could see these mountains were domed and had dark masses of lava-like rock, suggesting “craters of elevation.”

  Mt. Hillers from the west.

Five years later, Powell was in charge of the US Geographical and Geological Survey of the Rocky Mountain Region, and the Unknown Mountains had become the Henry Mountains, named for his physicist friend, Joseph Henry.  Powell directed Grove Karl Gilbert to take an exploratory expedition into the Henrys and perhaps settle the question as to whether volcanos were craters of elevation or of debris.  But the question was never answered, for Gilbert found no volcanos.

They left York, Utah, south of Salt Lake City, on June 20, 1875.  It took 20 days to travel the 200 miles to the Henry Mountains, by mule.  Trails ranged from good to unknown.  On August 22, they established Camp 38 very close to today’s Starr Springs Campground, where I found the mysterious notes and rock.

The old Starr Ranch.  The springs have long attracted humans; there's a BLM campground there now.

Gilbert spent two weeks in the Henrys in 1875.  He found no evidence of volcanic activity, and by the end of the visit was beginning to think about alternative models.  He returned the next year and stayed for two months.  When his report was published, the Henry Mountains became the type locality for a new geologic structure -- the laccolith.  Gilbert used “lacune” in his early field books, but then chose “laccolite” as the formal name. Later he was persuaded to change to “laccolith” because “-ite” typically applies to rock types rather than structures.

Gilbert’s laccoliths were distinct from other igneous structures recognized at that time.  Unlike volcanos, molten igneous material did not reach the surface.  Nor did it solidify deep below, as did masses of granite and related rocks.  Instead, magma was intruded into rock layers near the surface.  Furthermore, the “trap” deformed the overlying strata -- a novel concept.

"Half-stereogram" of a laccolith showing deformed sedimentary strata above, from Gilbert's report.  In his early field notes, he sometimes called these bubble or tumor mountains.

The deformed sedimentary rocks on the south flank of Mt. Hillers are especially striking.  "On this side a half dozen spurs show fragments of red rock as sketched ... inferior rocks tilted almost to the vertical and interspersed with dikes.  Moreover these sandstone hogbacks seem to trend in a curve around the mountain as far as they extend.”

Field sketch of the south base of Hillers, showing prominent hogbacks (click on image to view).

"Base crags of Hillers" from Gilbert's field notes.

Base crags of Hillers from the air.  Photo courtesy Jack Share of Written in Stone.

“revet-crags” and “bold spurs of trap” (dark rock near horizon)

"inferior rocks tilted almost to the vertical"

We wandered among the revet crags and bold spurs of trap for several hours.  They were extremely photogenic but it was getting hot.  So we headed back down to the car, and drove east and north to Hanksville.  With the Henrys in constant view, and Gilbert’s words and sketches bouncing around in my head, it was hard to keep my eyes on the road.

Eastern flank of Mt. Hillers.  Photo courtesy Jack Share of Written in Stone.

But while the Henry Mountains contribute almost nothing to our direct material interests, they offer in common with the plateaus which surround them a field of surpassing interest to the student of structural geology.  The deep carving of the land which renders it so inhospitable to the traveler and the settler, is to the geologist a dissection which lays bare the very anatomy of the rocks, and the dry climate which makes the region a naked desert, soilless and almost plantless, perfects the preparation for his examination.  -- G.K.G.

From "Map of the Henry Mountains by G.K. Gilbert.  From a model in relief." (1880)

Henry Mountains in southern Utah; click on image to view. 

Sources

Gilbert, G.K.  1880.  Report on the geology of the Henry Mountains, 2nd ed.  Dept. of the Interior, US Geographical & Geological Survey of the Rocky Mountain Region, Washington DC.  full PDF here

Hunt, C.B.  1988.  Geology of the Henry Mountains, Utah, as recorded in the notebooks of G.K. Gilbert, 1975-76.  Geol. Soc. Am. Mem. 167.

Share, Jack.  2011.  The Henry Mountains Laccolithic Complex on the Colorado Plateau at Written in Stone ... seen through my lens

And the answer is ...

A few days ago, I posted some mystery photos, which Silver Fox of Looking for Detachment identified correctly, in a general kind of way.  And yes, the location is south of Denver, as Ben Nevis pointed out, and a bit west as well.  The complete answer is ... Mt. Hillers -- one of the five Henry Mountains, south of Hanksville, Utah.

“beyond are red and white sands -- inferior rocks tilted almost to the vertical and interspersed with dikes.  Moreover these sandstone hogbacks seem to trend in a curve around the mountain as far as they extend."  Grove Karl Gilbert describing the south side of Mt Hillers, 1875.

South side of Mt. Hillers, from G. K. Gilbert's journal (1875).

Gilbert’s work in 1875 and 1876 was revolutionary.  He concluded that the Henry’s were not volcanic, but rather intrusive igneous features, which he called “laccolites” (today’s laccoliths).  These intrusions pushed up overlying sedimentary strata, forming the nearly vertical hogbacks at the base.

Mt. Hillers, viewed from south.

The Henry Mountains in southern Utah, south of Hanksville.  Waterpocket Fold on left, Lake Powell lower right.  Note sandstone “hogbacks” along south base of Mt. Hillers (click to view).  From ArcGIS online.

Stay tuned for more about the intriguing Henry Mountains, as well as the work of G. K. Gilbert -- perhaps one of the greatest field geologists ever.