Wyoming's Exotic and Enigmatic Phonolite

Folks, it’s phonolite!

In the late 1970s, I worked as a ranger naturalist at Devils Tower National Monument in northeast Wyoming. During my stints at the desk in the Visitor Center, I heard the Admatic Automatic Slide Projector’s short program about the Tower over and over. One phrase is especially memorable: “phonolite porphyry, named for the ringing sound the rock makes makes when struck.”

I thought it would be fun to demonstrate this for visitors, but I was never able to produce a ringing sound by striking the Tower rock. Interestingly, the Park Service now says that “phonolite (fō'nə-līt') refers to the mineral composition of the rock.” However “porphyry” hasn't changed; it still refers to rock texture—visible crystals (in this case feldspar) in a fine-grained matrix.

Northeast face of Devils Tower.

Futilely striking Tower rock.

Devils Tower phonolite porphyry.

Phonolite is an uncommon rock—“exotic and enigmatic.” Yet I encountered it again just recently, in the Rattlesnake Hills of central Wyoming only 175 miles southwest of Devils Tower. In both places the phonolite is estimated to be about 40 million years old, emplaced during the Eocene Epoch.

Phonolite is igneous—formed from magma. It’s often referred to as volcanic (e.g., here and here), which means it's extrusive—magma reached the surface. And yet the Devils Tower and Rattlesnake Hills phonolite bodies are considered intrusions that formed below the surface, though probably not deep. The phonolite in the Rattlesnake Hills has been called “subvolcanic.”

Devils Tower is just one of a collection of igneous intrusions of Eocene age in the northern Black Hills. Rock types include members of the phonolite-trachyte-latite series. They probably were emplaced late in the uplift of the Hills (part of the Laramide Orogeny, the building of the Rocky Mountains). The intrusions were later exposed by erosion, and now form prominent landscape features: Devils Tower, the Little Missouri Buttes, Sundance Mountain, Inyan Kara Mountain, Crow Peak, and more.

Sundance Mountain; note columnar jointing (click on image to view).

During cooling and contraction of the intruded magma, columnar jointing sometimes formed. As Evelyn Mervine has said, “Columnar jointing is always a joy to observe in rocks in the field. Stumbling upon perfectly geometric columns of rock can only be described as magical.”

Many northern Black Hills intrusions include columnar jointing, but never as extensively and dramatically as at Devils Tower—not even close! There the columns are as much as six feet across and 600 feet tall.

South face of Devils Tower.

Like the Black Hills, the Rattlesnake Hills were created during the Laramide Orogeny. They also include Eocene igneous intrusions. Composition is similar to those of the Black Hills, but phonolite is more common, found in 21 intrusions in the central core of the range (Hoch and Frost 1993). Columnar jointing occurs here too.

Igneous intrusions ahead! Dry Creek Road along the southwest flank of the Rattlesnake Hills.

Goat Mountain is latite.

Round Mountain is phonolite.

Columnar jointing on Round Mountain.

Columnar jointing east of Dry Creek Road; rock type unknown (phonolite-trachyte-latite series).

The same rare rocks of about the same age in close proximity demands an explanation! Are Black Hills and Rattlesnake Hills intrusions related? Sadly, it appears no one knows. Their origins are “poorly understood” and the phonolite remains enigmatic. Hoch and Frost (1993) think that Laramide structural features (faults, fractures) “enabled the alkaline and subalkaline magmas to ascend to shallow crustal levels.” But why would there be exotic magmas here?

Geochemical analyses indicated that the magmas must have involved some mixing. But Archean country rocks of the Wyoming craton—our several-billion-year-old basement rocks—are poorly represented in the mix. Hoch and Frost wrote that the magma “may reflect the composition of subcontinental mantle that has been isolated and chemically modified during the last 3.5 billion years.”

In other words, magma made from deep 3.5-billion-year-old rocks moved up through fractures 40 million years ago, and solidified before reaching the surface. Then starting around five million years ago, erosion carried away the overlying softer rocks, revealing the intrusions. If correct, this will be a really cool story! But more work needs to be done. The phonolites are not eager to give up their secrets.

North face of Devils Tower. “It is somewhat of a geological puzzle, standing alone as it does, and rising directly out of a country entirely made up of sedimentary rock.” Thomas Moran, 1894

Sources

Hausel, WD. 1996. Geology and gold mineralization of the Rattlesnake Hills, Granite Mountains, Wyoming. Wyoming Geological Survey Report of Investigations No. 52. http://www.wsgs.wyo.gov/products/wsgs-1996-ri-52.pdf Accessed 17 September 2017.

Hoch, AR, and Frost, CD. 1993. Petrographic and geochemical characteristics of mid-Tertiary igneous rocks in the Rattlesnake Hills, central Wyoming, with a comparison to the Bear Lodge intrusive suite of northeastern Wyoming, in Snoke, AW, Steidtmann, JR, and Roberts, SM, eds. Geology of Wyoming. Wyoming State Geological Survey Memoir 5:508-528.

Sutherland, WM, and Hausel, WD. 2002. Preliminary geologic map of the Rattlesnake Hills 1:100,000 quadrangle [Wyoming]. Wyoming State Geological Survey OFR 2002-2. http://www.wsgs.wyo.gov/products/wsgs-2002-ofr-02.pdf Accessed 17 September 2017.

For more on the puzzling geology of Devils Tower, see The many views of Devils Tower.

Devils Tower—What’s on Top? (updated)

Devils Tower is so striking that Henry Newton and William P. Jenney featured it as the frontispiece in their 1880 Report on the Geology and Resources of the Black Hills of Dakota.

Devils Tower is a sheer rock monolith that stands 1200 feet above the Belle Fourche River in northeast Wyoming. It’s about a mile around at the base, and tapers to a summit the size of a football field. Getting to the top isn’t easy—the only legal way is technical rock climbing. Less than 1% of visitors to Devils Tower National Monument make to it the summit, so perhaps it’s not surprising that the most common question about the Tower is “What’s on top?” (The most common question overall is either “How do we get to Mount Rushmore?” or “Where are the bathrooms?)

“You want to know what’s on top? Let’s go find out!”

So I guess I shouldn’t be surprised that my most popular blog post ever is Devils Tower—What’s on Top?, but I am. It’s not about rock climbing or spaceships, but rather plants and vegetation. How many readers really want to know that sagebrush, grass and wildflowers grow on the summit? I think folks land there by accident. In any case, it’s time for an update.

In 1941, George Hopkins parachuted onto the summit, where he hung out for six days in grass, sagebrush and rocks before being rescued. NPS archives.

This year, the National Park Service contracted with us to inventory plants and assess summit vegetation. In a sense the project is “almost” done. However, I have to finish the report and I hate writing reports. I’d much rather be blogging. A few days ago, I had a brilliant idea—do both at the same time.

This week I'm compiling information about summit plants that would be of interest to visitors. The Park Service will use it in interpretive materials available at the Monument and online, and may add it to employee training. So I’m wondering:

What would you want to know about what’s on top? What would you find interesting and memorable about plants on the summit? (please add Comments below)

Andrew and Cheryl commune with plants on top of Devils Tower.

This is what I’ve come up with so far (photos are for this blog post only, and not all are from the summit):

The top of Devils Tower is a rounded rocky hilltop. From the high point you wouldn’t know there were precipitous walls below if you hadn’t just struggled up one. Plants are common among the rocks—in fact plant cover roughly equals that of bare rock. Skunkbush sumac bushes and clumps of bluebunch wheatgrass grow scattered across much of the summit. In the northeast part, where the soil is deeper, there’s a small but vigorous stand of sagebrush and grass.

Andrew admires skunkbush and bluebunch wheatgrass among the rocks.

How do plants get up there? Wind is the most likely means of seed transport. But small mammals climb Devils Tower too—chipmunks, packrats, and deermice have been seen on the sides and summit. Maybe they haul a few seeds up there on occasion.

How can plants grow in rock? Over the long term, rocks are not as permanent as they look. They fracture, weather, and turn to dust. Dirt and debris slowly accumulate in small pockets, where seeds sometimes land. A few germinate; a few of these grow into plants. Their roots help break up the rock, one tiny bit at a time.

It may seem unlikely that seeds will land in just the right place and survive, but obviously they have. The top of Devils Tower is botanically diverse, with 53 species—impressive considering it’s only the size of a football field and really rocky.

At first glance, the summit looks dry and harsh—hospitable only to hardy drought-tolerant grasses and shrubs. But there are lots of favorable microsites. Thirteen plant species on the summit grow only in the shade of rocks and shrubs. Ferns grow in crevices with shade and soil … and a bit of flowing water when it rains.

Rocky Mountain cliff fern (Woodsia scopulorum). ©Adolf Ceska, used with permission.

In late spring, the summit turns green with grasses and wildflowers. Some of the showier species are sego lily, prickly pear cactus, harebell, western yarrow and wild onion. Common grasses include bluebunch wheatgrass, needle-and-thread, and blue grama. Shrubs are common too, especially skunkbush sumac and Wyoming big sagebrush.

Are any plants unique to the summit? No—all grow in the surrounding area too. However, Wyoming big sagebrush is much more common on the top of Devils Tower than anywhere else in the Monument (it’s extremely common further west in the basins of Wyoming).

Sagebrush grassland on top of Devils Tower; skunkbush sumac lower left.

Here are a few of the plants that greet rock climbers when they reach the top:

Plains prickly pear (Opuntia polyacantha) by Sally and Andy Wasowski.

When the large waxy yellow flowers of prickly pear cactus bloom, everyone notices. Otherwise it’s drab and easily overlooked, and painful when accidentally bumped!

Two different kinds of sagebrush grow on the summit: Wyoming big sagebrush and fringed sage. Sagebrush is not related to the sage we use in cooking, but was named for the sage-like fragrance of the leaves.

Wyoming big sagebrush (Artemisia tridentata ssp. wyomingensis) by Stan Shebs. Its leaves are 3-toothed at the tip, as the scientific name points out (“tridentata").

Fringed sage (Artemisia frigida) also lives up to its name. The leaves look “fringed” because they’re divided into fine segments (NPS).

The 3-parted leaves of skunkbush sumac suggest poison ivy, and indeed, the two are close relatives. But this sumac is not at all poisonous. In fact, its twigs  have long been used to make a citrus-flavored tea. The appealing fragrance explains the scientific name—Rhus aromatica.

Skunkbush sumac (Rhus aromatica) by Joseph A. Marcus.

Harebells look delicate, but they’re actually quite tough, surviving even where there’s very little soil. They grow in cracks on the sides of the Tower, and among rocks on top.

Harebells (Campanula rotundifolia). The scientific name refers to bell-shaped flowers and small round leaves.

One of the more common plants on the summit is rock spikemoss, but it’s rarely noticed. It’s a close relative of the resurrection plant, and behaves in a similar fashion. Rock spikemoss grows and produces spores during favorable conditions in late spring and summer. Then it lies dry and dormant, looking quite dead, as it awaits “resurrection” with next year’s spring moisture.

Rock spikemoss (Selaginella densa) is not a true moss but rather a relative of ferns.

No one expects to find trees on top, but there is one. A ponderosa pine has managed to survive on minimal soil and water in a crack. Never underestimate a plant!

• • •

One more question—what do you think about including scientific names? Botanists like them, as they’re information-rich. But others find them off-putting.

Here’s the current list of summit plants. About thirty were known prior to this project. After systematic careful searching—three visits scattered through the growing season, 20 person hours in all— the known vascular plant flora now stands at 53 species.

Grassland, Forest and an Igneous Intrusion

Devils Tower -- a puzzling igneous intrusion in northeast Wyoming.

Recently I spent a day wandering around Devils Tower looking for just the right shot.  It had to feature mixed-grass prairie and ponderosa pine forest, with Devils Tower in the background.

Replacement needed, in full color (from Knight 1994).

I was sent on this mission by Dennis Knight, Professor Emeritus of Botany at the University of Wyoming, who is revising his 1994 book, Mountains and Plains -- the Ecology of Wyoming Landscapes.  How fitting! ... it was an encounter with Dennis at Devils Tower that led to my enrollment as a Botany grad student at the University.  He was in the audience at an evening campfire program, and afterwards explained he had enjoyed my discussion of vegetation and bedrock.  The rest is history, as they say, and now 35 years later we’re back on the same topic.

At Devils Tower National Monument, ponderosa pine forest is best developed on sandstone ridges, and on buried talus around the base of the Tower.  With deeper roots, pines can tap into accumulated water among buried rocks or in fractures.

Grassland - pine forest mosaics are common in the northwest Black Hills.

Shallow-rooted grasses do well on fine soils that hold water close to the surface, for example soils derived from Permo-Triassic Spearfish red beds and shaley members of the Jurassic Sundance Formation.

What a nice orderly arrangement -- grasslands on fine soils, pines on buried talus and sandstone ridges.  Our pattern-seeking minds love such things!

From ArcGIS online; click on photo for details.

The Black Hills were named for the ponderosa pine forests that make them appear dark against the surrounding plains (below, from Google Earth).  Elevations are lower than in other mountain ranges in the region, and the relatively fast-growing ponderosas are the basis for a timber industry that costs tax-payers little or nothing (usually we subsidize timber harvest on public lands in our region).

But the Black Hills are not entirely forested.  Mixed-grass prairies are common.  These are indeed mixes -- of tall-grass prairie and short-grass prairie species.

From Regional Trends of Biological Resources  (USGS); mixed-grass prairie zone in purple.

The photo above currently is the front-runner for Dennis’s book.  The grassland on Joyner Ridge is an excellent example of mixed-grass prairie, with the following grasses common:

tall-grass species:  big bluestem, green needlegrass, needle-and-thread

short-grass species:  blue grama, buffalo grass

I especially like this shot because of the yellow coneflowers (Ratibida columnifera) in the foreground -- they’re classic prairie wildflowers.  But it will be Yale University Press that decides if the photo is appropriate.

Hiking trails at Devils Tower (NPS); click on photo to enlarge.

You can tour the vegetation of Devils Tower National Monument via hiking trails.  The Tower Trail goes through pine forest on buried and partially-buried talus.  There’s also an interesting rock to check out along the way.

Needle-and-thread grass on Joyner Ridge.

The Joyner Ridge Trail passes through mixed-grass prairie with great views of the Tower, and draws with oak, ash, chokecherry, wild plum and other hardwoods.  A connector trail hooks up with the Red Beds Trail through badlands of the Spearfish Formation.  From there, you can head down to Dog Town, where prairie dogs serenade you as you wander through the amazing diversity of plants they manage, both native and non-native.

Dog Town -- a highly-managed landscape.

Two of the managers.

I used to live within view of Devils Tower, and have spent a lot of time there in the years since.  Even so, I think it will always look improbable and mysterious to me.

thanks

Thanksgiving is a time to stop and appreciate the basic things that I sometimes take for granted -- family and friends, good health, enough to eat, a warm safe place to sleep.  But there's something else I’m especially grateful for too -- all those extra special things that Nature provides -- like beauty, fascination, awe, mystery, always something around the corner, something new, something different, or a different way of looking at familiar things.

Bur oak leaves cover the trail.

Contemplation of nature makes my life so much richer and enjoyable ... and is even a source of sanity at times.

Young ponderosa pine dusted with snow and sunshine.

“After you have exhausted what there is in business, politics, conviviality, and so on - have found that none of these finally satisfy, or permanently wear - what remains? Nature remains."  Walt Whitman

Ponderosa pine on Dakota sandstone.

"And this, our life, exempt from public haunt, finds tongues in trees, books in the running brooks, sermons in stones, and good in everything."  William Shakespeare

Late fall in the northern Black Hills.

Devils Tower on Thanksgiving day.

The many views of Devils Tower

Ponderosa pine, against a backdrop of massive columnar jointing.

It’s impossible to ignore Devils Tower if it’s anywhere in sight -- to say it dominates the landscape is an understatement.  It’s huge, rising 800 ft above the sandstone platform on which it appears to sit.  It measures roughly 3000 ft around at the base, tapering with graceful curves to ca 800 ft at the summit.  For this rock monolith to be standing above the nearly-flat sedimentary rocks of the Belle Fourche River valley seems so improbable, and continues to strike me as very bizarre, even though I’ve seen it many times.

“It is somewhat of a geological puzzle, standing alone as it does, and rising directly out of a country entirely made up of sedimentary rock.”  Thomas Moran 1894

The west face -- 600 ft from shoulder to summit.

The Tower’s form is graceful, and suggestive of symmetry from certain perspectives, but it’s not symmetrical, not even close.  All you have to do is walk around it, fly over it, climb it or drive the nearby roads to appreciate the many different views.

Aerial view of Devils Tower, courtesy ArcGIS online.

View from grasslands in the northeast part of the park.

As one approaches Devils Tower, the fluted texture of it’s sides becomes apparent, and its beauty is even more striking.  Up close, the overall form of the Tower disappears from view, replaced by giant rock columns.  They can be 6 ft or more across, and many are hundreds of feet tall with few horizontal fractures.  According to the literature, Tower columns typically are 5-sided but may have as many as 7 sides or as few as 4.  [I wonder who did the inventory, and how.]

Why columns?  As the Park Service explains, when the magma that formed Devils Tower cooled and began to solidify, it shrank and cracked.  Cracks grew out and down, intersected other cracks, and formed columns.

From bottom to top, there are notable changes in the structure of the Tower.  The base or shouder is massive, up to 100 feet in height.  Immediately above it are large columns that lie close to horizontal.  Moving up, the columns curve and become “vertical” ... but not quite.  They taper upwards, some ending, some merging, and so the walls slope inward, generally 75º to 85º.  The upper part of the Tower is much more fractured, vertically and horizontally.  The rock is less sound, cracks are wider, and pigeons (rock doves) and pack rats nest there.  The summit is rounded, covered with grass and sagebrush, and feels like a hill top.  Only from a few places around the edge are there precipitous views.

View from the south.  The leaning column of the Durrance Route, the "easiest" route on the Tower, can be seen about a quarter of the way in from the left edge, just above the curving lower columns (click to view).

View from halfway up the west face amid not-quite-vertical columns, looking down on curving columns near the base and talus field below.

The upper third of the Tower is more fractured.  Maybe it was exposed to erosion longer, or cooled under slightly different conditions ... another of the many mysteries.

And what's on top? ... plants and rocks!

The base of the Tower is surrounded by fallen column fragments.  Many are large and boulder-hopping is a challenge.  Most of the talus lies close to the base, though scattered boulders dot the slopes all the way down to the Belle Fourche River.

Below:  Large fallen column along the Tower Trail, just above trees; note tiny tourists on either end (click to view).

Phonolite porphyry of Devils Tower.

The rock that makes up Devils Tower is phonolite porphyry, part of a “continental phonolite-trachyte-quartze latite igneous association” (Halvorson 1980).  I know so little about petrology that I will go no further in this discussion, except to share a possibly mythical fact about phonolite porphyry.  When I was a ranger naturalist, back in the day, the Devils Tower Visitor Center was furnished with a giant box, The Admatic, that played a short slide show about the park every time a visitor pushed the button.  Obviously, those of us at the desk heard that program over and over, especially on busy days.  After three years, its phrases were pretty much hard-wired in my brain, most notably:

“phonolite porphyry, named for the ringing sound the rock makes when struck”

In fact, it was still in my head walking around the Tower thirty-plus years later, so I decided to put the authority of The Admatic to the test.  Sorry to say, the closest I could get to a ringing sound was a metallic “plink”.

The Origin of Devils Tower -- also many views

With its distinctive structure and rock type, surely the Tower’s origins would be decipherable by geologists.  But the words that William L. Effinger wrote in 1934 still apply:

“There is a good deal of uncertainty concerning the mode of origin of Devils Tower and the type of igneous body it represents.”

Effinger was employed by the Park Service under the Civil Works Administration to compile “such pertinent information as will be helpful in the preparation of geological museum exhibits at Devils Tower National Monument and more specifically to outline the story to be interpreted by such exhibits.”  Too bad the story wasn’t more clear.  Much of the material below comes from Effinger’s review paper, available at ParkNet.

Bear Lodge Butte or Mato Teepee, viewed from the west, Bear Lodge Mountains in distance.  Woodcut from Newton and Jenney (1880).

The first to formally describe Devils Tower were Henry Newton and Walter Jenney, who visited the Black Hills in 1875 with a military expedition led by Lt. Col. Richard Dodge.  They described the Tower, then called Bear Lodge Butte or Mato Teepee, as a “great obelisk of trachyte”.  They noted that the various igneous bodies of the region appeared to have been forced up through sedimentary strata which were upturned around them.  They found no evidence of volcanic activity.

Carpenter (1888) considered Devils Tower to be the remnant of a volcano, calling it a volcanic plug.  Russell (1896) concurred, but recommended the term plutonic plug.  Pirsson (1894) disagreed; he felt that volcanic origin and the shallow cooling it implied were unlikely, due to the Tower's massive columnar structure.  He suggested it was a remnant of a much larger and possibly laccolithic body.

View from the southeast.  Click to view remains of a wooden stake ladder to the summit, built by local ranchers in 1893, and fixed up by the Park Service in 1976 (near middle of face).

Jagger (1901) also concluded that Devils Tower, along with the Little Missouri Buttes to the west, were the remains of a large laccolith, no longer connected due to erosion.  He believed that massive columns required slow cooling deep underground; the neck of a volcano would have been too shallow.  However, Darton (1909) pointed out that prominent columnar jointing had been found in volcanic stocks in northwest New Mexico.  In addition, he considered the amount of erosion proposed by Jagger unlikely.  For these reasons, Darton favored volcanic origins.

Effinger (1934) ended his paper concluding that it was most reasonable to view Devils Tower as a remnant of a small independent laccolith.  He drew this diagram (right) to illustrate his take on the Tower's origins (click to view).

Northeast face.

Aside from Effinger’s review, Devils Tower received little attention from geologists from 1909 until the mid-1950s, when Robinson (1956) compiled a detailed map of the area.  He concluded that the Tower “is a body of intrusive igneous rock, which was never much larger in diameter than the present base of the Tower, and which at depth (1000 ft or more) is connected to a sill or laccolith type body.” (A well drilled in a structural dome between the Tower and the Little Missouri Buttes had hit phonolite at 1400 ft).  He also made clear that much uncertainty remained:  

“even today after detailed geologic mapping of the area, no conclusive proof of its mode of origin can be presented.”

The volcanic stock hypothesis did not die, however, and in fact made a strong comeback in 1980 based on work by Halvorson.  He cited as evidence the presence of alloclastic breccia with a volcanic glass matrix, the depressions surrounding both the Tower and the Buttes probably representing collapsed magma chambers, contemporaneous volcanic activity nearby, and the clear resemblance to known volcanic necks, like those in New Mexico pointed out earlier by Darton (1909).

Most recently, Zavada et al. (2009) suggested that Devils Tower is the remnant of an eroded lava lake “emplaced into a broad crater of a phreatomagmatic volcano”.  Their conclusion was based on suspected phreatomagmatic units around the Tower, gravimetric studies and modeling.  They proposed that columns formed by cooling on "two thermal fronts originally representing the bottom and flat top of the original phonolite lava lake."

View from the north ... of the remains of a phonolite lake?  How do we explain that to the public??

So in spite of its distinctive structure and composition, and much study, the origin of Devils Tower remains a puzzle.  The debate continues over the nature of the intrusion -- shallow or deep? volcano or laccolith?  Meanwhile, 400,000 people visit Devils Tower National Monument every year.  Many take photos, and quite a few pay the money to drive up to the Visitor Center where they can buy a few souvenirs before they head off to Mt. Rushmore or Yellowstone, their next stop.  A minority walk around the base, looking up in awe.  I suspect only a tiny percentage contemplate the origins of this improbable rock.

But in my cynicism, I tend to underestimate visitors and their appreciation of Devils Tower.  One evening in the campground, I watched as folks set up their tents (yes, there still are many tent campers) and arranged table cloths and dinnerware on their picnic tables.  A man was on his phone with a child at home, trying really hard to share his excitement:  “You aren’t going to believe where I am! ... can you guess?  I’m looking at a giant rock!!  [pause]  you don’t know?!! get your mother ... she’ll know!”  Mom did know, and got to hear how unbelievably huge and beautiful and mysterious Devils Tower really is.

The many faces of Devils Tower, courtesy Google Images; click to view.

Literature Cited

Carpenter, F.R.  1888.  Notes on the geology of the Black Hills. Prelim report of the Dakota School of Mines, Rapid City.

Darton, N.H.  1909.  Geology and water resources of the northern portion of the Black Hills and adjoining regions in South Dakota and Wyoming. USGS Prof. Paper No. 65, pp. 68-69.

Effinger, W.L.  1934.  A report on the geology of Devils Tower National Monument.  Berkeley, CA:  National Park Service, Field Division of Education.  Available at:  http://www.nps.gov/history/history/online_books/berkeley/effinger1/index.htm

Halvorson, D.L.  1980.  Geology and petrology of the Devils Tower, Missouri Buttes, and Barlow Canyon area, Crook County Wyoming: Grand Forks, University of North Dakota, Ph.D. thesis, 218 p.

Jaggar, T. A. Jr.  1901  Laccoliths of the Black Hills. 21st Ann Rep, USGS, Pt. 3.

Moran, Thomas.  1894 (January).  Artist’s adventures.  A journey to the Devil’s Tower.  The Century Magazine pp 450-455.  Available at:  http://www.unz.org/Pub/Century-1894jan-00450

Newton, H. and Jenney, W.P.  1880.  Rept. on the geology and resources of the Black Hills of Dakota. U. S. Geographic and Geologic Survey of the Rocky Mountains Region. Washington.

Pirsson, V. L.  1894.  Description of the character of the igneous rocks making up Mato Teepee and the Little Missouri Buttes.  Amer. Journal Science, Vol. XLVII, pp. 341-346.

Robinson, C.S.  1956.  Geology of Devils Tower National Monument, Wyoming.  USGS Geol. Surv. Bull. 1021-1.  Available at: http://www.nps.gov/history/history/online_books/deto/index.htm 

Russell, I. C.  1896.  Igneous intrusions in the neighborhood of the Black Hills of Dakota.  Journal Geology.  Vol. 4.

Zavada et al.  2009.  On the geological origin of Devils Tower, Wyoming; a new hypothesis constrained by field research, analogue and thermal modeling data, and gravimetric survey.  Abstracts with Programs - Geological Society of America 41: 444.