Missing Rocks Found

Sedimentary rocks in western Nebraska, made of sand, silt and clay from the Rocky Mountains.

We students of Wyoming geology are quite familiar with the Laramide Orogeny, the deformation of the Earth’s crust that produced the Rocky Mountains.  We’re surrounded by Laramide structures, and those of us older than a certain age drew many cross-sections through these mountain ranges in learning how they came to be.

Wind River Range, a typical Laramide uplift (Advanced Geomorphology course, 1984).

Laramide uplifts are fairly similar in structure.  Often there’s a steep reverse fault on one side, where the range was tilted up.  On the other, sedimentary rocks form flatirons, hogbacks and strike valleys.  These sedimentary strata are gone from the broad crest of the range, exposing ancient igneous and metamorphic rocks from the basement of the continent.

Two-billion-year-old quartzite in the high country of the the Medicine Bow Mountains.

But what happened to the old sedimentary cover? ... where are the missing rocks?  Though we often think of mountains as immutable, they are hardly permanent.  Erosion sets in as soon as uplift starts, and a range is worn down as it’s created.  The sedimentary rocks that covered the mountain ranges of the Rockies were deformed, fractured, weathered and carried away by water and wind.  In the case of our local uplift, the Laramie Mountains, they ended up where many of our plastic grocery bags go -- in western Nebraska.

I was in western Nebraska myself recently, and friends suggested I check out Toadstool Geological Park.  That’s the answer to the “where” part of my recent geo-challenge.  As to “why” -- I went to see the curious erosional features, fossils and preserved trackways of now-extinct mammals for which the park is known.

Courtesy USDA Forest Service, Oglala National Grassland.

When I arrived I discovered an even more compelling reason to visit -- rocks of the White River Group are particularly well-exposed in the park.  In fact it’s considered the “type section” for the group, the standard against which “all other similar-aged deposits in North America are compared” (from park brochure).  White River rocks are composed of sand, silt and clay, but not just any sand, silt and clay.  This is material eroded off the Rocky Mountains to the west, the remains of those missing sedimentary rocks!

The Laramide Orogeny ended around 45 million years ago, but erosion of the mountains continued.  The resulting debris accumulated in adjacent basins to depths of over a thousand feet in places.  By Oligocene time, sediments of the White River Group formed a plain sloping down from mountain ranges in eastern Wyoming into Nebraska.  The mountains were largely buried in their own debris, with just a bit of the crest exposed.

Laramie Mountains ca 30 million years ago.  Dashed lines indicate hypothetical height of uplift in the absence of erosion.  Yellow sediments include the White River Group (Advanced Geomorphology course, 1984).

It appears there were three of these episodes of deposition; White River sediments represent the earliest.  After each episode, deposition gave way to erosion.  Why?  Nobody's sure.  Perhaps there was regional uplift or climate change or both.  In any case, rivers were rejuvenated and they set to work carrying off material deposited just a few million years earlier, including White River sediments.

There's little left of the White River Group in eastern Wyoming and in Nebraska it’s largely covered by younger sediments.  That’s why I was so excited to find it at Toadstool Park!

"Toadstool Park" in Badlands (Brule formation) northwest of Adelia, Sioux County, Nebraska."
Photo by N.H. Darton, 1987; courtesy USGS.

The great geo-mapper Nelson Horatio Darton arrived here for fieldwork in 1897.  He found a picnic table, a sign that said “Toadstool Park” and much of interest, just as I did 106 years later.  In his project report, he described and mapped two formations (layers) within the White River Group -- the older Chadron (Eocene) and the younger Brulé (Oligocene).

Darton's geologic map of western Nebraska, north part.  Chadron Formation is orange, Brulé is the adjacent pale pink unit.  Arrow indicates area of Toadstool Park.

The badlands at Toadstool Geological Park are developed on the Brulé Formation (Orella member).  These sediments were laid down by slow meandering rivers in a hot humid environment -- quite different from today!  Now they are rock, alternating layers of sandstone and clay.  The sandstones were once sandbars and sandy river beds, the clays were adjacent floodplains and wetlands.  The contrasting hardness of the two rock types gives the area its charm -- sculptures produced through differential weathering.

Many little toadstools, with sandstone caps and clay stalks. 

Fallen toadstools.  The sandstone caps protect underlying softer rocks for awhile, but erosion always wins in the end and the structures collapse. 

I was intrigued by the fossilized trackways.  They look very much like modern-day game or stock trails, especially after a heavy rain or near streams.  I could easily imagine herds of large mammals walking along the old rivers.  The slab below is part of the most extensive Oligocene trackway known in North America, on the order of 3/4 of a mile long.

Footprints in sandstone.

It was a neat experience to walk on the rock slabs and think about ancient titanotheres (rhino-like beasts) and giant pigs passing by 30 million years ago.  Then it occurred to me ... the wet sandy mud in which they left their prints may well have come from the Laramie Mountains.  They in their time, and we in ours, were walking on the remains of the old sedimentary cover of some Laramide structure -- the missing rocks.

Sparky on the trail of extinct mammals.

How to get there

From Hot Springs, South Dakota take Highway 71 about 37 miles south to Toadstool Road.  Follow it 11 miles to Forest Service Road #902, then go 1.4 miles to the Toadstool Campground.  This route was clearly signed when I took it.  OR ... From Crawford, Nebraska go to the intersection of Highways 20 and 2, then north on Highway 2 to Toadstool Road (4.2 miles).  Follow Toadstool Road 11.4 miles to FS Road #902 and continue as above.  Some of the unpaved roads get quite slick when wet, especially those coming in from the north.  There is a small campground with picnic tables, sun shelters and outhouses (no water), and several hiking trails.

From Google Earth; click photo to view.

More Information

Roadside Geology of Nebraska has sections devoted to Toadstool Park and the nearby Hudson-Meng Bison Kill site.  I like this book because it has in-depth coverage of destinations as well as the usual roadside info.

You can read Darton’s description and discussion of the White River Group in his Preliminary report on the geology and water resources of Nebraska west of the one hundred and third meridian (1903).

Footprints in Stone: Trackways at Toadstool Park is a nice overview of the park’s geology and paleontology; by Steven Veatch (2000).

If you go to Toadstool Park, you might want to visit the University of Nebraska's Trailside Museum of Natural History at nearby Fort Robinson State Park.  The focus is Cenozoic mammal fossils of the region.

A Novel Discovery by an Accidental Botanist (a letter to the Earth)

By the late Aven Nelson, Professor of Botany, University of Wyoming, Laramie

Photo by Hollis Marriott, 2004.

The rugged high country of the northern Laramie Mountains is home to a rare columbine that grows nowhere else on earth.  It has showy flowers as do all columbines, but few people ever see them, for the plant is small and grows on all-but-inaccessible rock outcrops.  I was fortunate to have found it.

It was back in the early days of my botanical career, a curious story in itself.  I came to Laramie in 1887 to teach English at the new University of Wyoming, but the administration had inadvertently hired two English professors.  I had a Bachelor’s Degree from the Missouri State Normal School, whereas W. I. Smith had a Master’s Degree from Dartmouth College.  He got the job.

Fortunately the University needed an instructor for botany, zoology, physical geography, hygiene and several other subjects.  I presented my credentials -- an assistantship in biology, a love of natural history, several wildflower collections, and attendance at six lectures on plants.  I was promptly appointed Professor of Biology, Instructor of Calisthenics and University Librarian.  I was to study the flora (native plants) of Wyoming as well.  Thus began my career as an accidental botanist.

Wyoming was a wonderful place to botanize, for little was known about the state’s flora.  There were endless opportunities for exploration and adventure, and it was likely that “novelties” (species new to science) awaited discovery.  This was all very exciting for a young man embarking on new and unexpected career!

Research began in the summer of 1893.  I collected plant specimens in the vicinity of Laramie and spent the following winter identifying them.  Resources at the University were limited, so I sent my specimens to academic experts elsewhere for verification.  That was a learning experience!  I was told some were inadequate, and that my collection information was incomplete.  I was determined not to let that happen again.

The next two field seasons I traveled across Wyoming by horseback and wagon with a guide, an outfitter and a diligent student assistant.  We generally camped out and though we carried a tent we rarely used it, the weather being fine for sleeping under the stars.  Plants were carefully collected, pressed and dried, and all necessary information recorded.

Our collecting sites in 1894 and 1895 (click map to view).

In early August of 1895, we were traveling north along the east side of the Laramie Mountains, stopping periodically to collect.  We decided to climb Laramie Peak and approached it by way of Cotton-wood Cañon, where I collected a plant from dry crevices in abrupt cliffs.  It was a columbine, but not the common one of the Rocky Mountains.  This was a small delicate plant with white flowers.

My first collection of the Laramie columbine, from Cotton-wood Cañon in 1895.
Specimen from the Rocky Mountain Herbarium, University of Wyoming.

Study that winter revealed that the columbine was indeed a novelty.  I named it Aquilegia laramiensis.  Over a century later my choice of names would prove prescient, as surveys confirmed that the Laramie columbine grows only in the Laramie Mountains.

It has been quite entertaining to look down from my perch on high and watch today’s botanists search for the Laramie columbine!  It’s no easier to find now than it was back in my day, even with modern transportion.  There are only 51 known sites, all with scattered plants growing on hard-to-get-to rock outcrops where few people go.  Perhaps this is a blessing ... perhaps inaccessibility will ensure that the Laramie columbine thrives in its rugged rocky home for many years to come.  I certainly hope so!

Typical habitat of the Laramie columbine; photo by Dennis Horning.

[Editor’s note:  From his rather inauspicious beginnings, Aven Nelson went on to a long and productive career that earned him the title “Father of Wyoming Botany.”  By the time he died in 1952 at age 93, he had described numerous novelties, published over 100 academic articles, and mentored many students who would become prominent botanists themselves.]

Blogger Hollis Marriott has been crossing paths with Aven Nelson since she moved to Wyoming in 1977, most recently while doing surveys for the Laramie columbine.  This post was copied verbatim from a mysterious letter left on her field vehicle, while parked at the base of Laramie Peak.

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Note on Writing and Blogging

This post began as a sketch for an article for our local paper, the Laramie Boomerang, part of a series by the Albany County Museum Coalition.  I thought that if Aven Nelson could somehow tell me about the Laramie columbine, it would personalize the story and make it more engaging.  As I wrote, I became so taken by the idea of Nelson himself "writing" the article that I ran the concept by the editors.  We decided it didn’t fit with the style of the series, so I continued with a third-person version.

But of course there was no need to toss Aven’s story.  It’s right at home here in the blogosphere.  We bloggers have the freedom to write in our voice, in ways that excite us.  Most likely there will be sympathetic readers out there somewhere who will enjoy our creations.