In August 2025, work sent me on my most far-flung environmental due diligence site visit yet. I flew into Denver and drove four hours to the town nearest the site that had a hotel. This town was still an hour and a half away from the site. The exotic location: Western Nebraska. I’m never discouraged by remote areas, though, because often the landscapes tell amazing stories. Along this drive I also stopped to unlock some core memories.

I was back on the Oregon Trail. Now, I was not a talented pioneer back in 5th grade computer class. I rarely made it as far as Chimney Rock without dying of dysentery, and my wagon train usually met a tragic end rafting the Columbia River (https://www.died-of-dysentery.com/stories/rafting-columbia.html). I wasn’t going to miss a chance to see Chimney Rock in the flesh with only a brief detour!

I stopped by on a day so hot that the ranger assured me the rattlesnakes wouldn’t bother me – they were too smart to be out. With the disquieting knowledge that I was then stupider than a rattlesnake, I crunched down the gravel path towards the monument. A sign let me know that the “chimney” has shrunk by 80 feet since the wagon trains first rolled by. It’s been worn by the weather and, as legend has it, the occasional cannonball by a bored soldier. I was disappointed that barbed wire fences stood between me and a fascinating outcrop. Like thousands of people before me (97% of pioneer journals mention Chimney Rock) I marveled at the strangeness of nature’s formation here.

My inner geologist voice immediately started speculating about sedimentary formations. What made up the spire? Was it different from the base? What’s that white layer? I can’t help it. Let’s dive into some maps to get acquainted with the region. First, here’s an overview of Nebraska. My trip took me over to the Wildcat Hills on its western edge.

These Wildcat Hills are highlighted on the zoomed-in geologic map below. They’re bordered by the North Fork Platte River to the north and Pumpkin Creek to the south.

There are three geologic formations on the map, but only two matter for our Chimney Rock story – the Arikaree Formation (orange) and the Brule Clay member of the White River formation (tan). The third and youngest Ogallala formation (brown) is the star of the show further northeast in the Nebraska Sand Hills. The Brule Clay makes up the wide base of Chimney Rock, and the Arikaree Formation makes up the chimney itself as well as the spine of the Wildcat Hills.

Let’s map those two formations on Chimney Rock itself:

Modified from a photo by the Nebraska Historical Society.

Looking at layers like those in Chimney Rock, we can assume that they show a progression of time from the oldest on the bottom to the youngest on the top – the geologic “principle of superposition”. But how old is the bottom, and how young is the top?

Scientists have been able to use tiny amounts of uranium and lead trapped in zircon crystals (https://en.wikipedia.org/wiki/Detrital_zircon_geochronology) to date ashfall tuff such as this layer of Upper Whitney Ash. It’s a method of radioisotope dating. There is little doubt about ashfall tuff’s relationship to the date of the eruption, since it fell straight from the sky. The time between the cooling of the magma below the crystallization temperature of lead, the eruption of that magma as ash, and the ash landing on some poor oreodont in Nebraska is a mere rounding error in geologic time.

This method does not work with with the ash particles in the sedimentary portions of the Brule Clay or Arikaree Formation. That ash was deposited upwind or upstream, eroded, and carried to Chimney Rock on an unknown schedule. In these sedimentary formations, geologists have to rely on the paleontologists. Based on the principle of superposition, generations of paleontologists have meticulously cataloged and cross-referenced fossil layers across North America. The paleontologists then argue about where to draw the boundary between the geologic epochs based on the evolutionary progression of the fossils. The geologists try to impose some quantitative ages on the qualitative ages the paleontologists come up with by matching up fossils with volcanic ash deposits, but it can get contentious. So you’ll see on the timeline below that some formations with good radioisotope dates have ages in millions of years, and those only marked by fossils are labeled by ” ‘cenes”.

The timeline puts the Brule Clay and Arikaree formation in the context of bigger trends in Midwest geologic history.

Chimney Rock shows us a chunk of time somewhere between 34 to 19 million years ago, with a two million year gap (“unconformity”) between the base and the chimney. This is sedimentary rock – made of sediment that came from somewhere else – so where did it come from? We need to look at the context of the region to figure it out.

The rise of the Rocky Mountains slowed to a stop about 55 million years ago, followed by a pause in geologic uplift that sent only small amounts of sediment down from the new mountains. The combination of accelerating erosion from the Rocky Mountains and massive volcanic eruptions upwind in Utah created giant piles of eroded sediment including the Brule Clay and Arikaree.

The older Brule Clay at the base consists of silt-sized dust particles carried by the wind, called loess. More than 50% of this dust is of volcanic origin, carried from massive calderas erupting in Utah as the Basin and Range Province stretched apart. Most of the ash was deposited in Colorado and mixed with Rocky Mountain sediment by the streams that carried it to Nebraska. However, some eruptions were so enormous that airborne ash plumes stretched all the way to Nebraska. This airborne ash settled as “ash fall tuff” in stark white layers that are visible in the cliffs, and the two layers in the Brule Clay are called the Upper and Lower Whitney Ash.

Caldera locations from Utah DNR

The Brule Clay was formed on a high, dusty plateau where sediment was deposited effectively enough to preserve a host of fossils. It documents a steppe landscape occupied by oreodonts (https://www.nps.gov/articles/000/oreodont.htm) (a charming variety of ruminant pig, up to 5 feet in length), tiny deer called leptomeryx (https://en.wikipedia.org/wiki/Leptomeryx), and giant bathornithidae (https://en.wikipedia.org/wiki/Bathornithidae). These mercifully extinct flightless birds were up to 2 meters tall and ate the tiny deer for dinner. They liked to lived in marshlands, so I guess they were the alligators equivalents of their ecosystem.

The younger Arikaree formation is a silty sandstone that contains loess but also consists of river channel, floodplain, and pond deposits. This formation represents a more dynamic landscape with creeks and wetlands. About 25% of the sediment consists of volcanic ash, also imported from volcanoes in Utah. Arikaree fossils document a rolling grassland with herds of pony-sized horses, sheep-sized camels, the deer-sized Syndyoceras (https://en.wikipedia.org/wiki/Syndyoceras)with its knobbly face horns, and strange ancient critters called chalicotheres (https://eartharchives.org/articles/extinct-hoofed-animals-looked-like-gorilla-horse-hybrids/index.html). These fellows were larger than a large horse and had a horselike head, long front legs, and shorter hind legs. Each of their feet had three toes, each of which in place of a hoof bore an intimidating claw. Mammals grew larger in the Miocene than in the Oligocene and were approaching their modern forms, but are definitely strangers to our modern creatures.

While the Oregon Trail carried a stream of pioneers from east to west, all the sediment they marveled at in Nebraska came from the opposite direction – west to east.

Approximately 18 million years after the Arikaree Formation was deposited, uplift occurred under the high plains. The plains were jacked up so quickly that streams eroded vertically much faster than they could erode sediment laterally, resulting in broad plateaus separated by rivers.

The North Platte River and Pumpkin Creek drainages gnawed at the flanks of the Wildcat Hills over the past 5 million years to create the landscape of western Nebraska as we see it today. Much like the sea wears away at cliff to make sea stacks on the coast, the action of streams in this region has worn away at the Wildcat Hills until Courthouse Rock, Jail Rock, and Chimney Rock stand like land-locked sea stacks. Lime-cemented layers of sandstone in the Arikaree Formation are just resistant enough to form protective caps over the soft Brule Clay and other White River sediments.

Fewer pioneers wrote about Courthouse Rock and Jail Rock, although they would have come into view before Chimney Rock. Wagon train travelers would have watched Courthouse and Jail Rocks grow steadily larger for three days (https://historicoregoncity.org/2019/04/03/oregon-trail-mileposts/) before reaching them. The sun was low in the sky and the glare kept me from taking good photos of Courthouse Rock, but here’s the Jail:

I did meet a critter here – Nebraska’s state reptile (https://www.wowt.com/2022/10/28/ornate-box-turtle-named-nebraskas-state-reptile/). The ornate box turtle was not at all happy to see me and made the cutest hissing noises as he dove into the sagebrush. The evening was cooling to a pleasant temperature and I suppose he was enjoying his previously human-free stroll. I marveled at the extraordinarily flat landscape downstream of the Wildcat Hills and was only a little sorry to disturb him. So many little oreodont (and even turtle) fossils are sleeping beneath those rolling plains, waiting to be exposed over new epochs as erosion wears the Jail flat and topples Chimney Rock.

An oreodont, by Ray Troll

References:

• Nebraska State Historical Society. Marker Monday: Chimney Rock. https://history.nebraska.gov/marker-monday-chimney-rock/ (https://history.nebraska.gov/marker-monday-chimney-rock/)

• North Dakota Geological Survey. No date. X Marks the Sport: #13, Coffins Buttes. https://www.dmr.nd.gov/ndgs/SpotContest/contest13/ (https://www.dmr.nd.gov/ndgs/SpotContest/contest13/)

• U.S. Geological Survey. No date. Geological Survey Bulletin 1493, The Geologic Story of the Great Plains: Early History. https://www.nps.gov/parkhistory/online_books/geology/publications/bul/1493/sec3.htm (https://www.nps.gov/parkhistory/online_books/geology/publications/bul/1493/sec3.htm)

• Roger K. Pabian and James B. Swinehart II. 1979. Geologic History of Scotts Bluff National Monument. Educational Circular No. 3 Published by Conservation and Survey Division, University of Nebraska – Lincoln. February. https://npshistory.com/publications/scbl/geologic-history.pdf (https://npshistory.com/publications/scbl/geologic-history.pdf).

• C.G. Cunningham and T.A. Steven. 1979. Mount Belknap and Red Hills Calderas and Associated Rocks, Marysvale Volcanic Field, West-Central Utah. Geological Survey Bulletin 1468. https://pubs.usgs.gov/bul/1468/report.pdf (https://pubs.usgs.gov/bul/1468/report.pdf)

• Terri Cook and Lon Abbott. 2018. Travels in Geology: Western Nebraska’s Geoheritage Gems. May 24. https://www.earthmagazine.org/article/travels-geology-western-nebraskas-geoheritage-gems/ (https://www.earthmagazine.org/article/travels-geology-western-nebraskas-geoheritage-gems/)

• Edwin E. Larson and Emmett Evanoff. 1998. Tephrostratigraphy and source of the tuffs of the White River Sequence. Geological Society of American Special Paper 325. https://www.nrc.gov/docs/ML0911/ML091120799.pdf (https://www.nrc.gov/docs/ML0911/ML091120799.pdf) Pages 9 to 21 of PDF.

• Utah DNR Online Maps. 2017. Utah Volcanics. December 1. https://utahdnr.hub.arcgis.com/maps/acdc4d32419a43cda6e60595d17113fc/about (https://utahdnr.hub.arcgis.com/maps/acdc4d32419a43cda6e60595d17113fc/about)