To the northeast of Yellowstone National Park lies the Beartooth Highway, one of the most beautiful stretches of road in the Greater Yellowstone Ecosystem. Though this high altitude traverse is occasionally hairpin and hair-raising to some, these high turns and views offer a glimpse of some world-class scenery and classic Wyoming geology. Having been proclaimed America’s most beautiful highway, it is a truly jaw-dropping section of road that is as interesting as its expansive views.
This piece is focused as a broad overview between Beartooth Lake and Red Lodge, Montana, and perhaps these three aspects are the most crucial when considering this area’s natural history: the rocks, the Laramide Orogeny, and erosion/glaciers. In short, the rocks - they are some of the oldest exposed on earth, and the only reason they are exposed so high up is because of thousands of feet of uplift. This uplift was induced by compression of the continent during the Laramide 50-60 million years ago, and since then, the processes of time and erosion have continued their methodical handiwork. Large glaciers dominated the area as recently as 12,000 years ago, and wind, rain, snow, and ice continue to sculpt the Beartooths to this day.
Let’s start with the rocks. On this eastern portion of the Beartooth Range, 90% of the plateau consists of metamorphosed granodiorites that were intruded about 2.7 Ga during the Archean eon ("Ga" basically stands for "billion years ago/old"). That is literally two eons ago on the geologic timescale. Granodiorite is essentially a granite, containing the minerals feldspar (pink and white), quartz (clear, milky, or smokey), and the flaky micas biotite (black) and muscovite (clear). Being a granodiorite, it also contains the black iron/magnesium-rich mineral amphibole. Occasionally, you will see black dikes (magmatic intrusions) that cross-cut the granodiorite in various places. Shown to be younger by their cross-cutting nature, they are up to 20 feet wide and are usually around 1.7 Ga.
And so, this brings us to the present, but it should be noted: a trip over the Beartooth Pass is not only a soaring adventure via car, it is a trip back in time to some of the earliest glimpses of Earth history.
The other 10% of rocks is where it gets really interesting. Sandwiched between the granodiorites are the remnants of even older rocks that likely date back to 3.6 Ga. It’s not over yet though. Little recycled minerals of zircon - nature’s little timekeeper - in those rocks show ages of 3.9 Ga and older. Like clues for a detective, these rocks help piece together what Earth was doing only 500 million years after it formed (≈4.55 Ga). As it turns out, this is some of the only evidence we have from this time on Earth, and, therefore, these rocks offer glimpses into some of Earth’s earliest history and processes.
For example, at 3.6 Ga life probably did not exist. If it did, it consisted of cyanobacterial globs called stromatolites. That is it. Water is thought to have already been present, but there was no North America back then. Continents were smaller, and the rock here was part of a Wyoming-sized continent appropriately called the Wyoming Province. We have strong evidence that some form of modern plate tectonics was active by this time, and it caused the little bumper car continent of Wyoming to eventually crash and mold its edges with other small proto-continents. This kept happening for a while, and by about one billion years ago, these coalesced into most of what is today’s North America.
From there, starting about 540 Ma (million years ago), these clues to Earth’s early history got buried, and not lightly. Thousands of feet of sedimentary rock, caused by inundations of ancient seas and beaches, covered up the very rock you are standing on. Yet here you are, 9,500+ feet up and you are met with a plateau of glaciated tundra on top of multi-billion year old rock, which begs us to ask: “how?” In the Bighorn Basin just east of Red Lodge, these Archean rocks can be over 10,000 feet below the surface, as has been occasionally shown by oil drilling escapades back in the day. Continuing with this, the rocks of Beartooth Butte (at Beartooth Lake) give us a glimpse of consequential evidence: that deposited rock in fact once stood on top of the plateau as well (see photo). Thousands of feet of it that eventually eroded away as the Beartooths pushed up and out towards the sky.
This brings us to the Laramide Orogeny, the great upheaval that brought up many of Wyoming’s ranges. Indeed, so much of Wyoming’s geography was shaped by Laramide mountain building during the Late Cretaceous and early Cenozoic that the state’s motto could effectively be changed to “Wyoming: The Laramide State.” Named after the Laramie range, the Wind River, Teton, Bighorn, and Gros Ventre ranges all bear the stamp of “thick-skinned” Laramide uplift (the Absaroka Range just south of here are a volcanic story a few million years after the Laramide). These mountains are different than Glacier N.P., where thin sheets of rock overrode younger rocks. We geologists colloquially call this style of mountain building “thin-skinned” tectonics.
So what caused the 1,400 square mile Beartooth range (and other Wyoming ranges) to push itself through 10,000-15,000 feet of thick rock? Continental compression. Basically, if you squeeze a rock, it will break along discrete surfaces called faults. Keep squeezing the continent’s rocks for ten to twenty million years and you push up a mountain range along these faults. But not straight up, this squeezing formed faults at angles of 30-40 degrees (sometimes steeper), pushing the mass of rock outward as well as upward. You can visualize this relationship in the palisades near Red Lodge and along much of the Beartooth front. As the plateau’s rock rose up and out, it pushed the overlying limestones and sandstones out of the way, causing them to stand on-end.
Where are we now in Earth history? Dinosaurs have been roaming the land and are about to have a bad day. A giant swath of sea water has divided North America in two until the end of the Cretaceous. After the dinosaurs die out, mammals fill the niche and take over. Off the west coast, subduction of the Farallon plate has been active for millions of years in much the same way as subduction is occurring along Chile’s west coast today. At some point though, the slab went flat, propagating beneath North America at an angle between 10 and 20 degrees. Though how this happened is still a debate, the evidence is clear from imaging of modern flat-slab subduction beneath parts of South America: flat-slab equals Laramide-style mountains. The Sierra Pampeanas, mostly in Argentina, is evidence of this relationship, where thick sections of basement rock are being brought towards the surface by deep-seated, thick-skinned faulting.
In the Beartooth’s case, it took millions of years for this uplift to happen, but it would take over twice as long until the modern shape of Beartooth topography took form. This is where time comes in, and almost synonymous with time is erosion. We can see glimpses of erosion in real time, but over thousands to millions of years, erosion can do some hefty work. As mentioned, there was still a lot of rock sitting on top of the Beartooths that had to get removed, and this continued for millions of years. The arrival of the Yellowstone “hotspot” (thermal anomaly is probably a better term) in the last five million years probably gave erosion in the range a little push, and with the surrounding Greater Yellowstone area perched just a little bit higher, erosion has quickened by the simple effect of gravity.
Then came the ice. Lots of it. In the Yellowstone area, two major glacial periods are recorded. That is, there is evidence in the form of deposits from these massive glaciers. The first was the Bull Lake, approximately 150,000 to 160,000 years ago, but it is hard to say, given scant rock evidence, to what extent the Bull Lake glaciation shaped the Beartooths. However, glaciers likely covered large sections of Wyoming’s ranges at this time, spilling out into the basins and producing large bulldozed heaps of ground-up debris called moraines.
Evidence abounds for the most recent glacial period, however. Known as the Pinedale glaciation, this glaciation accounts for nearly all of the glacial features in Yellowstone and the Beartooth Range. At its maximum about 14,000 to 16,000 years ago, over 3,000 feet of ice covered Yellowstone. A lot of this ice came from the high Beartooths, where only the tallest peaks stuck out like sore thumbs (known as nunataks), almost like Antarctica today but only localized to high mountain ranges. From high up here, the glaciers flowed out onto the Yellowstone Plateau and cut out canyons all around the Beartooths and Greater Yellowstone area. Rock Creek on your way down from the pass is the perfect example of a canyon cut by glacial activity, given its U-shaped geometry and stair-stepping topography from the high plateau.
Since then, these masses of ice have all mostly disappeared, with the exception of some high isolated cirque glaciers (see photo; Grasshopper Glacier is also a famous example). A small event in the 1800’s, known as the Little Ice Age, probably added to these glaciers much like it did in Glacier N.P., but continued warming since the Industrial Revolution, likely induced by anthropogenic (i.e., human) factors, has shrunk what is left of these glaciers more and more each year with ensuing climate change.
Even the ecology and biology of the region is of great scientific interest in the coming years of documented climate change. Up here above 8,000 feet, pika, little mice-like creatures that scream at you, call this place home. These cute balls of fluff are experiencing more trouble as the climate begins to warm, pushing the high-altitude home of these buggers higher up to perhaps eventual extinction. The evidence is clear, as plants and trees rooted at lower elevations continue to climb with rising temperatures, so to do the homes of the pika, perhaps our best biological tickmark of climate change in the GYE.
And so, this brings us to the present, but it should be noted: a trip over the Beartooth Pass is not only a soaring adventure via car, it is a trip back in time to some of the earliest glimpses of Earth history. This is history you can stand on, with some of the oldest rocks on Earth, uplifted to their present height by tectonic forces and Yellowstone’s thermal buoyancy, and finally artistically shaped by persistent weathering and erosion. It is the Beartooth’s scenery, this product of weathering and erosion, that has proclaimed its grandeur to many, but this high range’s geology should be equally considered.
Editor's Note: Jacob Thacker became a geologist from his earliest curiosities of the mountains around his childhood home in Cody, WY. In 2014, he received his Master’s degree from Montana State University, where he researched the structural and mineralogical changes across a fault zone in the Stillwater mine at the Beartooth front near Nye, MT. Apart from geology, Jacob is an avid photographer and guitarist who spends the summer months backpacking and photographing the Greater Yellowstone area. He has worked for the Yellowstone Association, and is currently completing an internship with the NPS in Yellowstone, sponsored in part by the Geological Society of America, Americorps, and Conservation Legacy.