From the Gangplank to the Big Hole: Deep History in the High Plains

If you drive west along Interstate 80 from Cheyenne, Wyoming, you'll soon begin to cross the Rocky Mountains. But the strange thing is, you might not realize it. In this part of Wyoming, the high plains are really, really high. They form a great ramp that goes up to 7,000 feet--almost to the top of this section of the Laramie Range, which is low by Rocky Mountain standards. When you get to the place where the plains touch the granite of the mountains, you're higher than any point in the eastern United States, but it's mostly a big flat place. The only clue at first that you're crossing a mountain range are a few low granite hills that poke up through the plains.

This highest section of the high plains is called the Gangplank, and it played a key role in American history. When the Union Pacific railroad was looking for a way across the Rockies, they weren't interested in the soaring, craggy peaks to the north and south. That's much grander scenery, but you don't want to build a railroad through it. What they needed was a smooth climb to a gentle mountain pass, and this is where they found it. The trains still go this way today, sharing the Gangplank with Interstate 80.

The Gangplank isn't the most scenic entrance to the Rockies, but it's an fascinating place--once you know what what you're looking at. This is a landscape with a story to tell, and that story is much, much older than the Transcontinental Railroad. If you could go back in time about 30 million years, most of the Rocky Mountain region looked a lot like this stretch of I-80 today. The mountains were already millions of years old, and they had eroded until they were almost buried in their own debris, which formed sloping plains like the Gangplank. Only the tops of the mountains protruded, as the granite hills still do here. The Gangplank region is a fossil landscape; a window into a mostly-vanished world.

This world began to change a few million years ago, when rivers began to eat away at the plains and uncover the mountains. Geologists aren't sure if this happened because the whole region was lifted, making the rivers steeper, or because the climate grew wetter, making the rivers more powerful. Perhaps it was a bit of both. In any case, the mountains were uncovered, in an event known as the Exhumation of the Rockies. Because the Gangplank is between two rivers, it was left behind as one of the only places where the plains still rise to the mountaintops. The rocks here preserve an unusually complete record of the rise, burial, and resurrection of the Rocky Mountains.

It's an epic tale, and to understand it, a good place to start is to leave the interstate at Exit 345, and pull into a parking lot full of big rigs. At the east end of the lot is a sign about the Gangplank and its geologic history, with a nice (though slightly outdated) diagram, shown to the right.

As the image shows, three very different geologic structures come together here. The "You are here" marker is at the edge of the Gangplank, which is made of almost-horizontal layers of rock. At 5 to 37 million years old, these rocks are youngsters. But just a couple miles west, you enter a far more ancient landscape. Here the granite and metamorphic rocks are between 1.4 and 1.7 billion years old. Hard, ancient rocks like this are called basement rocks, because they form the foundation for most of North America (though they're often deeply buried below younger layers of sedimentary rock).

Now, if you look north or south, you may see layers of red and tan rock that poke up through the plains at a steep angle. These range from about 300 million to 65 million years old. They're mostly buried here, but in other parts of the Rockies they're exposed and and breathtaking. At places like Red Rocks near Denver, or the Flatirons near Boulder, you can see that they lean against the ancient basement rocks like a stack of plywood against a wall.

But they weren't always tilted like this. Before the Rockies began to rise around 70 million years ago, they were flat. Then the basement rocks rose from deep underground and tilted them upward; pushing them aside like a man coming up through a trap door. But what about the near-horizontal rocks of the Gangplank? Why weren't they shoved sideways? Because they weren't there yet. They formed as the mountains began to wear down, and that's why they're still relatively flat.

That's the basic story you get as you stand on the high, flat expanse of the Gangplank and read the sign above. But standing by the interstate looking at a sign isn't that satisfying. What we need is a place where we can descend down into the the rocks sketched on the sign, and see them in real life. Luckily, there's just such a place a few miles to the south, just across the Colorado border.

To get there, take the I-80 service road a couple miles west and head south on Harriman Road. It's a dirt road, but a relatively civilized one. To your left, you'll soon get a good look at the tilted sedimentary rocks that lean against the ancient granite. After driving among stubby granite hills for a few miles, you'll emerge into a sweeping landscape of red mesas and the high, snow-capped peaks of the Colorado Front Range. It's big, beautiful country.

Our destination is Red Mountain Open Space, a public park managed by the city of Fort Collins, Colorado. The park is in a valley the ranchers here call the Big Hole. As holes go, it is quite large, especially if you look into it from the edge of the Gangplank. To get a basic sense of the geology of the Big Hole, take a look back at the sign from I-80. South of the Gangplank is an eroded region of older, tilted sedimentary rocks. That's the landscape we're in now, but with one wrinkle.

No, seriously--there's an actual wrinkle in the rocks here, and it's a big one, called the Sand Creek Anticline. To see it, head east from the open space parking lot on Bent Rock trail, following Sand Creek. Soon you'll enter a canyon that cuts straight through a hill of red and salmon-colored rock. We'll call this Red Mountain. The rocks here rise and then fall again in a big arch, or anticline. The picture above shows the curve in the rocks inside the canyon. When you leave the canyon and look back to the south at Red Mountain, as in the picture below, you get an even better view of the anticline. The yellow lines show how the rock layers bend inside the canyon, while the steep layers in the foreground have eroded into formations called flatirons. This anticline is an asymmetrical one--the layers on one side are much steeper than the other. There's a reason for that, as we'll see.

If you turn around here and look north, you'll see that the Sand Creek Anticline runs for several miles through the Big Hole. The hill in the next picture doesn't have a name that I can find, so I'll call it Swoop Mountain, because the rocks here take a big swooping path across it. On the left side of Swoop Mountain, you can see the same red rocks that form Red Mountain, but there are several younger layers lying on top (these layers once covered Red Mountain, too, but have eroded away.) As the yellow lines show, all the layers across Swoop Mountain drop down from the west (slanting down off the Rocky Mountain front) and suddenly rise again steeply, like the rocks in Red Mountain (visible on the far right). Then they slope gradually to the east. Some of the yellow lines go through empty space, because some of the rocks have eroded away. The lines show how the rocks would bend if they were all still there. Swoop Mountain shows that surface landforms don't necessarily mirror the shape of the rocks below. Here the rocks bend downward right where the surface bends upward as a hill.

From down here in the Big Hole, we see that the sign on I-80 is basically correct, but simplified. The layers here do slope down to the east, but they have a sharp bend in the middle that doesn't appear on the sign. The reason for the bend, most likely, is that the basement rocks below the surface have a fault in them--they've broken into two blocks here. The block on the east side of the valley has lifted and rotated, leaving the sedimentary rocks draped over the fault like carpet over a cracked concrete floor.

But what about the Gangplank? How does it fit into all this? You can see it on the horizon in the picture above, but to get a better look, we need to hike to the other side of Swoop Mountain. From there, you see the view below. The cliffs on the right are the Chalk Bluffs; the escarpment formed by the southern edge of the Gangplank. The picture shows that these rock layers are basically flat. They weren't tilted and warped by the rising Rockies, because they formed afterward--from the eroding Rockies. The red layers on the left are tilted, though you can't see it very well in the picture.

The contact between these two rock layers is an example of an angular unconformity--a place where rocks of different ages and angles meet. There's no physical gap between the two layers, but over 150 million years is missing from the rock record. The deep red rocks are about 245 to 285 million years--older than the dinosaurs. The white rocks sitting on top of them here--more visible in the picture below--are called the White River Formation. They formed in a time when the dinosaurs were long extinct and strange prehistoric mammals roamed the West. But 150 million years is is just a minor gap. Further east, the White River Formation lies directly on top of basement rocks that are at least 1,400 million years old. That's a lot of missing history. Luckily, much of it can be found in other places.

I should mention one more feature, because if you take this hike, you'll notice it. Walking back toward the parking lot, on the east side of Swoop and Red Mountains, you'll see the weird crest of white rocks in the image below. Those are layers of gypsum from the Permian Period, when this part of the country was a shallow, salty lagoon near a hot desert--similar to the salt flats of today's Persian Gulf. Water evaporated rapidly in these conditions, leaving behind deposits of gypsum (or alabaster, if you want it to sound fancy). These layers erode and dissolve faster than most other rocks, often leaving cavities that can cave in as sinkholes. Rattlesnakes like to gather in one of the sinkholes near here to spend the winter in cozy heaps, so watch your step in these parts.

We've covered a lot of ground here, and an enormous amount of time, so maybe it would be good to step back and look at the big picture. That would be the image below. This is an aerial view of the whole region, copied from Google Earth and labeled by me. Now, I'm librarian, not a geologist, so take it with a grain of salt--the lines and boundaries are far from exact, and will likely make real geologists cringe.

Satellite Image from Google Maps. Amateur annotations by the author.

Here we see the Big Hole in the right foreground, the basement rocks of the Laramie Range to the left, and the Gangplank to the northeast. Interstate 80 is a few miles past the top of the image. In the Big Hole, you can see how the sedimentary rocks tilt to the east, pop up again at the Sand Creek Anticline, and then tilt eastward again. To the northeast, the Gangplank is much less tilted, but it also slopes gradually eastward to form the the western Nebraska plains. The caprock of the gangplank is a coarse sandstone called the Ogallala Formation. It's very porous, and it gets waterlogged below the ground to the east, where it forms the famous Ogalalla Aquifer.

This is vast, impressive country, spanning big distances and stupendous amounts of time. It's a whole series of ancient worlds, stacked one on top of the other. That's true in many places, of course, but here you can descend into the remnants of those former worlds, and think about how they rose and fell. It's a wondrous landscape that can tell many stories, and the more stories you learn, the more wondrous it becomes.

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To Explore Further:

Geology Underfoot Along the Front Range. Lon Abbot and Terri Cook

Roadside Geology of Wyoming. David Lageson and Darwin Spearing

Roadside Geology of Colorado. Felicie Williams and Halka Chronic

Geologic map of the Table Mountain quadrangle and adjacent parts of the Round Butte and Buckeye quadrangles, Larimer County, Colorado, and Laramie County, Wyoming. Courtright, T.R., and Braddock, W.A.

Geologic map of the Laramie 30' x 60' quadrangle, Albany and Laramie Counties, Wyoming: Wyoming. Ver Ploeg, A.J., and Boyd, C.S.

Stratigraphic Chart of Colorado.