What Makes the Teton Mountains so Steep?

Teton Mountains Rising Out of Jackson Hole
The Teton Mountains rising abruptly out of the valley of Jackson Hole below Crystal Butte. Gros Ventre Wilderness, Wyoming

The Teton Mountains are a striking mountain range whose eastern face is world-renowned for its stark beauty, shooting unusually steep out of the flat valley floor of Jackson Hole. Crowned by the Grand Teton at 13,770 feet above sea level, the peak rises over 7,000 feet above the smooth landscape that makes up much of the valley below. While most mountain ranges taper off in their foothills, the Tetons shoot directly out of the ground, their largest peaks visible for hundreds of miles in every direction. So how did they get that way?

The Teton Fault

The Teton Mountains are the result of what’s called a normal fault, in this case named, the Teton Fault. This is where the earth’s crust is pulling itself apart, causing one side to rise while the other falls. Based on best estimates, this process has been going on for as long as 10 million years, to as little as 5 million years, depending on which source you look to. In addition, for every inch the Tetons rise, the valley drops roughly three times the amount. Those quick with math and visuals, however, will say that valley doesn’t look like it’s actually dropped that much.

The valley certainly appears more flat than the deep descent that that math should add up to. This is because over those millions of years, and even as recently as 10,000 years ago, ice ages have carried rocks and sediment with them into the valley and beyond, slowly filling in the depth over that time. But just how much depth has the sediment filled in?

Thomas Murphy Homestead Below Mount Moran
The Thomas Murphy Homestead, comprised of houses and barns, resting on Antelope Flats below Mount Moran and the Teton Mountains. Grand Teton National Park, Wyoming

Sediment in the Valley

At the top of Mount Moran is a key layer of sandstone (visible in the photo above) that geologists have used to piece together many layers of geologic history in the region. Since that layer began to be uplifted millions of years ago, then that layer would also be on the valley floor below the rocks and sediment. The only real question is how far down would you need to go to find it. According to research, you’d have to drill a hole 20,000 feet into the ground before you would find that same sandstone layer! That’s how much “dirt” the valley has essentially filled in with, the deepest section of course just east of the fault line, which runs along the base of the Teton Mountains.

In terms of sheer steepness, the Teton Mountains appear to rise much more aggressively than other mountain ranges. This is due to the fact that in terms of ages of mountains, the Tetons are still just babies, despite being millions of years old. By comparison, the Rocky Mountains are about 50 million years old, meaning they’ve weathered at least 40 million more years of changing climates, glaciers, ice ages, and shifting tectonics. This is why the highest peaks of the Rockies require navigating through some foothills to access first. Likewise, the Appalachian Mountains are a staggering 300 million years old, which explains why they’re not nearly as sharp and rugged as the younger western mountains. In addition, the latter two ranges have also stopped growing, and are in the process of eroding down, while the Teton Range is still growing. That might raise the question though: if they’re still growing through a fault line, what’s the likely danger from a future earthquake?

Pronghorn Below Snow-Capped Tetons
A pair of pronghorn grazing below snow-capped Teton Mountains. Grand Teton National Park, Wyoming

The Teton Quake

After the last ice age ended over 10,000 years ago, there’s evidence of multiple earthquakes that spanned the next several thousand years; not just a few magnitude 4 and 5 earthquakes, but earth-rattling 6’s and 7’s. However as of a little over 5,000 years ago, they’ve since stopped. This has led many to wonder if we’re long overdue for another growth spurt in the Tetons.

Others still theorize that with all the ice and snow that melted off of the mountains after the ice age, the lifted weight allowed some pressure to be released, causing the massive earthquakes that followed. Only time will tell if we are in fact long overdue for another, or if enough pressure was released to keep us safe for our short geologic window.

Still have questions or want to know more about the geology of the area? Book a private Grand Teton National Park workshop with me and I’ll share all the knowledge I have about this area with you. You can also leave a comment and I’ll do my best to accurately answer any other burning questions you might have about the Teton Mountains!

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