Geological History of Grand Teton National Park

Grand Teton National Park has a little bit of everything – mountains, valleys, lakes, glaciers, forests . . . but where did it all come from, how was it formed?

The signs are all around you in Grand Teton National Park, if you just know where to look for them and what they mean. The geological history of the Teton mountains starts way before the mountains, the rocks are much older than the mountains are. It all began a very long time ago – around 2.5 billion years, give or take a million years or two, when sand settled on an ancient ocean with volcanic debris. Additional sediment was deposited over the next few million years, then buried beneath the earth’s crust. Lots of heat and pressure then changed that sediment into something called gneiss, the rock which actually comprises of the main mass of the Teton mountain range. The stress of this metamorphosis, or change, caused segregation of the minerals, and if you look closely as you’re exploring the Grand Teton National Park to this very day, you might notice the alternating dark and light layers which show the branded gneiss.

He seems to know what he’s talking about!

Geological History of Grand Teton National Park

Next thing to occur in the geological history of the Grand Teton National Park is when the molten rock or “magma” forced it’s way through the weak spots of the gneiss. This igneous rock which had been formed by heat, cooled down slowly and formed dikes of granite, some only inches thick, some hundreds of feet. You can see these larger dikes when you look at the Teton mountains from Jenny Lake and String Lake. The granite was exposed by many years of erosion and uplift, eventually becoming the central peaks of the Teton mountain range.

Shallow seas covered this region a few years ago too, well, when I say a few, I mean 600 million years ago to around 65 million years ago, that’s for around 535 million years, anyway, the sedimentary formations which were left from these seas can still be seen from both the north and the sounds ends of the Teton mountain range, as well as on the western slopes of the mountains. These seas advanced and retreated, advanced and retreated, advanced and retreated over and over again. During the periods of retreat the area was a low lying coastal plane where dinosaurs called home. East of the park close to Togwotee Pass, fossilized bones of the Triceratops (the one with the horns) have been discovered.

That’s ’em, well they used to live around here, a long, long time ago . . .

Building the Grand Teton Mountains of Grand Teton National Park

So what about the Grand Taton mountains? Well, around 80 million to 40 million years ago, the compression of the earth’s crust caused the uplift of the Rocky Mountain chain, stretching from what we now know as Mexico to Canada. The rise of the Tetons, however, had not yet started . . . they were still a glint in the eye creation (wow, not sure where that came from but I’ve just impressed myself).

Right, the Teton fault began only 6 – 9 million years ago (oh, is that all), with the thinning and stretching of the earth’s crust. Now, when this occurs, once every few thousand years or so the crust is stretched to the limit and, naturally, it breaks, a fault (or break) occurs of around 10 feet, which might not sound much in the big scheme of things, but this does relieve the stress of the earth’s crust. The blocks at each side of the fault begins to move, the block on the west side swinging skywards which formed the Teton Range, the east block dropping downwards to form the valley we now know as Jackson Hole.

Okay, I admit, this is the geological history of Grand Teton National Park “in a nutshell”, but next time you’re visiting the park do have a look around, remember what you’ve learned and it’ll all come together in your mind.

Ain’t they something?