Grand Teton National Park & John D. Rockefeller, Jr. Memorial Parkway - Geologic Activity (U.S. National Park Service)

NPS Photo

The Teton Range rises out of the valley floor, hinged along its eastern edge by the Teton Fault. The Teton Fault is a dormant fault that when active causes the mountains to rise and the valley floor to drop.

Imagine visiting Grand Teton National Park 85 million years ago. You would see a much different landscape. Stretching before you would be a great inland sea, which deposited layers of sediment on top of 2.5 billion year old granites and 2.7 billion year old gneisses. Jump forward 25 million years; those seas have retreated, and the collision of tectonic plates begin s to up-lift the Rocky Mountains. Another 45 to 50 million years pass before a different type of geologic force creates the Teton Range.

The Teton fault stretches the entire length of the Teton Range's eastern front, roughly 40 miles. Between ten to thirteen million years ago, this region began to stretch and the Earth’s crust broke along faults, tilting the mountains skyward and dropping the valley floor. Each time the crust broke; earthquakes up to magnitude 7.5 shook the land. These sporadic bursts of energy created the abrupt front of the Teton Range as it faces Jackson Hole. At 13,770 feet, the summit of the Grand Teton towers 7,000 feet above the valley floor. Total vertical displacement across this fault may be more than 23,000 feet. The floor of Jackson Hole dropped 16,000 feet, more than twice as much as the mountains rose. The beauty of the Teton Range lies in its majestic size and rugged appearance.

Beginning up to 2 million years ago, glaciers scoured and sculpted the Teton landscape. Large masses of ice flowed from the topographic high of the Yellowstone Plateau down into the valley of Jackson Hole numerous times. Fingers of ice, pulled by gravity, flowed from the high Teton peaks down into the valley. Grand TetonNational Park contains many features created during the Ice Age such as piedmont lakes, U-shaped canyons, knife-like ridges, kettles, and moraines.

Grand TetonNational Park, as it is today, boasts dramatic vistas and a geologic story that is by no means complete. Geologic forces such as erosion and glaciation still sculpt the landscape, but the Teton fault lies dormant. The last major earthquake happened between roughly 5,000 to 7,000 years ago,, but eventually the fault will lurch into action. Imagine a rubber band that is stretched to its limit; sooner or later it will break. For the Teton fault, it is not a matter of if, but when, it will move again. Adapted from "Windows into the Earth" by Robert B. Smith and Lee J. Siegel