That's why, even in the middle of rugged, mountainous country, the Tetons stand out. They seem to shred the clouds and steal scalloped bites of the moon. Like many mountain ranges in the Rocky Mountain region, the Tetons formed along a fault, still visible on its lower slopes, a few hundred feet above the flat valley floor. This weak line allowed a massive granite block 40 miles long and 15 miles wide to rise and the valley floor to drop. The uplift was much greater than the current height of the Teton range, perhaps by more than a mile.
Geologists find evidence of two ice ages here, although they say there have been many more.
The last ended a mere 13,000 years ago. Geologists believe that between 20,000 and 25,000 years ago, at the height of that last ice advance, some 90 percent of what is now Yellowstone National Park was buried under an ice blanket perhaps 3,000 feet thick. Most of the higher elevations in Greater Yellowstone were inundated. Slow-moving glaciers ground the valleys of Greater Yellowstone into big, rounded U's. Glaciers work like massive sandpapery sponges, scouring the earth across which they slowly creep and then incorporating the dislodged sediment into themselves.