After serving with the Lewis and Clark Expedition, John Colter journeyed to the Teton Yellowstone Country in 1807 and became the first mountain man to see the Teton Mountain Range. Early fur trappers gathered in the valley at the base of the range at the site known as Pierre's Hole for their annual rendezvous. Approximately 10 million years old, the Teton Range is the youngest mountain range in the Rockies. The three Tetons were once a single massive mountain of granite. Over the millennia, glaciers up to 3,000 feet thick carved steep canyons into U-shaped valleys, and freezing water broke off slabs of rock, creating the sharp ridges and pinnacles seen today. Early explorers called the mountains Pilot Peaks. Later, French trappers called the main peaks Les Trois Teton, or The Three Breasts. "Beaver" Dick Leigh was a well-known Idaho mountain man who lived and raised his family at the base of the Teton Mountain Range. The Teton Peaks served as a beacon; visible for miles, they guided mountain men to the Teton Valley, which became home for their annual rendezvous. The first rendezvous dates to 1832, when more than 200 mountain men gathered near the Teton River with the Nez Perce and the Flatheads to trade and celebrate in the wilderness.