Ages ago, these mountains were deep within the Earth's crust, and the area that is known today as the Bighorn Mountains was a basic. Beginning about 75 million years ago the land began to slowly rise above the sea bed reaching an elevation of nearly 20,000 feet. Since that time, the eroding forces of wind, water, and ice having removed thousands of feet of rock resulting in what you see today.
The cliffs of Tensleep Canyon are composed predominantly of massive layers of limestone. This limestone layer underlies the towns of Tensleep and Worland and serves as their major source of water. The water is removed from the layer by deep wells.
Glaciers carved out the valley of Tensleep Canyon within the last 250,000 years. Evidence of these ancient glaciers can be seen, in the U-shape of the valley bottom, and the piles of boulders, or glacial moraine, left along West Tensleep Creek. The "West Moraine" stretches for 10 miles, making it the longest moraine in the Big Horn Mountains. Weathering forces and the flow of the creek continue to wear away the rock in Tensleep Canyon. In the winter, ice flows can be seen on the canyon walls.