Shoshone Canyon is a gorge cut through Rattlesnake Mountain by the wearing action of the Shoshone River. Rattlesnake Mountain is a northwest trending uplift in the earth’s crust that rises 3700 feet above the surrounding terrain. Beds of sedimentary rock exposed on the mountain’s northeast flank slope eastward beneath the plains, while the same units, ranging in age from 205 to 570 million years, bend up and over the crest of the mountain and stand vertically along the southwest flank. Granite exposed at the west end of the canyon at Buffalo Bill Dam is two billion years old. North of the dam, a fault in the earth’s surface lies adjacent to the steeply inclined rocks, and the geologic compressive forces that formed the uplift also caused rocks to move vertically along the fault. The Rattlesnake Mountain uplift began to form 66 million years ago, about the same time dinosaurs became extinct, and the present topographic configuration of Rattlesnake Mountain and the formation of Shoshone Canyon result from erosional downcutting over the past three million years.