Bighorn Mountains

The Bighorn Mountains are a mountain range in northern Wyoming and southern Montana in the United States, forming a northwest-trending spur from the Rocky Mountains extending approximately 200 miles northward on the Great Plains. They are separated from the Absaroka Range, which lie on the main branch of the Rockies to the west, by the Bighorn Basin. Much of the land is contained within the Bighorn National Forest.

The Bighorns were uplifted during the Laramide orogeny beginning approximately 70 million years ago. They consist of over 9,000 feet of sedimentary rock strata laid down before mountain-building began: the predominantly marine and near-shore sedimentary layers range from the Cambrian through the Lower Cretaceous, and are often rich in fossils. There is an unconformity where Silurian strata were exposed to erosion and are missing. The granite bedrock below these sedentary layers is now exposed along the crest of the Bighorns. The precambrian formations contain some of the oldest rocks in the world, at 3.25 billion years old. Following the uplift, large volumes of sediments, rich in early Tertiary fossils, were deposited in the adjoining basins. The ice ages of the Holocene led to extensive glaciation. Though many cirques, U-shaped valleys and glacial lakes can be found in the mountain range, the only remaining active glacier is the Cloud Peak Glacier, which is on the east slope of Cloud Peak.

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