Big Horn Mountains

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The Bighorn Mountains are shown highlighted in red in the western United States

The Big Horn 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 (320 km) northward on the Great Plains. They are separated from the Absaroka Range, which lie on the main branch of the Rockies in western Wyoming, by the Bighorn Basin.

The Bighorn Mountains were uplifted during the Laramide orogeny beginning approximately 70 million years ago. The Bighorn Mountains 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. Following the uplift, large volumes of sediments, rich in early Tertiary paleontological resources, were deposited in the adjoining basins.

Cloud Peak (left) and Bomber Mountain (right) as seen from Lake Helen high in the Big Horn Mountains, Wyoming.

Though many cirques, U-shaped valleys and glacial lakes can be found in the mounatin range, the only remaining active glacier is the Cloud Peak Glacier, which is on the east slope of Cloud Peak.

The highest peaks within the Big Horns are located in Wyoming in the 1.1 million acre (4,500 km²) Bighorn National Forest. Two peaks rise to over 13,000 feet (3,960 m) Cloud Peak (13,167 ft, 4013 m) and Black Tooth Mountain (13,005 ft, 3964 m). There are a dozen more that rise to over 12,000 feet (3,650 m). From the east the mountains present a vertical relief of over 8,000 feet (2,450 m), rising abrutly from the plains. Overall, the Big Horns are more rounded than their sister mountain ranges to the west.

The range is the location of the headwaters of the Little Bighorn, Tongue, and Powder rivers.

Bighorn Canyon National Recreation Area consists of approximately 120,000 acres within the Bighorn Mountains. It includes Bighorn Lake, a reservoir damming the Bighorn River.

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• Paleontological resources doc. format