The Big Horn Mountains are a mountain range

Mountain range

A mountain range is a single, large mass consisting of a succession of mountains or narrowly spaced mountain ridges, with or without peaks, closely related in position, direction, formation, and age; a component part of a mountain system or of a mountain chain...

 in northern Wyoming

Wyoming

Wyoming is a state in the mountain region of the Western United States. The western two thirds of the state is covered mostly with the mountain ranges and rangelands in the foothills of the Eastern Rocky Mountains, while the eastern third of the state is high elevation prairie known as the High...

 and southern Montana

Montana

Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...

 in the United States

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, forming a northwest-trending spur from the Rocky Mountains

Rocky Mountains

The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...

 extending approximately 200 miles (320 km) northward on the Great Plains

Great Plains

The Great Plains are a broad expanse of flat land, much of it covered in prairie, steppe and grassland, which lies west of the Mississippi River and east of the Rocky Mountains in the United States and Canada. This area covers parts of the U.S...

. They are separated from the Absaroka Range

Absaroka Range

The Absaroka Range is a sub-range of the Rocky Mountains in the United States. The range stretches about 150 mi across the Montana-Wyoming border, forming the eastern boundary of Yellowstone National Park and the western side of the Bighorn Basin. The range borders the Beartooth Mountains...

, which lie on the main branch of the Rockies in western Wyoming, by the Bighorn Basin

Bighorn Basin

The Bighorn Basin is a plateau region and intermontane basin, approximately 100 miles wide, in north-central Wyoming in the United States. It is bounded by the Absaroka Range on the west, the Bighorn Mountains on the east, and the Owl Creek Mountains and Bridger Mountains on the south...

.

The Big Horns were uplifted during the Laramide orogeny

Laramide orogeny

The Laramide orogeny was a period of mountain building in western North America, which started in the Late Cretaceous, 70 to 80 million years ago, and ended 35 to 55 million years ago. The exact duration and ages of beginning and end of the orogeny are in dispute, as is the cause. The Laramide...

 beginning approximately 70 million years ago.