The northern end of the Bighorn Mountains have been cut by the Bighorn River which carved a canyon almost 50 miles long. The range runs for about 150 miles in north-central Wyoming and south-central Montana.
The mountains are made of over 9,000 feet of sedimentary rock formed mostly in marine and near shore environments. The deposition of the rocks began 600 million years ago in the Cambrian period and lasted until 65 million years ago at the end of the Cretaceous period. These rocks were deposited layer upon layer over older granite rocks that can be seen in the higher part of the range.
Layer Upon Layer
All these rocks were part of the Laramide mountain making deformation which is widespread across the western United States.