Within the boundaries of the Little Bighorn Battlefield National Monument, millions of years of geologic history have been preserved. The bedrock that underlies the monument is from the Upper Creaceous Period (100 million to 65.5 million years ago). It is made up of mud and sand left behind from a seaway that flooded west-centeral North America.
Throughout the past 2.6 million years (the Quaternary Period) streams have deposited gravel, sand, silt, and clay have helped to form the landscape that influenced the events that unfolded during the Battle of the Little Bighorn.
The natural features that dominate the landscape of the momunet consists primarly of ridges dissected byravines and coulees. “Coulee” is a French term applied in the western United States to a small stream (or the bed of such a stream) that is often intermittent. During the Battle of the Little Bighorn, ridges offered defensible high ground for the soliders of the 7th Cavalry. While ravines aided the advance of the Indians, and the steep banks of the Little Bighorn river limited access and escape from the floodplain.