Heart Mountain is an 8,123-foot klippe just north of Cody in the U.S. state of Wyoming, rising from the floor of the Bighorn Basin. The mountain is composed of limestone and dolomite of Ordovician through Mississippian age, but it rests on the Willwood Formation, rocks that are about 55 million years old—rock on the summit of Heart Mountain is thus almost 300 million years older than the rocks at the base. For over one hundred years, geologists have tried to understand how these older rocks came to rest on much younger strata.
The carbonate rocks that form Heart Mountain were deposited on a basement of ancient granite when the area was covered by a large shallow tropical sea. Up until 50 million years ago, these rocks lay about 25 miles to the northwest, where the eastern Absaroka Range now stands.
Between 75 and 50 million years ago, a period of mountain-building called the Laramide Orogeny caused uplift of the Beartooth Range and subsidence of the Bighorn and Absaroka Basins. Just south of the Beartooth Range, this orogeny uplifted an elongate, somewhat lower plateau which sloped gently to the southeast toward the Bighorn Basin and to the south toward the Absaroka Basin.