Heart Mountain Detachment

The Heart Mountain Detachment is the fault surface beneath Heart Mountain, an 8,123-foot klippe north of Cody, Wyoming, produced by the largest rockslide known anywhere on the land surface of the Earth. Between about 50 and 48 million years ago, during the waning phase of the Laramide Orogeny that had uplifted the nearby Beartooth Range and dropped adjacent basins, a sheet of rock covering roughly 500 square miles broke free and slid at least 25 miles southeast into the Bighorn and Absaroka basins, on a slope of less than two degrees, reaching speeds estimated above 100 miles an hour and completing its slide in perhaps 30 minutes. The mountain's summit, made of Ordovician through Mississippian carbonate rock some 500 to 350 million years old, now sits directly on the much younger Willwood Formation, laid down around 55 million years ago, a roughly 300-million-year age gap between the rock on top and the rock beneath that first alerted geologists to the scale of the slide. This entity documents the Heart Mountain Detachment specifically as a geological formation, the fault and landslide surface beneath Heart Mountain.