Heart Mountain in May. The rock on the summit of Heart Mountain is almost 300 million years older than the rocks at its base. Geologists believe that between 50 and 48 million years ago a giant sheet of rock about 500 square miles in area detached from the plateau near Cooke City, Montana and slid more than 25 miles to the southeast and south into the Bighorn and Absaroka Basins. This sheet, overlying Absaroka volcanic rocks, was probably originally about 14,000 feet thick. Despite the slope...
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Heart Mountain in May. The rock on the summit of Heart Mountain is almost 300 million years older than the rocks at its base. Geologists believe that between 50 and 48 million years ago a giant sheet of rock about 500 square miles in area detached from the plateau near Cooke City, Montana and slid more than 25 miles to the southeast and south into the Bighorn and Absaroka Basins. This sheet, overlying Absaroka volcanic rocks, was probably originally about 14,000 feet thick. Despite the slope being less than 2 degrees, the front of the landslide traveled at least 25 miles and the slide mass ended up covering over 1,300 square miles. This is by far the largest rockslide known on land on the surface of the earth and is comparable in scale to some of the largest known submarine landslides.
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