|Heart Mountain near the Montana-Wyoming border once moved 100 kilometres in a half-hour in a catastrophic scenario during the early Eocene .|
The Mountain originally formed with the Absaroka Range, to the West, about 50 million years ago.
However, lava from deep underground slid the mountain to its present position.
The Heart Mountain block slide of northwestern Wyoming and southwestern Montana is one of the largest slides known to have occurred in Earth's history.
The block slide covered an area of over 3400 km² and moved a minimum of 45 km across open terrain. The initial 2- to 4-km-thick Heart Mountain block slide moved on a slope of about 2°, detaching for half its length on a nondescript bedding plane in the Ordovician Big Horn Dolomite (BHD).