Lewis Overthrust

The Lewis Overthrust is a major thrust fault exposed in Glacier National Park, Montana, and Waterton Lakes National Park, Alberta, formed roughly 59 to 75 million years ago during the Laramide Orogeny, when tectonic collision drove a several-mile-thick wedge of Precambrian rock some 50 miles eastward over much younger Cretaceous rock, placing strata as much as 1.3 to 1.4 billion years older on top of far younger ground. The resulting Lewis thrust sheet extends more than 280 miles, from Mount Kidd near Calgary, Alberta, southeast to Steamboat Mountain west of Great Falls, Montana, and seismic studies put its total displacement at about 115 kilometers, roughly 75 kilometers of direct movement along the fault plus another 40 kilometers taken up by duplex folding in the rock beneath it. The decollement surface where older rock overrides younger is visible from Marias Pass as a thin tan line running roughly halfway up Summit Mountain and Little Dog Mountain. This entity documents the Lewis Overthrust specifically as a geological formation.