The Lewis Overthrust is a geologic thrust fault structure of the Rocky Mountains within Glacier National Park in Montana, United States and Waterton Lakes National Park in Alberta, Canada. A collision of tectonic plates about 170 million years ago drove a great wedge of rock several mile thick some 50 miles (80 km) eastwards, causing it to overly softer Cretaceous age rock that is 140 million years younger. Chief Mountain in Glacier National Park is a clear example of the result of erosion of these rock layers.