High Quality Content by WIKIPEDIA articles! The Lewis Range is a mountain range located in the Rocky Mountains of northern Montana, U.S. and extreme southern Alberta, Canada. Formed by the Lewis Overthrust beginning 170 million years ago, an enormous slab of Precambrian rocks 3 miles (4.8 km) thick, 50 miles (80 km) wide and 160 miles (260 km) long faulted and slid over newer rocks of the Cretaceous period. In this relatively rare occurrence, older rocks are now positioned above newer ones.