Gunsight Mountain is a 9,263-foot peak in the Lewis Range of Glacier National Park in Montana. It takes its name from nearby Gunsight Pass, named by George Bird Grinnell in 1891. Like many mountains in Glacier, it consists of sedimentary rock laid down in ancient shallow seas from the Precambrian to Jurassic periods. Its larger geologic setting was shaped beginning about 170 million years ago, when the Lewis Overthrust fault forced an enormous slab of much older Precambrian rock, about 3 miles thick, 50 miles wide, and 160 miles long, over younger Cretaceous rock.