Glenwood Canyon is a 12.5-mile gorge in western Colorado carved by the Colorado River, with walls rising up to 1,300 feet and exposing rock layers from Cambrian, Mississippian, and Pennsylvanian periods, including the prominent Leadville Formation along the upper rim. Formed relatively recently in Pleistocene time, it is the largest canyon of its kind on the Upper Colorado and one of the region’s most dramatic river landscapes. Its narrow passage through rugged mountains made it an important transportation corridor, carrying the Denver and Rio Grande Western Railroad, the 1906 Taylor State Road, and now Interstate 70, whose scenic but slide-prone route shares the confined gorge with rail lines.