Red Rock Pass is a low mountain pass in eastern Idaho, south of Downey in southern Bannock County. It is geologically significant as the spillway of ancient Lake Bonneville. It is located along highway US-91 at an elevation of 4785 feet (1458 m) above sea level, bounded by two mountain ranges; the Portneuf to the east and the Bannock to the west. The pass was cut through resistant Paleozoic shale, limestone, and dolomite, and forms a narrow gap two miles (3 km) in length.At one time the pass was 300 feet (100 m) higher, where the shoreline of Pleistocene Lake Bonneville stood.