In July 1805, the Lewis and Clark Expedition passed through this canyon of “nearly perpendicular rocks” on its journey up the Missouri River. Although the men complained about mosquitos and prickly pear cactus, the Corps of Discovery was clearly impressed by the Adel Mountain Volcanics, the eroded remains of a mass of volcanic rocks that flank the river for forty miles. The volcanic rocks beside the Interstate consist mostly of fragments from violent, explosive eruptions that began about 75 million years ago and continued for several million years. The arrival of the Montana Central Railroad in 1887 helped open the region to settlement, but not until the early 1930s did a modern paved highway provide a direct connection between Helena and Great Falls through this area. U.S. Highway 91 followed the Missouri River through the canyon’s volcanic outcrops and crossed the river on two large steel truss bridges that still carry traffic near Hardy and Wolf Creek. Though bypassed by Interstate 15 in the 1960s, old U.S. 91 still offers travelers a Great Depression-era road through one of Montana’s most spectacular landscapes.