Deep in Montana's Big Belt Mountains, Prickly Pear Canyon cuts through vibrant red and green Spokane Shale, sedimentary mudstones and sands deposited more than a billion years ago in the Precambrian Era, then crumpled, folded, faulted, bent, broken, and tilted about 70 million years ago during formation of the Rocky Mountains as the floor of the Pacific Ocean slowly collided with the western margin of North America. The greenish layers formed from tiny amounts of iron minerals on low-oxygen mudflats, while pinkish layers oxidized where oxygen was slightly greater. These rocks, which thin eastward where they overlie hard granite-like basement rocks, are especially pronounced in the overthrust belt near the eastern edge of the Rockies. Long known to Native Americans, the rugged canyon was first recorded by road-builder John Mullan in 1859, who called it "a perfect defile" and the most difficult point along the road from Hell's Gate to Fort Benton. In 1865 the Territorial Legislature licensed the Little Prickly Pear Wagon Road Company to build a toll road through the canyon, and in 1866 Helena merchants James King and Warren Gillette bought the road and spent $40,000 upgrading it. Heavy traffic between Fort Benton and Helena allowed them to recover that cost within two years, and by the early 1870s the route had become part of the Benton Road, an important freight and passenger road in the territory.