Sixty million years ago, this area had no high mountains or deep canyons, but streams meandered through broad valleys and low hills that may have resembled Kentucky's Cumberland Valley. Rocks, gravel, sand, clay, and silt eroded from the hills into streams, accumulated, and were cemented into layers of coarse conglomerate and sandstone visible in the face of Tower Rock across the highway. William Clark's "dark brown flint" was actually Precambrian quartzite, some of the oldest rock in Idaho, which can be seen better at outcrops between here and North Fork. On August 21, 1805, Clark wrote of this "clift" as reddish brown in color, with rocks falling from it of a dark brown flint tinged with that color, and with gullies of white sandstone and sand fine and as white as snow.