Sixty million years ago, you wouldn't have recognized this place. There were no high mountains, no deep canyons. Streams meandered through a landscape of broad valleys and low hills that may have resembled Kentucky's Cumberland Valley.
The rocks, gravel, sand, clay and silt that eroded from the hills into streams accumulated and were cemented into layers, coarse conglomerate and sandstone you can see in the face of Tower Rock across the highway.
Clark's "dark brown flint" is actually Precambrian quartzite, some of the oldest rock in Idaho. It can be seen better at outcrops between here and North Fork.
This clift is of a reddish brown colour the rocks which fall from it is a dark brown flint tinged with that Colour. Some Gullies of white Sand Stone and sand fine & as white as snow." - William Clark, August 21, 1805