The mountains in this corridor are made of the Prichard Formation, a rock unit dating back 1.5 billion years and among the oldest exposed sedimentary rocks in western Montana. At the confluence of the Clark Fork and Flathead rivers, the Prichard is more than 4 miles thick and its base has not been discovered. Uplifting and shifting mountains across the river tilted the formation on its side, and large rocks at this turnoff preserve ripple marks and mud cracks from their sedimentary origin. The formation began as layers of sand, silt, and clay deposited by a river delta into a sea that covered parts of what is now Montana, Idaho, Washington, and Canada, and over millions of years heat and pressure changed those sediments into the hard metamorphic rock argillite. Its mudstone layers also split into flagstone used for hearths, stone paths, and building facings, while a slate-like rock from the formation was used by early people to make tools and weapons.