With a few very rare exceptions the oldest rock within an hour of Spokane is the Prichard Formation. It is the basement of the so-called Belt Supergroup, which includes the Wallace Formation that surfaces out near Four Lakes. The Prichard is typically dull silt and mudstone (argillite) but it really lights up under stress. What you see in this photo is a closeup of Hauser Lake Gneiss, which is what happened to a large segment of the Prichard when it was heavily stress by faulting south of Mt. Spokane during the Eocene epoch, some 50 million years ago. Hauser Lake Gneiss, streaked with amphibolite is a beautiful rock, and here’s a close-up of it in a road cut out at Hauser Lake in the Idaho panhandle.