This valley is the surface trace of the Leech River Fault, a major regional fault that marks the contact between the oceanic basalts of the Crescent Terrane (part of Siletzia) to the south, and the metamorphic rocks of the Pacific Rim Terrane to the north.
The valley in question is the exposed surface of the Leech River Fault, a major fracture that separates the oceanic basaltic rocks of the Crescent Terrane from the metamorphic rocks of the Pacific Rim Terrane to the north and south, respectively.