A large fault system, here named the Foothills fault system, is the dominant structural feature of the western Sierra Nevada. The steeply dipping to vertical component faults trend northwestward through an area about 200 miles long and 30 miles wide north of 37°30′ north latitude. The faulted Paleozoic and Mesozoic rocks are overlapped by unfaulted younger rocks, and the total extent of the fault system is not known. It is probably not limited to the western Sierra Nevada. Faults are marked by belts as much as 4 miles wide of cataclastically deformed and recrystallized rocks and by truncated folds. Along one fault, Upper Jurassic rocks are juxtaposed against Paleozoic rocks for at least 100 miles. The direction of fault movement has not been determined. Net displacement on some of the component faults exceeds 3000 feet and may be measurable in miles. Major faults cut beds of Late Jurassic age and are in turn cut by plutonic rocks of probable Late Jurassic and Middle Cretaceous age. Faults that controlled deposition of quartz veins and gold ore bodies of the Mother Lode belt are apparently younger and structurally less important features superimposed on one of the fault zones of the large system.