Geologic mapping in the extreme northwest portion of the Sacramento Valley and southern Klamath Mountains has revealed basement exposures of the metamorphosed Triassic (?) Applegate Formation which are unconformably overlain by sedimentary rocks of portions of the Knoxville (?) Formation (Upper Jurassic, Portlandian) and the Budden Canyon Formation (Lower Cretaceous, Hauterivian). The greenschist facies metamorphic rocks which form the southern end of the Western Paleozoic and Triassic belt of the Klamath Mountains consists of quartzites, metacherts, and greenstones, with minor amounts of slightly altered volcanic rocks, limestones, argillites, and conglomerates. These rocks were intruded by Jurassic peridotite (now mostly serpentinite) followed by gabbro. In the southwestern portion of the map area the basal polymictic conglomerates of the Knoxville (?) Formation, averaging about 2,800 feet in thickness, are separated from the basement complex by a marked nonconformity. A wedge of dark shale, thinning to the north, separates the Knoxville from the Budden Canyon Formation. The basal conglomerates of the Rector Member of the Budden Canyon Formation are lithologically similar to the Knoxville conglomerates. These outcrop intermittently with variable thickness (0 to 900 feet) in the north and thicken to about 3,000 feet in the south where they overlie the Knoxville. The Ogo Member of the Budden Canyon Formation, which overlies the Rector Member and locally rests on the basement, consists almost entirely of thinly bedded, dark marine shales which are intertongued with minor, relatively thin, coarse clastic units which had their current source from the north. In this area the Ogo Member may be as thick as 10,300 feet. Sediments of both these Lower Cretaceous members also occur as small outliers in the northwest part of the map area. The sedimentary rocks are exposed in an easterly dipping homocline, while the complex litho-structural grain of the basement units trend northwest-southeast. Opposing sets of high angle to vertical, probable reverse faults form acute angles of intersection of approximately sixty-five degrees; the northeast trending set has left lateral separation while the northwest trending set exhibits right lateral separation. In the northeastern portion, two moderately large reverse faults have uplifted blocks on their hanging wall side which have over 600 feet of topographic relief. Major folding is represented only in the gently southeasterly plunging syncline of the sedimentary Platina outlier.