During late Tertiary time a thick section of coarse and fine continental clastics accumulated in Ridge Basin. Deposition was concurrent with movement on the San Gabriel fault zone which delimited Ridge Basin on the southwest. Movement on this fault ceased in late Pliocene time and the younger sediments overlapped southwestward across the fault and onto a pediment cut into the crystalline basement. The low-angle Frazier Mountain thrust moved in the Pleistocene relatively southeastward across the veneer of sediments on the pediment and in part onto the thick Ridge Basin deposits. Subsequently the thrust, veneer of sediments, and pediment were folded and faulted along an east northeast trend whereas the thick sediments in Ridge Basin were folded along trends nearly at right ngles to this and subparallel with the basin margins. At this time the San Gabriel fault zone was reactivated in the southern part of the area.