During late Tertiary time a thick section of coarse and fine continental clastics accumulated in the northwestern part of the Ridge basin. Deposition was concurrent with movement on the San Gabriel fault which bounded Ridge basin on the southwest. Movement on this fault ceased in early Pleistocene (?) time and the younger sediments overlapped southwestward across the fault and onto a pediment cut into the basement. Later in the Pleistocene the low-angle Frazier Mountain thrust moved 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 and west-southwest trend.

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