This formation, long considered synonymous with Oligocene Series, was never adequately studied in its type area, resulting in falacious concepts of its faunas and correlations. The name was extended to rocks as far north as British Columbia and as far south as southern California. Subdivided, the older Twobar Shale Member appears to have been deposited slowly in a bathyal open-sea environment, the younger Rices Mudstone Member rapidly in a restricted basin. Most of the so-called characteristic mollusks are from the Rices Mudstone Member of overlying Vaqueros Sandstone. Subdivision of the San Lorenzo Formation and discovery of glauconitic marker beds provide additional stratigraphic control in an oil province with poor exposures.

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