[1][2] It overlies the San Joaquin Formation, and can be up to 4,000 feet (1,200 m) thick.
[1][2] Its sediments consist mainly of unconsolidated deposits of clay, silt, sand, and gravel.
[1] Many freshwater fossils are preserved in the formation, dating back to the Neogene and Quaternary Periods of the Cenozoic Era.
[3] They include the largest fossil assemblage of clams and snails known on the Pacific Coast.
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