The middle Miocene Topanga formation at its type locality in Topanga Canyon, 10 miles northwest of Santa Monica, Los Angeles County, California, consists of more than 8,000 feet of conglomerates, sandstones, and shales with intercalated basalts. An abundant molluscan fauna, obtained from four different horizons within the formation is divisible into two major faunal assemblages separated stratigraphically by 1,200 feet of barren beds. The lower assemblage ranges through approximately 200 feet of strata at the base of the formation and is characterized by the presence of Turritella ocoyana s. s. and Pecten (Lyropecten) crassicardo n. var. The upper assemblage ranges through the three upper horizons and is characterized by the presence of numerous specimens of Turritella oc yana topangaensis and Turritella temblorensis.

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