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Many times in the geologic past, the area that is now San Diego has been partly or wholly beneath the sea. The most recent time in which that occurred was about 100,000 years ago, toward the end of the Pleistocene Epoch, and the oldest for which we have a fossil record was in the Jurassic Period, about 140 million years ago... These submergences are recorded by sedimentary rocks containing marine fossils. Such fossils may be collected in and near San Diego from rocks of those ages and of Cretaceous, Eocene, and Pliocene age, about 80,000,000, 45,000,000, and 5,000,000 years old respectively.... Not all the fossil mollusks found in the San Diego area can be described in this guide. Those species chose for inclusion are common in the area, highly distinctive, or were first collected in or near San Diego.... A fossil mollusk collected in the San Diego Area can be dated on the basis of the maps in this book that show the distribution of rocks of various geologic age. --- excerpts from book's Introduction.

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