Diagram of the marine embayment during the Pliocene, about 5.3 million years ago, along with approximate locations of familiar cities and Diablo Mountain.
Diagram modified from Bartow, J.A. 1991, U.S. Geological Survey, Professional Paper 1501 EXPLANATION TO SYMBOLS Nonmarine deposition -- Dotted line indicates inferred extent;queried where uncertain Fracture trace of Neogene San Andreas fault
Marine deposits -- Solid line indicates inferred shoreline; hactures indicate inferred shelf edge; queried where uncertain
Emergent area -- Queried where uncertain
Fault -- Probably active; arrows indicate direction of relative movement
Marine Embayment Once Covered Forest Land
Water has covered the forest land at different times throughout geological history. Beginning as early as 59 million years ago, water inundated the land and then receded millions of years later. About 16 million years ago, during the Middle Miocene, a shallow marine embayment reaching depths of only 8 to 10 meters (26 to 32 feet), covered the area which lasted through the Early Pliocene (about 3-4 million years ago). The Pliocene marine embayment left sediment that geologists call the Purisima Formation.
Land that Marcel's Forest sits on became dry again in the Late Pliocene (3 to 4 million years ago) when the Pacific Plate began to move obliquely past the North American Plate instead of sliding past it. This oblique movement caused the plates to converge. The compression created by this convergence squeezed up the Coast Range (including the Santa Cruz Mountains).
The Santa Cruz mountains are still uplifting. During the 1989 Loma Prieta Quake, the land surface on the west side of the San Andreas fault lifted up approximately 35 cm (almost 14 inches).
Evidence of Embayment Remains
Evidence of the shallow Pliocene embayment can still be seen in some of the rocks found Along Aptos Creek. Sedimentary rocks (rocks made from layers of minerals and rock fragments deposited as the seawater broke down the parent rock over time) contain the shells of mollusks which once lived in the sea. Salt-and-pepper-colored granodiorite, a type of granite from the basement rock called the "Salinian Block," is also found along the creek's banks. (See Pictures on sidebar Geologic Foundation).
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