Pliocene and Pleistocene Coastal and Shelf Deposits of the Merced Formation and Associated Beds, Northwestern San Francisco Peninsula, California

Abstract This field-trip guidebook discusses the Merced Formation (of Pliocene and Pleistocene age) in its type section (Lawson, 1893) and associated Pleistocene beds in the same sea-cliff outcrops. The outcrops extend from Mussel Rock on the south to Fort Funston on the north (Fig. 1A). This section is notable for its thickness and excellence of exposure and for the wide variety of shallow marine and coastal depositional environments represented. Although the section contains a potentially important record of changes in relative sea level, the record has not yet been fully interpreted because of a dearth of precisely dated horizons. On this field trip we will proceed stratigraphically upward through the section, starting at Woods Gulch, proceeding northward past Thornton Beach State Park (presently closed because of landsliding and gullying), and ending at the northwest corner of the old Fort Funston military reservation, now part of Golden Gate National Seashore (Fig. 1A). We will not see the lower part of the section between Mussel Rock and Woods Gulch. Although the exposures are often excellent where newly eroded by ocean waves, they tend to disappear rapidly because of landsliding and sand deposition on the beach. Because of the rapid changes, all the features described in this guidebook cannot be expected to be seen at any one time. The exposures are usually best at the base of the sea cliffs in winter, when much of the beach sand has been removed by erosion. The Merced Formation and associated Pleistocene beds crop out in a belt that trends northwest-southeast for a distance of 25 km across the northern San Francisco Peninsula (Fig. IB).