The oldest rocks exposed are Upper Cretaceous foraminiferal mudstones, graded sandstones, and conglomerates (9,500 feet) occurring in a fault slice along the coast south of Pescadero. Not in contact is the Butano formation (5,000 feet) of Eocene age, consisting of interbedded sandstones, siltstones, and mudstones. The sandstones are thicker and cleaner in the upper part of the Butano formation and produce oil in the La Honda field. Conformably overlying the Butano formation are 2,500 feet of San Lorenzo mudstones and siltstones (upper Eocene A-1 zone to lower Zemorrian) which are cut by diabase sills and dikes. These dikes were feeders to basalt flows which poured from subaerial volcanoes into shallow water. The volcanic rocks are interbedded with upper Zemorrian and Saucesian mudstones, quartzose sandstones, and organic calcarenites. This sequence totals 2,000 feet in thickness and is overlain by 500 feet of brown chert and laminated mudstone (Relizian ?). Transgressing all older rocks are the upper Miocene cherts and diatomaceous mudstones (0–9,000 feet thick) of the Monterey formation. The Pliocene Purisima formation (5,650 feet) overlies the Monterey comformably and is still transgressive. It is characterized by the first influx of andesitic debris, probably from the Sierra Nevada. The Purisima is divided into five mappable members, which from the base upward are: tuffaceous siltstone and sandstone member containing small amounts of oil in the La Honda field (2,150 feet); siliceous mudstone member (2,300 feet); pebbly sandstone member (150–350 feet); mudstone member (450 feet); fine sandstone member (400 feet). Pleistocene terraces, recent alluvium, and landslides complete the stratigraphic column.

It is believed that the Butano, the lower Miocene volcanics and the Purisima formation can be directly correlated across the present San Andreas fault into the Stanford-Woodside area. The correlation suggests that lateral displacements along the fault in this area may be a mile or two, but not hundreds of miles.

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