Geologic units in San Mateo county, California

Pliocene marine rocks (Miocene to Pleistocene) at surface, covers 21 % of this area

Sandstone, siltstone, shale, and conglomerate; in part Pleistocene and Miocene.

Miocene marine rocks (Oligocene to Pliocene) at surface, covers 14 % of this area

Sandstone, shale, siltstone, conglomerate and breccia; in part Pliocene and Oligocene.

Quaternary alluvium and marine deposits (Pleistocene to Holocene) at surface, covers 14 % of this area

Alluvium, lake, playa, and terrace deposits; unconsolidated and semi-consolidated. Mostly nonmarine, but includes marine deposits near the coast.

Eocene marine rocks (Paleocene to Oligocene) at surface, covers 13 % of this area

Shale, sandstone, conglomerate, and minor limestone; in part Oligocene and Paleocene.

Older Quaternary alluvium and marine deposits (Pleistocene) at surface, covers 7 % of this area

Older alluvium, lake, playa, and terrace deposits.

Franciscan Complex, unit 1 (Coast Ranges) (Jurassic to Cretaceous) at surface, covers 6 % of this area

Franciscan complex: Cretaceous and Jurassic sandstone with smaller amounts of shale, chert, limestone, and conglomerate. Includes Franciscan melange, except where separated--see KJfm.

Mesozoic granitic rocks, unit 1 (Salinian Block) (Early to Late Cretaceous) at surface, covers 5 % of this area

Mesozoic granite, quartz monzonite, granodiorite, and quartz diorite

Franciscan melange (Jurassic to Cretaceous) at surface, covers 4 % of this area

Melange of fragmented and sheared Franciscan complex rocks.

Upper Cretaceous marine rocks, unit 1 (Upper Great Valley Sequence) (Late Cretaceous) at surface, covers 4 % of this area

Upper Cretaceous sandstone, shale, and conglomerate

Mesozoic volcanic rocks, unit 1 (Coast Ranges) (Jurassic to Cretaceous) at surface, covers 3 % of this area

Undivided Mesozoic volcanic and metavolcanic rocks. Andesite and rhyolite flow rocks, greenstone, volcanic breccia and other pyroclastic rocks; in part strongly metamorphosed. Includes volcanic rocks of Franciscan Complex: basaltic pillow lava, diabase, greenstone, and minor pyroclastic rocks.

Tertiary volcanic flow rocks, unit 6 (Page Mill-Mindego) (Tertiary (12-15 Ma)) at surface, covers 3 % of this area

Tertiary volcanic flow rocks; minor pyroclastic deposits.

Oligocene marine rocks (Eocene to Miocene) at surface, covers 2 % of this area

Sandstone, shale, and conglomerate; in part Miocene and Eocene.

Plio-Pleistocene and Pliocene loosely consolidated deposits (Miocene to Pleistocene) at surface, covers 1 % of this area

Pliocene and/or Pleistocene sandstone, shale, and gravel deposits; in part Miocene.

Ultramafic rocks, chiefly Mesozoic, unit 3 (Coast Ranges and Western Klamath Mountains) (Middle to Late Jurassic) at surface, covers 1 % of this area

Ultramafic rocks, mostly serpentine. Minor peridotite, gabbro, and diabase. Chiefly Mesozoic unit 3

Paleocene marine rocks, unit 1 (Central and Southern California) (Paleocene) at surface, covers 1 % of this area

Sandstone, shale, and conglomerate; mostly well consolidated

Quaternary sand deposits, unit 1 (coastal) (Quaternary) at surface, covers < 0.1 % of this area

Extensive marine and nonmarine sand deposits, generally near the coast or desert playas