Pescadero State Beach - sedimentary rock (sandstone with fossils)

Sandstone preserving the imprint of a large scallop shell from the north part of Pescadero State Beach in San Mateo County.

The low, wave-cut cliffs along the northern part of Pescadero State Beach

in San Mateo County are made up of Oligocene (34-23 million year old) sandstone (shown as Os on the geologic map). This sandstone is characterized by large fossil scallop shells.

Sandstone is a sedimentary rock Sedimentary rocks are formed when accumulations of sediment (small bits of rocks and minerals produced by weathering and erosion, often with some plant or animal remains mixed in) are buried, compacted, cemented, and slowly converted to rock. Plant and animal remains are incorporated into the rock as fossils. Some fossils preserve the original shells, bones, wood, or other material, but more often the original material is dissolved or decayed, leaving a mold or imprint in the stone. The Oligocene sandstone at Pescadero State Beach formed from nearshore accumulations of beach sand.