Geologic Setting

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Stability and Erosion

Heavy surf and rains severely erode beaches and fragile sea cliffs along the coast of San Mateo County, California. During a storm season, the main coastal highway gets damaged and closes for a considerable period of time due to landslides, and many buildings and homes are damaged or threatened by sea-cliff retreat and slope failure.

Coastal Stability

Segments of the San Mateo County coast are classified as stable and unstable based on the inherent resistance of the exposed rocks to wave erosion and slope failure. The stable coastal segments in the northern part of the county are backed by resistant granitic rocks, which form high bluff to low cliffs. The stable coastal segments in the southern part of the county are backed by resistant sedimentary rocks that form low cliffs.

Unstable coastal segments are backed by weakly indurated or highly fractured sedimentary rocks that form high bluffs, weakly indurated terrace deposits that form low cliff, and loose beach and dune sand that forms spits and low cliffs. Along much of the San Mateo County coast, fragile terrace and loose dune deposits overlie resistant to moderately resistant bedrock. In those areas the base of the cliff is stable and the upper part is unstable. If the cliff is sufficiently high that wave surge does not erode the terrace deposits exposed in the upper part of the cliff face, that coastal segment is classified as stable.

Natural coastal processes such as cliff erosion, slope failure, wave erosion, landslides, block falls, debris slides, and cave collapse are illustrated diagrammatically on the cross sections. Wave erosion is the primary or initiating erosive process along the San Mateo County coast. Waves erode the base of the sea cliffs or coastal bluffs, destabilizing the slopes above. Landslides, debris slides, and block falls are secondary erosive agents that move loosened rock material down the slopes to the beaches below, where the material is eventually eroded away by wave activity. Along most of the San Mateo County coast groundwater that seeps from the cliff face softens and loosens even otherwise resistant bedrock material, thus contributing to the erosion process (Jacobs Associates 1998).

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