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 bluffs (maps 5 and 6) to low cliffs (map 7). The stable coastal segments in the southern part of the county (maps 20 to 24) are backed by resistent sedimentary rocks that form low cliffs. Coastal segments of intermediate stability are backed by moderately resistent sedimentary rocks that form high bluffs (maps 4 and 16) and low-to-high cliffs (maps 7 to 9, 12 to 18, 25, and 26).

Unstable coastal segments are backed by weakly indurated or highly fractured sedimentary rocks that form high bluffs (maps 1 to 2, and 8), weakly indurated terrace deposits that form low cliffs (maps 3, 4, 6, 7, 9 to 12, and 23 to 25), and loose beach and dune sand that forms spits and low cliffs (maps 4, 9, 10, 19, and 23 to 25). Along much of the San Mateo County coast, fragile terrace and loose dune deposits overlie resistent to moderately resistent 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 (maps 7, and 18 to 23).

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