San Francisco Bay Region Geology and Geologic Hazards

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San Mateo County Coast - A Small Fault

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This photo shows a small fault along the San Mateo County coast. Matching layers across the fault shows that it has offset the sandstone (lighter layers) and shale (darker layers) about ten feet.
A fault is a fracture in the Earth’s crust where the rocks have moved on either side.  Some faults have offset of hundreds of miles, but some have much smaller offset, like this fault north of Highway 1 near Devil’s Slide (between Pacifica and Half Moon Bay).  Ironically, the small faults are in many places easier to see, because the faults with large offset grind up the rocks nearby.  The broken rock is easily eroded, leaving a recognizable faulted landscape (for more about the faulted landscape, click here (howfind.doc)), but obscuring the fault itself.  Small faults don’t break up the rock as much, and so can be seen in roadcuts and natural outcrops.



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