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Slumped boulder on Marshall’s Beach, San Francisco

By Paul Wehking posted 01-06-2016 04:10 PM

  

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Check this out:

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That’s a big boulder (with my GigaPan for scale) at Marshall’s Beach, San Francisco, right around the corner from the Golden Gate Bridge:

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The big boulder is mantled in colluvium derived from the weathering of the serpentinite mélange exposed along the beach here. I don’t know whether this boulder is close to its bedrock source point, or whether it’s a “knocker” that weathered out some tens of meters uphill, became incorporated into the colluvium, and has been on its way downhill for some time. Regardless, it’s on the move, and it has reached the beach. Unusual levels of rain and wave action the previous week undermined the boulder’s support from below, at the same time it “slushified” the colluvium above. The boulder then slipped a couple of feet downhill.

 

 

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