6 January 2016

Slumped boulder on Marshall’s Beach, San Francisco

Posted by Callan Bentley


Check this out:


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:


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.


The evidence of this recent slip is the relatively pristine scarp above the boulder, wherein you can see serpentinite colluvial / soil horizons exposed in cross-section:


Let’s zoom in:



I shot two GigaPans of this scene:

Link Image by Callan Bentley

Link Image by Callan Bentley

While the GigaPan worked (several hours of time), I poked around and observed. Uphill and to the right (south), I noticed that the colluvium showed a lot of other small scarps that showed up when the light was right. Check them all out:



This is a hillside that is on the move! You would not want to be walking Marshall’s Beach during or immediately after a big rainstorm.