29 December 2015

Serpentinite mélange on Marshall’s Beach

Posted by Callan Bentley

Another geological pilgrimage I made in San Francisco earlier in the month was to Marshall’s Beach, just south of the Golden Gate Bridge. I had first visited this site 5 years ago with my friend Alan Pitts. It is a great place to see a tectonic mélange of serpentinite.

Looking toward the Golden Gate from the fortification called Battery Crosby, you can see our destination – the greenish slopes along the shore.


See the cluster of rocks directly between this perspective and the bridge? That’s where we are headed next…


Some of the fresh exposures of serpentinite mélange make my heart swoon…


Here is a relatively coherent block of former seafloor, surrounded by a goop of sheared out and wet-metamorphosed rock that probably once looked much the same.


Blocks like these are frequently sheathed in slickensides, such as this one here:


It’s a beautiful place to visit. But if you can’t make it yourself, here’s a GigaPan of the outcrop above to explore:

Link Image by Callan Bentley