29 December 2015

Serpentinite mélange on Marshall’s Beach

Posted by Callan

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.

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

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

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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.

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

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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