26 April 2019

Friday fold: Black Sands Beach

Posted by Callan Bentley

This Friday, let’s hearken back seven weeks, when a similar Friday saw me and the ‘Streetcar to Subduction’ team in Marin Headlands, north of San Francisco. Black Sands Beach there is a great place for pillow basalts. You can see the knobby texture of the pillows on the far side of the beach, but today, let’s look at the big boulders in the foreground…

Let’s scoot over to the left a bit to that big boulder with the vertical stripes…

Close-up, these stripes reveal themselves to be upright and folded chert layers, with gazillions of tiny fractures perpendicular to the bedding planes:

The cherts here are gray, almost bluish-green, in stark contrast to the brick-red variety further to the east, near Kirby Cove and the Golden Gate Bridge.

On the underside of a cliff overhang at the west end of Black Sands Beach, there are more folded cherts of this gray color:

Happy Friday, all!