Geology Along the Coast of San Francisco

The geology along the cliffs at Fort Funton seems fairly straightforward …until it isn’t.

On top, it’s loose sand held in place by vast carpets of South African Ice Plants, and large thickets of tall Coyote Brush (Baccharis pilularis), a resilient evergreen shrub native to California, Oregon, Washington, and Baja California.

As you descend to the beach, the stripped patterns of sedimentary rock are self-evident, with changes in color giving some evidence of climate change. 

These cliffs are pretty soft stuff and are only a few million years old -- formed long after the last Ice Age. This is the Merced Formation , and considering how close it is to the San Andreas fault, there's far less folding and uplift than we see in Appalachia, which is 100 times older.

So simple, right?

Well, it was until I spied a very familiar rock -- a small piece of meta-basalt, i.e. greenstone. 

That stuff is volcanic, and at least 190 million years old.  

Where did it come from?  The answer:  a bit of a ways.

The piece of greenstone in my hand most likely broke off from pillow basalt heads that were formed by underwater volcanic eruptions far out in the Pacific, and which were subsequently brought to their current location through continental drift and plate tectonics.  Apparently the Merced Formation (sedimentary) has some meta-basalt mixed in with it, and the result is called (of course) the “Franciscan Complex”.