22 December 2010

GoSF4: Kirby Cove

Posted by Callan Bentley

“GoSF” = Geology of San Francisco

I’m taking this week to write up my three field trips examining the geology of San Francisco and neighboring areas. My plan is to cover:

Introduction and overview

Seafloor basalt

Deep sea chert

Kirby Cove, Marin Headlands (today)

(wildlife interlude)

Graywacke turbidites

Serpentinite and mélange

Fractures and the chemistry along them

Pleistocene dunes

The San Andreas Fault and Mussel Rock

Kirby Cove is a cove between two rocky headlands on the south shore of the Marin Headlands section of Golden Gate National Recreation Area. Here’s a view over Kirby Cove, looking to the east. You can just make out the northern tower of the Golden Gate Bridge in the fog at the far right, in the distance.

The yellow and orange things on the eastern side of the beach are two kayaks. The thing backing up the beach, looking like a marine terrace, is in fact an artificial structure — a bunker built by the U.S. armed forces to defend San Francisco from Japanese invasion during World War II. It was designed to look like a line of sand dunes. Pretty decent job they did, too… except for the brick-lined drainage duct which funnels stream water out to the ocean.

Our explorations of Kirby Cove were guided by trip #5a in Clyde Wahrhaftig’s Streetcar to Subduction guidebook (1984). It directed us first to the eastern side of the beach, where we found some lovely chert with thin shale interbeds. Here, the pounding action of the waves had etched away the shale preferentially, highlighting the lovely folds in the chert.

Here’s a pretty cool fold in the chert, looking down on the “back” of the fold hinge:

Rotating the perspective point to the left, we can “look down the barrel” of this fold hinge:

You can see how it’s etched out underneath this fold. If you duck in there, there’s a small cave going back about 8 m into the rock face:

More folded cherts:

On the western side of the beach, we found pillow basalts galore, nice and clean thanks to the scraping effects of the incessant waves. Check out this pulchritudinous population of pillows (proportions by pencil pieces):

And lastly, Lily takes a brief nap on one of these soft, fluffy pillows:

Next up — a brief wildlife interlude…