Sunday Sand: Marshall Beach, San Francisco

I’ve had the idea of starting a regular Sunday post simply showing off the
character of a different sand each week. And, following on from my recent visit
to the San Francisco Bay area, where better to start than on Marshall Beach, in
the shadow of the Golden Gate Bridge.

I took the bus (San Francisco’s public transport system is a delight, and
the 511.org website a brilliantly designed
guide to it) to the toll plaza parking at the south side of the bridge, and
wended my way along well-signed trails, through the massive remains of the
military installations; the Presidio had been a strategic military complex since
the days of the Spanish and was handed over to the National Park Service in
1994 – they have done a typically great job of rehabilitation and making the
place a pleasure to visit. Among the efforts of the Park Service has been the
remarkable installation of stairways on many of the trails that lead down and
through the landslipped bluffs to the shore:

(Since, for reasons that escape me, I neglected to take any photos en route,
perhaps preoccupied with my cranky knees and the prospect of the return journey,
these are by Ingrid Taylar from the “Batteries to Bluffs” guide at about.com:San
Francisco).

The rocks along the trail are chaotic and rotten,
disintegrating, crumbling, and sliding – and most of them are
serpentinite, the much-vilified and possibly about-to-be-illegal State
Rock of California; perhaps I should have worn breathing apparatus. Will the
Park Service be required to remove the steps since they wind amongst
what is portrayed as a public health hazard? Throwing caution to the winds, I
stopped several times to admire the gloriously varied hues of these rocks –
apple-green, blue-green…

At the foot of the crumbling and colourful bluffs is Marshall Beach.

Rocks
along the shore tell the tortured story of California’s geological past: they
are part of the Franciscan mélange. The word is French for a mixture, a term
that bestows some class on what are, in fact, very messy rocks (the sort of
thing that field geologists refer to as “fubrite,” the etymology of which I
shall leave up to your imagination to deduce). The mélange is, indeed, a
mixture, lumps of different rock types, large and small, embedded in a swirling,
fractured, vein-shot matrix. This and the serpentinite, together with the other
ingredients of the rocks in the, area tell the torrid story of the chaos of
subduction 100 million years ago, oceanic crust and mantle, along with deep-sea
sediments, bulldozed beneath, and scraped up on to, the edge of the old North
American continent to form the tangled tectonic scrap heap that would be California.

The sand of Marshall Beach is, in itself, a microscopic museum of this
geology, a kaleidoscope of colourful grains of different rocks and minerals,
including the sparkling quartz grains that quite likely were born in the
granites of the Sierras, weathered out and washed down into the sands and
gravels that also contained gold, were flushed into the San Francisco Bay by the
wholesale manmade erosion of the Gold
Rush, and eventually swept out to the coast to end their journey in my
ziploc bag.

[See also Andrew Alden’s guides
to the local geology at About.com, and, for more detail, the USGS field
guide to the Geology of
the Golden Gate Headlands.] SIGNATURE

Comments

Callan Bentley (2010-08-22):

Great post! I love it.
At some point, perhaps you could describe or demonstrate your sand microscope photography set-up. That might be interesting for folks interested in following your example…
C

Sandglass (2010-08-22):

Thanks, Callan - glad you liked it. And, yes, good idea - I’ll put something together on my far-from-sophisticated “lab”!

F (2010-08-22):

I would certainly be interested in the equipment and methods used in your microphotograpy as well. The general principles are somewhat obvious, and certainly I could look up setups provided by others, but you write so well, and I’m sure you have unique insights to share.
I’m always amazed by your sand photos. Now, I’m disappointed that I had not collected any sand when I was in California. However, places like Manhattan Beach and Venice beach are quite likely artificially nourished, so I could not be sure as to where the sand really came from.

Sandglass (2010-08-22):

True, a degree of nourishment has probably been administered - but the process generally involves pumping sand from just offshore, so there’s a good chance it will still be local, even if not naturally beach sand!

Originally published at: https://throughthesandglass.typepad.com/through_the_sandglass/2010/08/sunday-sand-marshall-beach-san-francisco.html