As you walk about the hills of San Francisco, you can consider many things: the blue sky, the fog rolling in, the birds tweeting about you, a beautiful building, a tree, a garden, the fabulous inhabitants. Even living in earthquake country, it is easy to think of the ground under our feet as something immutable. Dr. Jean DeMouthe, Senior Collection Manager at the California Academy of Sciences, on March 21st, 2013, brought us around to thinking about what and how the rocks of San Francisco came to be under our feet.
Fifteen thousand years ago, one could walk to the Farallones Islands, the golden gate a river . 125,000 years ago, San Francisco was an island, Lake Merced a remnant of the Colma Straight. The rocks of those days were “forged” much much earlier as the pacific plate crashed into the north america plate, subducting under it, and giving rise the the volcanoes in the (now) east bay and to the north. Out of that, and the movement of plates came our main rocks: serpentenite, chert, and sandstone. Our serpentenite (not as nice looking as in other areas of the world) is a metamorphic rock formed under heat and pressurel; chert is remnants of biological activity — shells of radalarians that accumulated on the sea floor, and buried, and under pressure solidifies the silica in the shells recrystallizing, separated by layers of clay and siltstone; our sandstone is graywacke, the weathered remains of volcanoes washed out to sea. The latter two, laying as they did on ocean floors, owe their place in our hills due to the movement of earth.
Today the subduction is done, the plates now moving in opposite directions (there’s still subduction going on in the Pacific Northwest part of the “ring of fire” around the Pacific). But those rocks are now with us (at least for our short stay on this world) and help give rise to the unique landscape of our city. Serpentenite, for instance, is detrimental to most plants, it is poor in calcium and magnesium, the opposite of what most plants need to thrive, but of course evolution comes in to play and there are plenty of endemic species that survived the harsh climate and adapted to the conditions.
Not much of our rock was particularly good for buildings, (like our wood presumably) a lot of that rock came other places where there was volcanic rock to be had like from Marin. In other places, serpentinite might have been used as a decorative piece, but ours is not as pretty as some.
Our city is dotted with little rocky outcroppings, along the shore, at the tops of many of our hills. You can see the layers thrust up and out by the movement of the earth. If you look closely and in the right places, you might see the remains of other processes, rare materials, and fossils (be warned that it is not longer legal to take, so just look!).
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