View of Golden Gate Bridge from Fort Mason
It seems that Callan Bentley of Mountain Beltway and I had similar intentions while attending the fall AGU meeting in San Francisco last week. We both went on our own separate geologic excursions using Doris Sloan's "Geology of the San Francisco Bay Region" and Clyde Wahrhaftig's "A Streetcar to Subduction" guidebook. I've wanted to make use of this guide since hearing about in my graduate level igneous petrology course. Luckily it seems Callan and I decided to explore different sections of the city (with some overlap) and I think he had better luck with the weather!
View of Crissy Field Looking Towards Downtown San Francisco
I decided to walk down to Fort Mason, along the eastern coast via Crissy Field, under the Golden Gate Bridge on-ramp, through the Presidio (confusing!) and then down the Coastal Trail to Marshall and Baker Beach. The weather cooperate only in that it didn't rain on me while I was walking and I had about 40 minutes of sun right before sunset. I was a little underwhelmed by the Fort Mason sandstone exposures, they are relatively small and not as interesting as other localities in Wharfhaftig's book (I should have paid better attention to the descriptions). However, the walk along Crissy Field was quite scenic (even with the fog) and I even encountered a friendly Blue Heron!
The following slideshow contains photos associated with Trips #2 and #3 in A Streetcar to Subduction, which focus on Fort Mason on the eastern side of the city and the Coastal Cliff Trail along the the western edge. This tour visit sandstone within the Alcatraz Terrane, exposures of serpentinite, chert and both Marshall's and Baker Beach. My favorite 'finds' were an excellent exposure of ribbon chert and a serpentine boulder with a highly fractured surface likely caused by the differential weathering of chrysotile, both near Marshall's Beach. This is a simplified geologic map of the Coastal Trail area from Stoffer and Gordon (2001) that provides a nice overview of the geologic relations and geography within this area:
For more information on the images in the slideshow, click on full screen and then Show Info in the upper left hand corner. Enjoy!
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