San Francisco lies in close proximity to the San Andreas Fault, an active boundary between tectonic plates. The fault zone runs just under four miles to the west beneath the Pacific Ocean, beyond the left edge of this image. I wanted to find a place in the city that resonated for me both aesthetically and geologically. The Sunset District, on the western edge of San Francisco, slopes gently toward the ocean.
On the day I photographed this image, I walked along the beach and drove through the neighborhood, where I encountered an excavation site for the installation of flexible gas pipe. The trench exposed the uppermost of two sand layers that blanket this part of the city. Not far beneath these sands lies bedrock that records a much older tectonic story.
The darker rock shown in the image formed during a time of plate collision, when oceanic crust was forced beneath the advancing North American plate. This material—serpentine, clay-rich rock, and fragmented stone—preserves the disrupted remnants of that Cretaceous-era convergence.
Today, the Sunset District sits near a different type of plate boundary, where the Pacific Plate slides northwest past the North American plate. The houses along this street are approximately four miles from the San Andreas Fault, where movement along the plate boundary is occasionally felt as earthquakes.