We visited Fort Funston on a beautiful, warm February day (trust me, this locale is never this warm — summer or winter), and above is a panorama taken from the cliff top. The promontory peeking out from behind the cliff in the photo’s the far right and left sides is Point San Pedro, which is the seaward foot of Montara Mountain. Mt. Tamalpais, in Marin county, is in the photo’s center. Barely visible to the left of Mt. Tamalpais is Point Reyes National Seashore. The San Andreas Fault runs from the gap between Mt. Tam and Pt. Reyes offshore to Mussel Rock, which is visible at the foot of Montara Mountain (it’s the dark sea stack visible to the right of the apex of the shoreline’s bend). San Francisco is visible just to the right of the large, sandy, grassy hummock in the photo’s center. Fort Funston is located in San Francisco’s southwestern most corner, and is part of the Golden Gate National Recreation Area.

Two storms, one with six meter waves (as measured at the San Francisco Buoy), removed about a meter of sand from the beach at Fort Funston in the span of ten days. Don’t worry — this happens every year, and then the gentle waves (comparatively gentle) of summer put the sand back by season’s end.

By the way, the blackness of the beach in these photos has nothing to do with the Cosco Busan oil spill of about a month ago — what you are seeing are magnetite grains that have been sorted to the surface of the beach by the storm.

The presence of magnetite, along with the absence of a local source rock, plus the medium grained sand at Ocean Beach, of which Fort Funston is a southern extension, indicates that this sand is relatively young. Curiously, there is also no local riverine source for this sand, so one is naturally led to ask of its origin.

The San Joaquin / Sacramento rivers, which drain the 700 km length of the Sierra Nevada, do not empty sediment into the ocean at the Golden Gate, as one might think. Instead, they drop their sediment load near Sacramento into one of the world’s rare inland deltas, the product of a sea level rise 8,000 to 15,000 years ago, at the end of the last ice age.

So what explains the mystery of the sand field at Ocean Beach? I say sand field, because Ocean Beach is actually in the middle of one of the largest sand fields on Earth. It extends 40 km east to west, from the continental shelf near the Farallone Islands (which lie about 35 km due west of San Francisco) to half way across the city of San Francisco.

This vast dune field is the product of glaciation during the last ice age, and was deposited when the San Joaquin / Sacramento rivers reached a shoreline that 18,000 years ago lay near the Farllones. Sea level was 100 meters lower than today, and the rivers were swollen with snow melt, and carried a much larger burden of sediment that they deposited on the then exposed continental shelf west of the Golden Gate. The sand, not having a local source river or rock, is termed ‘relict’ sand by geologists. One can discern this entire story by simply looking with a hand lens at the sand grains themselves — each beach reveals the story of its life and travels through its sand.

A footnote: I was reading in Wikipedia about magnetite, and via a footnote discovered an article titled Ferrous Nonsnotus by Bob Moriarty. Ok, 25 cubic km of 10% magnetite bearing sand is impressive, but what got me were the 2000 meter sand dunes — that’s just insane.

We visited Kirby Cove on 23 November, three weeks after the Cosco Busan struck the Bay Bridge on 7 November. There was some oil on the west end of the beach, but otherwise it looked clean. Some of the shallower tidepool areas looked scoured of life, but I’m not certain of that without checking earlier photos. Deeper tidepool areas (we were there right at an exceptionally low -1.7 meter tide) still had plenty of anemones, feather boa kelp, and other organisms. The spill happened at neap tide, so the lowest tidal areas probably escaped exposure to the oil.

Kirby Cove lies west of the Golden Gate bridge on the north (Marin) side of the strait. One can see downtown San Francisco, the San Francisco Bay, the entire length of the northern, strait facing shore of San Francisco, and the Pacific ocean from it. It is part of the Golden Gate National Recreation Area, and as such is managed by the National Park Service.

Clicking on one of the images below takes you away from this blog to images for that day.

Fort Funston lies on the Pacific Ocean in the southwest corner of San Francisco. I often go there, and photographs from a recent visit follow. An early winter storm had left its mark on every corner of the beach and cliffs; in addition, a nice low tide exposed lovely, reflective expanses.