Gorgeous and grounded, my recent love interest gets all around the city. Sometimes, though, my crush remains hidden, seen only in select places or even, occasionally, falling to pieces.
I am smitten, yet it’s not some human heartthrob I desire. My passion is directed toward a target more solid than any fallible person. In need of a pursuit this Valentine’s month, I fantasized: Could I follow the seam of serpentinite, our state rock, that stretches from the peninsula’s northwest corner all the way to Irish Hill in the southeast?
To fulfill my geologic curiosity, I cobbled together a haphazard ramble on foot and bicycle, an alternate version of the new Crosstown Trail, with a nerdy twist. Follow along, and you too could fall in love with a rock that, depending on its particular composition, the moisture around it, even the quality of the daylight at the moment, can glow like jade, shine like turquoise, or glint like obsidian.
Like a quirky lover, it can even offer bouquets of flowers that are hard to find elsewhere, but only if your timing is right. I’ll show you where to look.
The Serpentine Gate
You may have heard serpentinite called “serpentine,” a more common name referencing its three primary minerals — chrysotile, lizardite, and antigorite — and several rare ones. Serpentine is usually a distinctive blue-green color, but can be white, yellow, or even black. Its stand-out hue and “soapy” or “greasy” texture make it easy to recognize.
Geologists swoon over San Francisco for having some of the best serpentine exposures in the world. This is hardly a surprise, considering the rock’s earth-shaking origins. The city is built at the dynamic edge of a tectonic boundary, where the heavier Pacific plate began subducting beneath the lighter North American plate 100 million years ago.
As material from the sinking oceanic crust scraped off, it accreted into terranes — rock formations that have a geological history distinct from their neighbors. Three terranes make up the bedrock of San Francisco. Two mélange zones, or crushed-up mosaics of deformed rocks, separate these terranes. Serpentine is one of those mélange zones and runs northwest-southeast, from Fort Point to Hunters Point.
(For more, dig into Doris Sloan’s Geology of the San Francisco Bay Region.)
There’s no better place to start exploring it than the Golden Gate Bridge. The iconic span is stuck, literally, in the stuff. The south anchorage and Fort Point are built atop serpentine. Critics in the early 1930s doubted the strength of the flaky rock as a foundation for the bridge. The project’s geologists responded by drilling more, determining it was abnormally solid, and they anchored the south pier 16 feet deeper than originally designed.
Like a quirky lover, serpentine can even offer bouquets of flowers that are hard to find elsewhere, but only if your timing is right.
Just north of Battery Cranston, the last battery along the cliffs before the paved trail ducks under the bridge toward the bay side, I kept a careful eye on the cyclist traffic and spotted serpentine boulders near the cliff’s edge.
It’s so prevalent around the bridge that David Rains Wallace, in his book Mountains and Marshes, rechristens the Bay’s front door, from golden to green, as “the Serpentine Gate.”
The Presidio: A wild infatuation
Heading back south along the batteries, swaths of the cliffside were the unmistakable shade of pale turquoise, crumbling into the sea. To get up close to a serpentine landslide, I turned right to mosey down the steep Batteries to Bluffs Trail, a 1.7 mile trip through some of the city’s most intact natural habitat.
Portions of the trail were paved with serpentine cobbles like a medieval European alley. Others had blue-green pebbles embedded in the compacted soil like gems shining in ore. Pelicans flew over my head in a chevron, and the poison oak was prolific, intertwined with periwinkle ceanothus blossoms that complimented the blue-green rocks and frothing winter surf. As often happens, the fog blurred the lines.
Soon I was at Marshall’s Beach, a serpentine wonderland. My visit was timely: The blocks are best exposed during a low spring tide in the second half of winter when steep breakers can lower the beach as much as two meters.
This was a chance to bust out the magnifying lens to examine the mineral veins and variations making each wave-polished boulder unique.
Back at the main trail, spring water trickled down a gully known as the Valley of the Serpent. The water transformed the serpentine soil into blue-tinged mud, demonstrating how prone this relatively soft rock is to erosion. The seep is home to several rare plants, such as the Franciscan thistle.
Serpentine is high in magnesium and other toxic heavy metals but low in calcium. This untypical ratio freaks out most plant species (including many exotic weeds) but supports much of California’s endemic flora, which thrives in the harsh habitat. While only one percent of the state’s soil is serpentine, nearly 13 percent of plant species diversity is associated with these areas.
The Valley of the Serpent isn’t the only spot in the Presidio with serpentine-loving plants. Of the 50 acres of the 1,500-acre park commandeered by serpentine, another accessible outcrop is along the Ecology Trail below Inspiration Point. To get there from the southern end of Batteries to Bluffs is a 20-minute walk, even shorter by bike.
Though originally quarried, this serpentine grassland now is an ecological island, restored by park staff and volunteers with native species since the mid-1990s. If you visit in April, you might catch glimpses of the delicate and threatened Marin dwarf flax, and in May, the hot pink and carmine petals of the rare Presidio clarkia.
Central SF: Playing hard to get
My next stop, a quick trip from the Presidio, was Laurel Hill Playground. In the mid-1940s, when it was a soon-to-be-demolished cemetery, nursery woman Lester Rowntree snuck in to rescue the world’s last known Franciscan manzanita. She kidnapped it in a gunnysack to replant in her personal garden in Carmel. The species was assumed extinct in the wild until a solitary specimen was spotted by a conservationist driving by the Presidio’s Doyle Drive reconstruction in 2009 — the loneliest example of another species predominantly associated with serpentine outcrops.
Laurel Hill Playground offers nothing so geologically impressive as the epic exposures of the Presidio bluffs, but serpentine still peeks out from the slope above the ball field. I looked for the ghost of the Franciscan manzanita to no avail, and instead envisioned a future restoration project with cuttings propagated from the famous 2009 find.
After a brief examination, I hopped back on my bike and wended my way down The Wiggle in search of one of the city’s biggest chunks of serpentine. Unfortunately, the U.S. Mint is perched on top of it, stretching the entire block behind the Safeway at Market and Duboce Streets, and other parts of the outcrop are overlaid by houses and streets. Even the parts left exposed are inaccessible, protected by a chain link fence and sun-faded NO TRESPASSING signs.
Again, I was left mainly to imagine — this time, a past before Europeans arrived. Rocks only earn their shape through constant contact with wind and water, and I remembered that the Wiggle bike route follows the bygone waterway through the former Sans Souci Valley.
All this made me think of “shifting baselines,” a concept from conservation psychology suggesting that each successive generation inherits a diminished landscape. What seems normal to me — my baseline — is bereft of species and relationships from the world my parents experienced, and would have been unrecognizable from the ecologically more intact planet into which their grandparents were born.
The southeast: I just can’t get enough
Pedaling toward Potrero Hill, my eye was now attuned to aqua. Passing Franklin Square on 17th Street, I spotted hunks of serpentine, interspersed with agave swords in the same muted jade hue. Then I hit another mother lode. Serpentine was everywhere: jagged rubble in tree wells, smooth stones in gardens, the hill behind the housing projects that tumbles toward Cesar Chavez Street.
My friend who lives there said portions of Vermont Street were recently dug up and whole sections underground were nearly pure serpentine. And in all my years seeing shows at Bottom of the Hill, at the base of Potrero’s north slope, I never noticed that part of the building’s foundation is a serpentine outcrop, bulging out into the vacant lot next door.
hough the former grazing land — “potrero” means “pasture” — is now densely developed, there is a demonstration plot of a serpentine meadow at the new Eco-Patch test garden near the 101 overpass at 17th Street. Establishing habitat connectivity with the nearby Starr King Open Space, which offers 3.5 acres of serpentine grassland, could help animal populations such as the rare bay checkerspot butterfly. This insect’s survival depends on dwarf plantain and owl’s clover, both which flourish in serpentine soil.
The final push brought me downhill to the east, to Dogpatch, once industrial and characterized by extensive serpentine exposures. Both the serpentine and the industry are hard to find these days. The shipyards are nearly gone, the American Can factory is now a warren of offices and studios, and the old PG&E station is about to become a new neighborhood.
Across the street from the American Can building is what once was called Irish Hill, a remnant of serpentine that was surrounded by working-class cottages in the late 1800s. I closed my eyes and imagined the Irish immigrant men who’d congregate every Saturday for bare-knuckle boxing. (“The losers bought the winners five-cent steam beers at Mike Boyles’ popular saloon.”)
As I rode away at the end of my date with the serpentine trail, I was certain that more remains to discover. San Francisco is like many a romantic partner — maddeningly frustrating but with a beauty revealed throughout the years, making you surge with appreciation again and again.
Rock your own adventure
There are infinite ways to access the serpentine trail. If cycling or walking is your thing, the SF Bike and Walking Guide is an indispensable map. If you’d rather bus it, the 28 gets you to the southern terminus of the Golden Gate Bridge, and the 29 to Baker Beach. All Muni train lines travel along Market Street within a block of the Mint. The 19 and 22 buses go through Potrero Hill, and the 55 gets you to the Dogpatch.
