Seashells unearthed during summer gardening. Masses of granitic rock that formed alongside the Sierra Nevada, since stranded on coastlines. Sedimentary rock, originating from the seafloor, perched 800 feet above San Francisco. The jumbled mélange of Bay Area geology makes sense only when seen through the lens of plate tectonics. Many of these formations fall within the Franciscan Complex, the diverse array of rocks delivered from far away by the chaotic movement of Earth’s plates. Some are quite old, from as long as 200 million years ago, making our own lives seem as momentary as the passing of a bird’s shadow.
The Bay Area’s geologic past has culminated in a playground for rock climbers—a chance to get up close with formations that have scarcely changed in centuries but on geologic timescales remain ever in motion. Here are five Bay Area sites offering a range of climbing experiences, from basic scrambling and bouldering to challenging roped ascents, while revealing a slice of the region’s rich rocky past.
San Francisco: Glen Canyon Park
Glen Canyon Boulders
In one of San Francisco’s wildest places, home to raptors, coyotes, and a restored stream corridor, there are also magnificent rocks—layered, folded outcroppings jutting from the slopes of Glen Canyon Park. There are many exposed crags here, though some are off-limits due to the presence of sensitive plant species. The rocks where climbing is allowed, and that offer the best experience, lie at the north upstream end of Upper Islais Creek and are sometimes referred to as the Main Cliff Band.
You can enter the canyon from above, at the trailhead on Turquoise Way, or from below, via Bosworth Street or Elk Street, by the public baseball field. The lower route leads through a mixed landscape of eucalyptus forest and a restored riparian corridor of willow, native blackberry, dogwood, and elderberry. After crossing a wooden bridge, as the canyon floor begins to rise, you arrive.
The rock here is chert, a layered sedimentary rock that formed from tiny silica-shelled organisms called radiolarians that settled on the ocean floor between 100 and 200 million years ago.
“[Those layers] were obviously laid down like pages of paper on top of the bottom of the ocean—it would have been horizontal,” says Richard Nevle, codirector of Stanford University’s Earth Systems Program. “But the fact that they’ve been contorted into those accordion-like folds tells us the story of what has happened to those sediments . . . as a result of the ongoing collision of the Farallon Plate with the North American Plate.”
The layered and folded nature of such chert is a stunning geologic signature of the high hills of San Francisco. For climbers, too, they can be loads of fun. At Glen Canyon, the primary face is almost 100 feet long, with ragged lips, cracks, and ledges formed by the exposed chert layers running the entire distance. Much of the outcrop is prominently overhanging, with solid, fist-filling grips—what climbers call jugs—that provide cliff-hanging thrills without the need for cumbersome ropes or technical gear.
This is a bouldering site, but don’t treat it too casually. Some routes can take you 20 feet off the ground, should you wish to climb all the way up. Think twice before you do, and, as you always should, wear a helmet and use a crash pad.
Elk and Chenery streets, San Francisco. Free street parking.
San Francisco: Corona Heights Park
Beaver Street Wall
At Corona Heights Park, just off Beaver Street, visitors will find a stunning piece of urban geology so smooth, glossy, and vertical that it could almost pass for a human-made structure.
In fact, the Beaver Street Wall is what geologists call a slickenside—a rock surface polished smooth by the lateral movement of two opposing masses of rock along a fault. A slickenside can be almost any rock, but this one, like the outcrops in nearby Glen Canyon, is chert.
Rock climber Quinn Todzo, who lives near the Beaver Street Wall and hosts a podcast called The Rock Behind the Climb, calls this site one of the “gems of San Francisco.” He pointed out that a rock’s geologic composition isn’t the only factor that affects the experience of climbing on it.
“It’s a lot about what happens to the rock after it gets formed,” he said.
The Beaver Street Wall demonstrates this clearly. Unlike the dynamic and jagged outcrops at Glen Canyon, the chert face on Beaver Street is mostly too smooth to climb, and even the bouldering options along its base are limited. While climbing here is allowed, the San Francisco Recreation and Park Department urges climbers not to chip or alter the rock in any way.
The main draw is a route called Crack Direct, which runs up the prominent split in the 60-foot face. It’s rated a modestly hard 5.9 on the Yosemite Decimal System, the standard U.S. scale that ranks hikes, scrambles, and climbs by difficulty, from 1.0 to 5.15. The wall offers at least two alternative routes running straight up the polished face, one of them a challenging 5.12.
Beaver and 15th streets, San Francisco. Free street parking.
TIBURON: RING MOUNTAIN PRESERVE
Turtle Rock
If you approach from the west, you may recognize the shape of the namesake reptile of this geologic landmark. It’s a snapping turtle to my eye, its pointed nose jutting northward, its shell formed by the boulder’s slanted rear and its wrinkled neck suggested by folds and shadows in the rock.
As you draw nearer, the turtle disappears and details emerge. This feature is blueschist, a metamorphic rock formed through a long and stressful journey: As basalt, it was attached to a tectonic plate that sank deep into the earth’s mantle, where it was exposed to what Nevle describes as an unusual combination of great pressure and relatively low “bread-baking temperatures,” transforming it into blueschist. Eventually, through the messy process of plate tectonics and uplift, the turtle arrived at its current perch atop the Ring Mountain Preserve.
The beautiful stone displays fine layers, like rings on a tree stump, swirled and contorted by nature’s ancient abuse; polished dimples and divots; and glistening mineral assemblages peppered across the charcoal-blue surface. There are chalk smudges, too, marking the paths of least resistance for rope-free bouldering. Handholds like 4-by-4 beams are interspersed with others that scarcely accommodate a fingertip. Large cracks zigzag skyward. In at least one place, most handholds are “underclings,” which require an inverted grip, with your palm facing upward.
Turtle Rock is a favorite climbing site of Todzo, who describes one overhanging route as remarkably tricky, smooth, and lacking in toeholds, until you cross a prominent horizontal lip and reach up.
“You get over the top, and it’s amazing—it’s like jugs everywhere . . . and that’s because you’re climbing across the foliation lines,” he says, referring to the parallel mineral streaks in the rock’s surface created through the metamorphic process.
If you go to Turtle Rock in the morning, the sun will soon warm the east-facing overhang where you’ll find the most engaging “problems”—what climbers call bouldering routes; go in the afternoon, and you’ll be climbing in the cool shade of the turtle’s flank.
From Paradise Drive take Taylor Road for 0.4 mile until it ends in a cul-de-sac. Free parking.
JENNER, SONOMA COUNTY: GOAT ROCK STATE BEACH
Sunset Boulders
Along the Sonoma Coast, geologic processes are shaping the land at a human timescale. The cliffs supporting the region’s iconic terraces are collapsing, threatening homes built just a couple generations ago. As softer substrate washes into the ocean, massive boulders and sea stacks resembling anvils and chimneys are left behind.
The climbing attraction here sits 100 feet above sea level, about halfway between Shell Beach and Goat Rock—a cluster of house-size monoliths named Sunset Boulders. These rocks are considered fossil sea stacks, since they once protruded from the ocean but, after some continental uplift, now sit high and dry. A short walk from the road leading to Goat Rock Beach gets you there. You can also approach from the south via the Kortum Trail, starting at the Shell Beach parking lot—a hike of about a mile.
John Wakabayashi, a geology professor at California State University, Fresno, describes these monoliths as metamorphic “garnet amphibolite with a blueschist overprint.” They began their current lithic life as seafloor basalt, he says, before embarking on a long and chaotic metamorphic journey. This included getting scraped off one tectonic plate and onto another roughly 170 million years ago, being subjected to extreme pressure 25 miles below the seafloor, and tumbling back into a fault-line trench during an underwater landslide. Much later, through the combined action of wave erosion and tectonic uplift, the rocks were revealed where we see them today.
Up close, the rock’s surface is coarse, veiny, rough like a shark’s skin, and with linear striations that cling to the rubber of one’s shoes. Chalk marks in the many splits, cracks, and fissures, and on knobs and jagged ledges, reveal the most popular routes and problems. Bolted anchors above allow for top-roping, though it means an unprotected scramble up the backside to set the rope. Tread carefully. The named and rated routes include the 5.6 Sunset Slab, the 5.8 Sunset Corner, and the much trickier Sea Breeze, rated 5.11c.
In the way of bouldering, the lower quarters of this rock jumble are a wonderland, with caves, ledges, sharp corners, and sweet overhangs. Look closely, too, for the polished patches about 12 feet up that scientists believe may have once been used by mammoths for rubbing their backs.
Goat Rock Road, Jenner. Free parking.
SUNOL REGIONAL WILDERNESS
Indian Joe Cave Rocks
The hike to Indian Joe Cave Rocks runs a little over a mile through a compact wilderness area of canyons, meadows, and hills. You park at the Canyon View Trailhead, off Geary Road, and follow Indian Joe Creek Trail uphill, beside the stream, ascending some 600 feet.
Almost at the creek’s headwaters, amid elderberry, live oak, sagebrush, and some dense poison oak thickets, you’ll find a spectacular pile of huge split and fractured boulders, and a jungle gym of climbing opportunities. Roped climbs, like the Arete and the Crack, range from 20 to 50 feet, ascending a sheer cliff with bolts leading the way.
The bouldering problems, like Western Gold, Fool’s Gold, and Indian Joe, can take you quite high, 15 to 20 feet up, if you follow the chalk marks. Edges and corners are unforgivingly jagged, and there are no soft landings here, so climb carefully.
“Know your limit, and don’t try anything too scary without the proper people and protection below,” as Todzo says, referring to bouldering in general.
Like Sunset Boulders, the rock here is blueschist. Bluish gray and splashed with puma-brown blotches, it got its start as basalt after lava hardened on the seafloor, underwent metamorphosis deep in the Earth’s mantle, and finally came up for air.
“At some point it was involved in a terrestrial landslide as it slid from its source somewhere above,” Wakabayashi explains. “The erosion of this landslide by water winnowed out the less resistant sandstones and other rocks.” This left the pile of blueschist rubble we know as Cave Rocks.
1895 Geary Road, Sunol. $5 parking fee on weekends and holidays.
Though we see these features in a geologic freeze-frame, they are still changing. Uplift and subduction continue, radiolarians still rain to the seafloor, and new slickensides are being polished as we speak. As we live, climb, and fall, Earth’s journey goes on.
