A ridge of deep-sea chert sets off the Golden Gate and its signature bridge. Photos by Andrew Alden except where noted.
Thousands of people, tourists and locals alike, come here to gaze over the Golden Gate. But if you turn your back to it, the Marin Headlands are a textbook location to see some classic rocks of the Franciscan Complex. Rocks like these set apart the landscapes that incorporate them, literally shaping San Francisco’s worldview.
The Marin Headlands have such a distinctive set of Franciscan rocks that their geologic package is named the Marin Headlands terrane, a geologist’s word that means a well-defined piece of land and the closely related rocks that underlie it. The Franciscan has about a dozen terranes represented in the Bay Area, and while the Marin Headlands terrane appears in several places, this is where to see it best.
The Headlands couldn’t be easier to get to—the entrance is just north of the Golden Gate Bridge. Head west and start on Conzelman Road, and stop at the first turnout (Battery Spencer) if you can find a parking spot; if there isn’t try the next, because all display the same rocks. The roadcuts on Conzelman Road have just been renewed as the roadway is improved, so it’s a great time to study them. (Road work goes on during the week; check projectheadlands.gov for updates.) Here’s what you’ll be looking at. Remember that no hammering or collecting is allowed in the Golden Gate National Recreation Area.
The reddish-brown, swirly rock is ribbon chert. This hard sedimentary stone started out in the Pacific as deep-sea clay full of the microscopic shells of radiolarians, lying on the volcanic rocks of the seafloor. It was once flat beds, but as the seafloor was subducted beneath California the chert was crumpled into a picturesque state.
The crust itself was also crumpled, sliced into ribbons and stacked up. That accounts for the look of the geologic map of the Headlands (click it to see it bigger).
The map depicts three main rock types from the Franciscan: the oldest lava of the ancient seabed of Jurassic age (Jfv), the deep-sea chert that sifted down onto it over a period of some 100 million years (JKfc), and the sandstone that collected on top of that as the seafloor approached California during the Cretaceous Period (Kfs). Melange, as I’ve explained in my post about Shell Beach, is an intimate mixture of all three (and more) Franciscan rock types. Alluvium is the young sediment that lies in river valleys.
The lava that once underlay the chert now crops out next to it. At the end of Conzelman Road, at Point Bonita (south of the “Kfs” label), there’s a spot where you can see spectacularly preserved pillow lava, looking as pristine as it did 150 million years ago when it erupted into the deep sea.
Photo courtesy Ed Bierman of Flickr under Creative Commons license
You can see this lava close up on the south end of Rodeo Beach (northwest of “Kfs”) in its mildly metamorphosed form known as greenstone.
While you’re at the beach, take care NOT to resist the excellent pebbles, sourced from the whole range of the Franciscan. I’m told that Kirby Cove’s pebble beach is just as good.
If you’re up for some hiking, there’s a lot more to see. For instance, north of Rodeo Beach the view up the coast shows off the sandstone and melange.
And if you keep your eyes on the ground, you may spot black manganese minerals coating the chert. Parts of the Headlands were exploited for this strategic mineral during World War II. The most likely source for the material is the formation of manganese nodules, back when this chert was abyssal mud. To impress your friends, be sure to call out, “Look! Psilomelane!” when you find some. That’s the name of this sooty-looking mixture of manganese oxides.
If you still must admire the Golden Gate instead of the rocks, let my colleague Brian Romans show you some of the wonderful geology under that impressive strait.