Chert Roadcuts

Look to the roadcuts along Conzelman here, where the road was carved through the ridge and laid the rock bare. That ribboned, rusty-red stone, folded and buckled like a crumpled rug, is one of the great stories in the whole Headlands. It's called radiolarian ribbon chert, and to read it you have to think small and think deep. Way out in the tropical Pacific, in water far from any shore, the surface swarms with radiolarians, tiny single-celled plankton that build glassy shells of silica. When they die, those shells drift down, a slow snow of them settling on the deep ocean floor, year after year, until they pile up as a fine silica ooze. Squeeze that ooze under its own weight over ages, harden it, and you get chert, laid down in thin beds, one ribbon at a time.

Now here's the part worth holding in your mind as you drive. This rock didn't form here. It formed far out on the floor of the equatorial Pacific and rode the ocean plate slowly north, carried along over something like a hundred million years. When that plate finally reached the edge of the continent, it dove down beneath North America, and as it went, slices of the seafloor got scraped off and plastered onto the coast, like mud scraping off a boot. The reddish tint is generally put down to iron in the rock. The folds are the record of all that squeezing at the plate boundary.

What you're seeing is part of what geologists call the Franciscan Complex, a jumbled stack of ocean-floor rock, all of it hauled in and mashed together at that ancient collision zone. Chert, greenstone, sandstone, the bones of a vanished ocean, welded to Marin. So the ground under your wheels is genuinely well-traveled.

One request as you take it in. This is protected national park land, so it's look-don't-take. No chipping off a souvenir, no pocketing a red chunk of it. Leave the ribbons in the rock for the next traveler, and for the folks a hundred years from now who'll want to read the same deep-sea story you're reading right now.