California Geology: Touring Coyote Hills

Hiking in California provides endless amusement for the geology-minded adventurer. Ribbon chert is a lovely rock formation found all over the coast in the Bay Area, and is a testament to the dramatic geologic history of the state. California’s coastline sits on the San Andreas fault, where continental and oceanic crust slide against each other in a transform fault. Much of California’s geology is leftover from related tectonic tension, with something as big as the Sierra Nevada Mountains actually created by a subduction zone, as oceanic crust slipped under the North American continent (I can talk about that when I find some serpentinite to share here).

Coyote Hills is a regional park in the Bay Area known for its windy outlook points over the bay (where we found a Peregrine Falcon very dramatically enjoying the view) as well as a biologically diverse marsh (where you can find northern shovelers, American white pelicans, yellow rumped warblers, great blue herons, swallows, etc., etc.)

And it is here that you can also find ribbon chert, a formation of rock strata formed on the ocean floor. Radiolarians, a siliceous plankton, grew in large numbers in the surface water. When they died, their silica-rich tests sank to the bottom of the sea floor. Thus, these layers contain radiolarian fossils, hardened into sedimentary rock.

Now, however, they sit above the water, and not horizontal and flat like the lazy layering of seafloor mud. Instead, this sedimentary rock has been uplifted and rotated to the vertical formation we see here. Throughout Coyote Hills, you can find it laid in all sorts of directions – horizontal, vertical, diagonal, and crinkled. The rock was formed on the ocean floor, and enormous tectonic forces lifted it up above sea level, (in many cases) rotated it from its original horizontal orientation, and squished it out of the original perfectly flat layers.

We call it Ribbon Chert for a reason – the strata aren’t straight, but contain waves, curves, sometimes even 90° twists! The tectonic activity that uplifted the seafloor sediment onto California coastlines managed to squish, bend, and deform the chert into layers of red ribbons, all without metamorphosing it into a molecularly different kind of rock.

There are lots of ribbon chert outcrops throughout the trails, although some of them aren’t their original red. Physical and chemical weathering has discolored the surface of some outcrops; you can still see the red rock underneath if there’s a fresh cut on the surface. Colorful gray, orange, and lime green lichens are also scattered throughout the park, masking the redness (and contributing to the erosion) of the rocks they’ve made a home on.

Some of the formations also contain cracks filled with quartz. This is another clue to its marine origins. Water flowing under the oceanic crust is heated by geothermal activity and is rich with minerals. When the water flows up through the crust and into the ocean through cracks in the seafloor, the minerals solidify into quartz veins, captured as crystalline gray streaks through some of the chert formations. Of course, this hydrothermal water flow is also responsible for the abyss’ famous hydrothermal vents.

Happy hiking, and keep an eye out for the stories nature has to tell!