Geological Outings Around the Bay: Rodeo Beach

The strait called the Golden Gate has beaches on both sides of its seaward end. On the south side, in San Francisco, are Baker Beach and Ocean Beach, which are made of fine sand derived mostly from the Sierra Nevada. On the north side in Marin County is little Rodeo Beach, which is not. Rodeo Beach is small but has a lot to see.

Rodeo Beach is next to old Fort Cronkhite in the Golden Gate National Recreation Area, situated between the sea and the valley of Rodeo Creek. The beach qualifies as a bar, the geographer's term for a pile of sediment that crosses the mouth of a river. As you can see in the top photo, Rodeo Creek is dammed behind the beach to form Rodeo Lagoon, but some of the water from the lagoon manages to spill across the beach at its north (left) end into Rodeo Cove.

Rodeo Creek supplies no sediment to the shoreline. Instead, the beach's sand and gravel is manufactured on the spot from the local bedrock of the Franciscan Complex, which crops out in steep coastal bluffs on either side. Before you study the beach itself, take a look at these rocks. There are three main Franciscan rock types at Rodeo Beach that you can easily distinguish: on the south side (see below) are chert and basalt and on the north side is sandstone.

Chert is the most obvious of the three, a hard, flinty stone arranged in hundreds of thin layers. This is the same deep-sea ribbon chert visible along Conzelman Road as you arrive through the Marin Headlands, but here parts of it have been turned green and even bluish colors by the pressure and chemical action of metamorphism.

Basalt here is not the black lava you might expect in Hawaii or the Cascade volcanoes. It's largely deep-sea pillow lava that also has been metamorphosed into greenstone. The original pillow shapes persist in some places here.

The sandstone of the northern bluffs is not the red or brown rock of the Grand Canyon, but a dark grit with its own metamorphic overprint. Of these three rocks, basalt is the weakest, the sandstone grains are the hardest and the chert is the toughest. In Rodeo Cove, the vigorous Pacific surf tosses and sweeps and grinds them together, turning out batch after batch of coarse dark sand mixed with polished chert pebbles. The north end of the beach is the best place to admire (and not collect) this exquisite gravel.

Geologists have studied this sediment closely and matched its ingredients to nearby rocks, although one rare constituent used to puzzle them: round pebbles of translucent carnelian, a red-orange gemstone. In Geology of the San Francisco Bay Region, Doris Sloan states that in the winter of 1967 after an especially strong storm, the outflow channel from Rodeo Lagoon dug its way to basalt bedrock six feet down. In that deeply buried stone the source of the carnelian grains was found. They had formed when bubbles in the lava filled with silica—in geologist's jargon, they were amygdules or mineral-filled vesicles.

Ever since learning this, I've watched for another such coincidence of storm and tide, although a lifetime might not be sufficient. As could be said of various things, the most severe times may produce the most sublime experiences.