Mysterious and epic dramas take place at the bottom of the sea, where massive forces meet microbiology. In the red peaks of Corona Heights we can read an engrossing chapter: this stark hill with the distinctive flora and the unbeatable view over San Francisco is composed of the fossilized shells of prehistoric sea creatures. Formerly quarried for bricks, the site today hosts the Randall Children's Museum, a coterie of dog walkers, and a remarkably strong community of California native plants.
The back-story: single-celled organisms ruled the Ocean Deep during the Cambrian period, some 500 million years ago. Radiolaria, the most prolific of these protozoa, grew microscopic silica shells of fantastic and unlikely beauty, often spherical or helmet-shaped with delicate spines radiating from the center, in thousands of distinct variations; they dominated the primordial brine for over 300 million years, and still survive in diminished numbers today. As their spent shells sank over eons in what Silent Spring author Rachel Carson calls "the long snowfall," countless layers of fine sediment accumulated into a thick coat of radiolarian ooze on the ocean floor. Time and pressure turned it to stone.
Flinty chert, the rock used by native peoples to make arrowheads and knives, is in fact this same petrified radiolarian ooze disrupted by plate tectonics and lifted to the surface. It forms San Francisco's system of radiolarian peaks, that scimitar of red chert cutting through the city: Lone Mountain, the camel humps of Buena Vista and Corona Heights, Sunset Heights, Twin Peaks, Mt. Davidson, the San Miguel Hills, Bernal Heights, and Bayview Hill.
Witness the raw power of the ages in the striated rock of Corona Heights -- particularly the 40-foot cliff face near 15th and Beaver Streets, where the impressive red layers are twisted and warped, testifying not only to hundreds of millions of years of "radiolarian rain" and the crushing weight of the ocean, but also to the unspeakable violence that ripped this stone from its bed in the sea and thrust it into the sky.