Like other large bodies of water, San Francisco Bay takes on every color at some time, from black at midnight to white at noon. To me, the most characteristic one is a milky bluish green that I see on summer evenings when I cross the Richmond Bridge going east. There is a sense of celestial depth about it, paradoxical as that might be. It seems very alive after the baked brown of the inland hills.

      It is the color of serpentine, which is appropriate. Serpentine is a common rock around the Bay Area, but a strange one. It comes from earth’s mantle, a hot layer of heavy metals many miles beneath the surface, and it reaches the sunlight only after millions of years of geological processes that would be cataclysmic if they weren’t so slow.

      Huge plates that form the planet’s crust collide and slide against each other, dragging slabs of the underlying mantle along. When one plate rides over another in the tectonic process called subduction, mantle material scrapes onto earth’s surface, forming an igneous rock called peridotite, which weathers to red or black. But some mantle material mixes with water during subduction, forming a metamorphic rock that weathers a slick, milky bluish green at the surface. It is called serpentine because its color and texture seem snakelike.

      Some serpentine further metamorphoses into jade, a semiprecious stone thought to have magical properties. Serpentine has its own magical property. Because of its heavy metal chemistry, its soil resists the weedy exotic vegetation that has preempted much of the Bay Area, allowing many beautiful native plants and the animals that depend on them to survive. For plants that have been evolving here for millions of years to persist on rock that has been forming for an estimated 200 million years seems to embody the evolutionary nature of this place. And for that rock to be associated with the serpent, the creature that—more than most—connects with the depths of earth and time, seems to embody the Bay Area’s mythic nature. It’s easy to imagine some sinuous ridgetop of slick blue-green rock as a coil of a snake, so big and old that its movements are too slow for human perception—a World Serpent.

      The Golden Gate is a misnomer in geological terms. The only natural gold I know of in the Bay Area is the residue of Mother Lode mining scraps that rivers have washed into the Bay. It’s really the Serpentine Gate.

      Of course, the Chamber of Commerce wouldn’t like calling it that. Prejudice against serpents and anything associated them has been endemic to Western civilization since Genesis. Prejudice persists even in these “green” times. The illustrator of my book The Klamath Knot made a wonderful jacket design of a World Serpent coiled around the Klamath Mountains. But market research at Sierra Club Books rejected it for a less inspiring one of an anthropomorphic myth, the giant Bigfoot.

      The prejudice is recent. Most cultures, including Western ones, have revered snakes because of their associations with depths and origins. The ancient Greek oracles and Eleusinian mysteries centered on snakes. The greatest prophetess, the Delphic oracle, was the Pythoness. Many learned volumes have been written about snake mythology. But the most interesting way into snake lore is through the snakes themselves, preferably the local ones.

      The Bay Area isn’t the snake capital of the world. It doesn’t, for one thing, have green snakes, the color that people archetypically associate with them. (Children usually color snakes green.) The local rattlesnake’s scientific name used to be Crotalus viridis, the green rattler,