Shale and chert in the Claremont Shale

In preparation for my upcoming talk (billed as “drunken education”) for Nerd Nite East Bay, I took a three-hour tour of the Oakland hills to take fresh photos of my favorite rocks. The tour was a whirlwind one.

This is the classic exposure of the Claremont Shale in the valley of Claremont Creek, which I’ve featured here before. On this day, the light on the freshly rain-washed stone was irresistible.

In these hills, the Claremont is mostly chert — that’s the lighter colored, blocky stuff. But it’s larded with thin, soft layers of brown shale, a rock type defined as a more or less pure claystone that breaks in sheets. The layer at my hammer’s point shows how quickly the shale weathers into flakes. Most exposures don’t offer such a clear view of the shaly portion.

Clay is a sediment that comes only from the land. The sediment that makes chert is the microscopic shells of diatoms; you could say that chert is a pure precipitate of seawater. It’s intriguing to see the two so close, and so separate. Not to mention the force that so carefully tilted all of this rock upright from its original horizontal position.

With ten places to visit that afternoon, I couldn’t linger. An hour here to ponder the details would be more to my liking.