Just up the hill from the Paleocene sandstone I was looking at last week, on Woodrow Drive, all sign of bedrock disappears. But I kept looking and eventually spotted a dark pod of rock—a concretion. I banged it open and this is what I saw.
The piece in the middle shows the dark outer surface, probably a thin layer of iron or manganese minerals. To its right and above it is the core of the concretion. Its outer surface was pure clay that stained my fingers and got all over my pants. But the definitive field test is to nibble it. Pure clay, as found in shale, is not gritty but smooth as chocolate against the teeth. A mudstone or siltstone is definitely gritty. (Mudstone is a sedimentary rock with a mixture of particle sizes, clay and silt and sand.) And of course, sandstone you just don’t bite.
I don’t know much about concretions, really. Some have an obvious fossil or foreign stone in the center, which releases or attracts elements that create shells of harder minerals around them. In others, I suppose the central object dissolved away completely into the outer layers (this one had none). They can be little things or a meter or more in size. Bowling Ball Beach, up in Sonoma County, is full of them.
Anyway, this concretion told me that the rock along this part of Woodrow Drive is really shale, even though it’s hidden. Shale turns back to clay so readily that you don’t get good exposures of it except in a fresh roadcut. The rock here is mapped generally as Eocene mudstone, Eocene meaning about 55 to 35 million years old.