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 cen