Up in Huckleberry Botanic Reserve my eye was caught by a stone in the path—a clast of Claremont chert, to be exact—bearing this fine example of a slickenside. Slickensides are the polished, striated surfaces on a rock that result from rubbing together, usually by movement along a fault or, in this case, by internal deformation when the rock was being squeezed and uplifted into the Oakland Hills. Little ones are more common than you might think, but ones the size and quality of the one in San Francisco’s Corona Heights are exceedingly rare. I shot this one because there aren’t a lot in the Claremont chert. Usually deformation is accommodated in the shaly beds between the cherty layers, so there’s little chance to get a good rubbing going. The bluish color is reflected skylight, not the chert itself.