They’ve been building some new parking in Lakeside Park, near the boathouse, and the work opened up the subsoil to view. You’re looking at a hard, pure marine clay from a time when the sea rode high, late in the Pleistocene Epoch.
Lakeside Park sits on the flat surface of a terrace a little over 20 feet elevation. It matches other surfaces in the Uptown/Valdez area and in old Clinton, all of which are assigned an age of around 125,000 years.
The scenario is this: As the great continental glaciers melted well beyond their extent today, the sea brimmed over and rose about 6 meters above its present level. The advancing waves carved their way into the Oakland landscape, creating a wave-cut platform, and then all was peaceful for a few thousand years. That’s when the nice clay topping was laid down. Only the finest sediment could get this far away from the new shore because all the stream valleys were drowned.
Later the Earth’s orbital cycles proceeded, the climate cooled, the glaciers grew back and the sea retreated. On this side of the bay, far from the Pacific surf, the process was gentle enough to leave behind much of the evidence. Elsewhere in the East Bay, the clay beds from this time were thick enough to support a brick industry.