Geologic Tour of Oakland, California
Lake Merritt area in downtown Oakland. Map units: af, artificial fill; Qms, Merritt sand; Qmt, marine terrace; Qpaf, recent or late Pleistocene alluvial fan. Images on this page (c) 2003 Andrew Alden, licensed to About.com, Inc.
Adams Point is actually the name of the neighborhood at the top center of this map, but I'll use it for the peninsula pointing southward into Lake Merritt. This point is the only place left on the whole Oakland shoreline where the natural deposits are still exposed. You can see it at the left edge of the Lake Merritt photo.
One of the problems of urban geology is finding undisturbed land. Much of the mapping in a place like Oakland is based as much on topography and old photos as on proper outcrops. Fortunately there are a few outcrops here.
None of the rocks along this shoreline are native. The small boulders are from old walls, probably built in the 1930s along the high water mark. Some of them are concrete while the rest come from a quarry near Piedmont. The large boulder is a decorative stone rolled here by vandals; others just like it sit in the parkland above the trees. But the sand and pebbles are local, derived from the hillside. Under its blanket of quackgrass and shrubs, the earth in this locality has been left alone since before the Spanish land grants and the Anglo invasion.
If this were out in the woods somewhere, a real geologist would dig into the hillside to uncover clean exposures. But in a city park we must accept what nature gives us, and in only two small spots can we see what's under this land. If you visit this spot, please treat it with the same respect I did.
Mixed sand, silt and gravel appear here in classic alluvial beds. Because the sediment is not well sorted or well rounded, we can surmise that it was carried here by vigorous currents after a fairly short trip. The steepness of the bedding, descending from right to left, also fits that scenario. I confess I picked out one pebble from this spot: it was a fragment of volcanic ash, from the other side of the Hayward fault.
This other outcrop is hidden under tree roots. Here are larger pebbles, showing the variety of rock types in the hills that shed this debris long ago: red chert and blueschist and various other things. (Rock gardens in the city are based on this same set of Oakland rock types.)
Although the map shows this spot as a marine terrace deposit, these sediments don't seem to have been subjected to the conditions of a beach. That would have made the pebbles more rounded and better sorted. Perhaps when sea level was temporarily higher, just a few thousand years ago, the waves shaved the top off the alluvial fan (Qpaf on the map) that was here without removing it entirely.
Download a 2400 by 2400 geologic map of Oakland (2 MB) for all the details.