Oakland Geology

Downtown Oakland sits on an unusual bit of geology — a large dune field mapped as the Merritt Sand. San Francisco is famous for its sand dunes, of course, and Points Reyes and Año Nuevo have some too, but the dune fields of Oakland and Alameda are the only ones within the bay. Here they are, labeled “Qds” on USGS Map OF-00-444, which shows the young (Quaternary) deposits of the Bay area.

They’re just like the dunes in San Francisco. They formed during the ice ages, when the shoreline was out near the Farallon Islands, the Bay was totally dry and the Sierra was full of permanent glaciers (on not quite the same schedule as the great continental ice caps). The rivers carried huge amounts of fresh-ground rock dust from the glaciers to the Bay and beyond, and the Pacific beaches of the time must have been formidable. Think of the brisk summer days we have when the sea wind is being sucked into the Central Valley, and now multiply that. Those winds blew all that sand here.

Brian Atwater of the U.S. Geological Survey mapped ice-age deposits in the Delta, and in 1982 he summarized the overall picture as “a stage on which three related and repetitious plays are presented simultaneously. In one play, wetlands and flood plains appear and expand as tidwater invades from the west, then become sites of erosion after the tidewater retreats. In another play, glacially eroded detritus from the Sierra Nevada builds alluvial fans and, reworked by wind, creates extensive sand dunes. In the third, little-understood play, streams draining the Sierra Nevada foothills and Coast Ranges episodically build alluvial fans. Spanish- and English-speaking persons enter during a major incursion of tidewater and find most of the stage covered with tules.”

Our dunes may sit higher than the buried dunes of the Delta because the conditions that built them were more stable here. There was always a wind gap at the Golden Gate and always lots of sand available on the other side.

In detail, the Merritt Sand (Qms) reaches the edge of Lake Merritt.

It differs from the marine terrace deposits (Qmt) that I’ve described before. It consists of very fine sand, with a little clay. It’s also higher and less flat. Apparently the original, undisturbed surface featured yardangs — elongated ridges of sand running in the direction of the wind — whereas the dunes of Alameda were the more typical arc shapes known as barchans. All of that is obliterated today.

You can see the edge of the Merritt Sand platform from across the lake where the streets rise abruptly away from the shore. Snow Park is probably the least disturbed exposure.

Another exposure stands out between Jackson and Madison streets, although it probably has also been excavated.

It’s the back yard of an apartment building at 160 17th Street. The view is nice from there.

That same agreeable elevation attracted Oakland’s early elite, who put up a row of mansions overlooking the lake. Of those great homes, only the Camron-Stanford House survives.

This entry was posted on 22 August 2016 at 8:03 am and is filed under Oakland sediment and soil. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.