Even before I knew what it really was, this short pier at Middle Harbor Park stood out.
This is clearly not a typical groin or breakwater, something that is dumped into place, but a carefully laid work of drystone masonry. It feels absolutely solid to walk upon. And then there are the rocks in it, a rich assortment of large and pristine Bay area specimens. There are tuffs composed of volcanic pyroclastic flows,
colorfully metamorphosed volcanics,
slickensided serpentinites,
and other metamorphics whose colors could inspire a fabric maker.
Also sandstones and even a few ringers of Sierran granitic rocks, perhaps from the old quarries of the Rocklin area. But it’s not typical of a modern marine rockwork—those use stone trucked in from a single quarry to save money and control the quality.
An interpretive sign explains that it’s a replica of the old “training wall” on the north side of the shipping lane to Oakland Inner Harbor, where the signature gantries load and unload big freighters. Training walls are jetties designed to turn a shipping channel into a flume during ebb tide, keeping the bottom clear of mud. The walls were built between 1874 and 1894, using shoreline quarries around the Bay and shipping the rock here by barge. That explains the unique variety and distinctive regionality of the material.
The north training wall was removed when the shipping lane was enlarged in 2001, but they saved some of the stones. Masons installed them over a core of rubble, and here they are. The south training wall remains in place.
Oakland Geology 
5 July 2011 at 2:33 pm
I really love getting the Oakland Geology updates! i’m one of those geeks who is completely fascinated by everything historical, geological, botanical, anthropological/archaeological about the place where i live and your site is right up my alley. you may find these photos of tafoni interesting: http://www.flickr.com/photos/eastbaywilds/sets/72157626357964562/with/5554119107/
under the comments, there’s a link to an interesting video about tafoni too.
thanks for the wonderful posts!
pete veilleux
east bay wilds
5 July 2011 at 9:58 pm
I have always loved the idea of training the ocean. What hubris! Thank you for the great post.
[You might call it hubris, but I would call it good engineering. — Andrew]
6 July 2011 at 8:11 am
Moving stones is very expensive even now, and fantastically expensive in the 19’th century. Perhaps the granite came from ships ballast, many ships were marooned in Oakland during the gold rush. Rocklin could be a source though. Prisoners were building the Folsom prison from granite at Folsom. State Legislators went to see the construction of the prison the found the prison to be prettier than the state capitol. The Legislature ordered the prison construction to be stopped and the prisoners to quarry granite for the state capitol building. The quarry in Rocklin is the source of the granite for the state capitol although the granite would have to be hauled overland for a good distance from Rocklin to Sacramento as Rocklin does not have any rivers. The quarry in Rocklin is now a climbing park called Deer Creek Park.
14 October 2018 at 2:49 pm
More information on the old training wall: Photos at https://www.loc.gov/resource/hhh.ca2606.photos?st=list and documentation buried in the Naval Air Station EIR vol. 1 starting at page 653. Most of the rock came from Yerba Buena Island from a new quarry opened in 1874; rock also came from “other local quarries at Second Street and Telegraph Hill in San Francisco, Angel Island, San Bruno, and Point Pedro in San Pablo Bay.” The boulders “were placed closely together by masons to provide a high degree of interlocking, producing greater strength with smaller, lighter stone.” But rising labor costs put an end to this technique. “Few, if any, examples of this kind of maritime construction survive today.”