Whenever I ride my bike to work, I come across a semi-circle of what appear to be old cobblestones at the intersection of Frederick & Shrader Streets.
I never thought much of it until I noticed a full circle of similar stones on 13th Street, at Folsom, near Rainbow Grocery.
My curiosity was piqued. What do they mean? Why are they there? Are they the remnants of an ancient civilization, a decorative flare from a bygone era of well funded city government, or simply the remains of an old cobblestone street?
An e-mail to the San Francisco Department of Public Works solved the mystery. Chris McDaniels of the DPW quickly responded to my query:
“The cobblestone circle in the roadway is there to identify an underground San Francisco Fire Department water storage tank or cisterns. In emergencies, the SFFD rely on these indicator to assist in fire fighting when hydrant are non operational. The tank hold several thousand gallons of water and are installed at various location around the City. Modern cisterns use brick instead of cobblestones.”
As to the cistern at Frederick & Shrader, DPW Mechanical Engineer Michael B. Smith went on to say:
“Our records indicate it was constructed in 1908 of reinforced concrete and has a capacity of 75,000 gallons.”
He also sent me these fascinating DPW cistern construction drawings:
The first document appears to be the original plans/work order for the Frederick & Shrader cistern dating back to 1909/1910. The second is a somewhat more recent diagram from 2007 outlining basic cistern construction guidelines.
So there you have it! The Secret of the Stones. Hopefully they’ll never be needed, but it’s good to know they’re there.
Many thanks to Chris & Michael of the SFDPW for making this post possible.