Where the subsurface is inaccessible, you learn to prize the opportunities to sample it. A house on the crest of Montgomery ridge is upgrading its foundation, and the contractor has helpfully kept the tailings in this handy bin. It’s mostly sand and mud, but there’s a good deal of the Franciscan sandstone in it as well, plus hunks of the lovely red chert that seems to be part of the youngest detrital beds all over this hill. I restricted myself to collecting just one chert piece to add to the pile in the front yard.
I keep thinking that a concerted effort by enough Oakland citizens could help ensure that these ad-hoc trenching studies are properly exploited for their scientific value.
Oakland Geology 
21 November 2009 at 10:23 am
Any chance of a photo of that red chert? I would rather see it as an
illustration for your excellent blog than a heavy-load dumpster full of
grayish stuff :-)
[Well, there’s here and here.]