as we stopped at the formation known as Rock City we discussed if the formation was Glacial. I think jobob thought it looked glacial, my theory was either sedimentary or volcanic. For the tie breaking vote I went to a friend of mine, Scott Hassler, a Phd in Geology. Scott writes

Hi Dita,

No worries wrt rock question. No glaciers in this part of California, at least in the past 900 million years. Rock City is composed of sedimentary rocks, specifically layers of sandstone and shale (originally muds) that formed in deep marine environments. Anyway, at this part of Mt. Diablo, the originially horizontal layers have been folded and faulted to near vertical orientations. What you all saw is
1) differential weathering, i.e., the sandstone beds are more resistant that are the shales, so the former erode slower, giving you "fins" of sandstone and valleys of shale,
2) the cavity like in your picture is a
reflection of the same process - it's where, at the bottom or top of a sandstone bed, there's some mud mixed in, and it erodes easier. Then wind/rain/frost/people gradually make it bigger. Technically these are called "cavolli" tho I forget why.

Hope this helps, see you soon I hope.
Cheers, Scott


so folks...now you know...the things you see and learn cycling!!