Oakland Geology

As you go up Tunnel Road past the turnoff to Route 13, on the left is a long expanse of bedrock. Right now, in midsummer, it seems to be at its best.

Click the photo for an 800×600 version. This is mapped as a little outlier of the same rock exposed in the big Leona Quarry, a keratophyre of Late Jurassic age (that would be about 150 million years old) also known as the Leona volcanics. It’s pretty hard to study, but easy to just admire. Watch the traffic, though. A bit further up is this thick fault zone:

It has signs of more than one episode of motion at some long-ago time: cracks recracked, layers sheared. If you aren’t required to decipher it as part of some devilish class exercise for a Cal geology course, it’s fun to contemplate. Hammers are OK, too, but won’t do you much good. This rock is heavily altered, but it apparently was originally volcanic and shallow subvolcanic material, of a composition richer in silica than seafloor basalt — perhaps part of an island arc that was carried up against North America back in the late Mesozoic.

This whole hillside was quarried during construction of Route 24 and the approaches to the Caldecott Tunnel. It yielded rugged yellowish rock useful for filling things in and not much else.

This entry was posted on 13 August 2009 at 7:33 pm and is filed under Leona volcanics, Quarries and mines. 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.