The Tunnel Road quarry

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

tunnel road quarry

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 Leona Rhyolite. It’s pretty hard to study, but easy to just admire. Watch the traffic, though. A bit further up is this thick fault zone:

vein

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 at one time before it became part of a major highway route. It yielded rugged yellowish rock useful for filling things in and not much else. The stone of much of the WPA hardscaping around town may have come from here.

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One Response to “The Tunnel Road quarry”

  1. farmlady Says:

    I just love your Blog. I was raised in the Bay Area and have family there. I read your posts all the time. It’s so interesting even though I don’t always understand the technical info. I’m learning all kinds of things. Keep up the great work. Thanks.

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