Blueschist on Old Redwood Road

How many people are going to visit a quiet dead-end up a steep hill just to see if any rocks are there? Not too many, but I’m one of them. The street is Old Redwood Road, a short arc overlooking the Munck Elementary School that doesn’t quite return to Redwood Road. It has a handful of homes and all of this blueschist.

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The exposure is about 100 feet long and in good condition. I didn’t inspect it with minute care, but it shows evidence of lots of shear, which you’d expect in this rock and this setting.

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It tapers off at each end and rises to about 10 feet in the middle. This view of the bottom end shows an interesting feature: there appears to be an irregular pod of serpentinite in it, on the right-hand side.

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A closer look at it reveals some typical serpentinite features: a gleaming surface, a relatively green hue, and balls of partially remineralized stuff that are polished and shaped like balls of clay rolled between the hands.

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I didn’t find any reference to it online, but perhaps the real experts know all about it. If so, they haven’t published anything. Or maybe they don’t know it’s there—the geologic map says that this is a sliver of “Leona rhyolite” at a contact with the Knoxville Formation (which is indeed exposed downhill). No way.

While I’m talking about this area, I have to say that the Munck School grounds, and Pinto Playground next to it, look like yet another former quarry, although they may have been excavated between 1959 and 1968 when the new Redwood Road was built, isolating Old Redwood Road (and the loop around the Hebrew Day School farther uphill).

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6 Responses to “Blueschist on Old Redwood Road”

  1. Michael Kelly Says:

    Wouldn’t the serpentine indicate slipping?

  2. Andrew Says:

    The whole outcrop indicates shear. Serpentinite simply is easier to shear, and on a larger scale, serpentine rock is a favored place for faults to persist. In Oakland, the Hayward fault does not follow serpentine. Serpentinite is associated with other parts of the San Andreas fault system, though, and the New Melones fault in the Sierra foothills, where the Mother Lode mines are concentrated, is full of it.

  3. Scott Bowman Says:

    Went there this AM on my way to work. Nice outcrop, but I don’t think it is schist. The color and texture in places looks like blueschist, but I took some pieces home and under the microscope it just looks like serpentine. It has none of the glaucophane crystals which give blueschist its color and has the greasy, pearly look of serpentine even at the granular scale. BTW, the formation appears to continue up the hill to the area above Redwood Road and below the Oakland Hills Tennis Club. I didn’t brave the whizzing traffic to inspect it there, but may do so another day.

  4. Andrew Says:

    Thanks for checking it out. The same rock does appear on the other side of Redwood Road uphill from the tennis club, although I didn’t push very far in that direction.

  5. bob Says:

    nice blueschist not far away – see article – Melange and Fault Rocks Exposed In and Around Abandoned Quarry at the Schmidt Lane Recycling Center, El Cerrito, CA -by Wakabayashi et al

  6. Andrew Says:

    I’ve been to that locality with Wakabayashi. It’s everything he says it is. However, most of the good part is fenced off and getting more overgrown each year. Funny, just today I was reading Waka’s interpretation of the northern San Andreas fault system.

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