Rocks of Upper Rockridge III: Blueschist

At the uppermost top of Upper Rockridge is Contra Costa Road, where amid the fine homes and gardens is this knocker of real blueschist.

blueschist knocker

Knockers are the California geologist’s nickname for blocks of resistant rock in Franciscan mélange, the smorgasbord of rock types (chert, basalt, greenstone, serpentinite, etc.) bound by shaly matrix that is common throughout the Coast Ranges. Knockers are too big to be called boulders but too small to be mapped. Anyway, this knocker is a tough stone of a deep indigo color from the high-pressure mineral glaucophane, which if you remember your Greek simply means “blue in appearance.” I couldn’t resist taking home a chip.

rockridge blueschist

Unlike the garnet-mica blueschist of Joaquin Miller Park, this outcrop is almost monomineralic except for some white veins, probably quartz. It gleams like leather in the magnifier, with intricate crenulations and understated foliation—not a real schisty schist, but layered enough to qualify. I’m in love with it.

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7 Responses to “Rocks of Upper Rockridge III: Blueschist”

  1. Naomi Schiff Says:

    I thought glaucus meant white, not blue.

  2. Andrew Says:

    “Glaukos” means blue-green or simply gleaming. Minerals and rocks with glauc- in their names are blue or green.

  3. Mary Beth J-T Says:

    I want you to come and live with me and do a running commentary on the geology in my life. I am SO grateful whenever anyone can tell me something about the geology of a area. Like why are those lone buttes on the way to Chico? and why is the dirt red in Redbluff and Redding and nowhere else? What are those yellowish grayish rocks in the clay in my back yard that shatter so easily? You get the picture. You would have to move to San Mateo…

  4. m. Carey Says:

    Mary Beth:
    Those lone buttes on the way to Chico are the “Sutter Buttes” – An old eroded volcano. It is the southern point of the Cascade Range, which includes Shasta and Lassen. These mountains extend north to Oregon and Washington State.

  5. Andrew Says:

    Thanks for responding. I have a whole photo-tour of the Buttes on my About.com site

  6. artisancrafts Says:

    https://www.evernote.com/shard/s138/sh/e5ee4871-1315-47c0-bb94-050a76b5ffcc/

    I am fairly certain that I have found a Blueschist knocker beneath an extinct streambed on the Mills College campus. I’ve been doing historical research in this area. I spotted it on a eucalyptus lined trail about 150 meters NNE of Trefethen pool. It’s really quite pretty. Also, there’s an Oakland Survey Marker embedded in the trail very near by.
    Can anyone tell me more about this formation and a history of the area?

  7. Andrew Says:

    There’s no bedrock mapped there, but boulders from upstream in Lion Creek could have come down to the campus.

    The link you posted is no good.

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