Rocks and land of south Mountain Boulevard

First things first: I’m leading a “fieldwork-style outing” at Knowland Park on Sunday the 20th, from 2 to 4 pm. Details and tickets at Wild Oakland. This will be an experiment in having people learn about how geologists do their jobs and experience the landscape.

The southern end of Mountain Boulevard, between the zoo and the former Leona Quarry, is a little-traveled piece of road. I walked it the other day simply because I’m walking every road in Oakland, but it gave me a Eureka moment to share in this post. As usual, here’s the topography, with asterisks at the localities featured.

southMtnBvd-topomap

And here’s the corresponding geology. Jsv is the Leona “rhyolite,” which is actually a metamorphosed body of erupted volcanic material with a rhyolitic composition. This will come up again later. Jpb stands for Jurassic pillow basalt. They’re the two rock units I’ll be showing.

southMtnBvd-geomap

But first, a longing look through the fence at the empty piece of property where the Oak Knoll Naval Hospital once stood.

OakKnollvalley

It looks like a peaceful valley, but the aerial view shows that it’s full of old concrete. When it gets redeveloped, which will happen some day, a portion will surely be kept woodsy. I hope the friends of nature are being vigilant about retaining as much habitat as they can here.

Also visible is the clubhouse of the short-lived Oak Knoll Golf Course that preceded the naval hospital.

OakKnollClub

There is little significant bedrock geology on the property, which is almost all Leona “rhyolite,” although how it managed to get so deeply eroded here is a puzzle. It would be interesting to get a look at the ground as it’s being excavated. The valley here is drained by Rifle Range Branch.

riflerangebranch

As is common in our relatively arid climate, the stream runs in a deep-cut bed or arroyo in a wider floodplain.

The second rock unit crops out along Mountain Boulevard at the western edge of the Oak Knoll property. It’s very different from the Leona and not really like the other brown rocks of Oakland either.

J-basalt-MtnBlvd-at-Fontaine-overcross

It’s mapped as pillow basalt, which is not at all evident right here. Basalt I can buy, although it’s pretty shattered by exposure and multiple tectonic insults since its eruption about 145 million years ago. Will definitely visit again for a closer look.

Farther north in the residential areas, you start seeing a lot of Leona rock in the landscaping, including some big boulders.

Leonarhy-decoboulder

And I couldn’t resist a close look at this nice actinolite boulder across the street.

actinboulder

Farther north, Rifle Lane strikes up into the hills next to the Leona Quarry development. It’s secluded and rustic and full of rocks.

Alturasboulder

I also noticed a fair number of stones with greenish bits, which I’ve seen in many places in Oakland. This time I realized that they must originate in the Leona. Here are two examples. The left-hand one is from Dunsmuir Ridge, and the right-hand one is from a hillside on Outlook Avenue.

Leonarhy-celadonite

Clifford Hopson, one of the greats of California geology and a close student of the Coast Range Ophiolite of which this rock unit is a part, wrote in 2008 about these rocks, “Devitrification of once-glassy tuffaceous and fragmental siliceous rocks, including silicification that accompanied devitrification, accounts for their hard flinty character. Local turquoise-green beds mark pervasive celadonite, a typical low-temperature devitrification product of rhyolitic/dacitic tuff and pumice.”

And at the top of Altura Place is a colorful boulder of this stuff (1000px).

Leonarhy-celadonite-boulder

This rock superficially resembles the greenish metachert of the Franciscan Complex, shown here and here and here and here. But it forms in a fundamentally different way, and once you’re familiar with both rocks they’re easy to tell apart.

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One Response to “Rocks and land of south Mountain Boulevard”

  1. nbschiff Says:

    Oak Knoll CC was for years used as the Officers’ Club. It is graffitied and ill-maintained but it should be reused; it’s a really fine building with a good deal of surviving detail, large event rooms with big stone fireplaces. Unfortunately the current developers want to knock it down, though their earlier pre-2008 plan was to retain it.

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