Archive for the ‘oakland rocks’ Category

Blueschist on Old Redwood Road

27 April 2015

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).

Rocks of the Crusher Quarry

11 April 2015

This former quarry in Laundry Farm encompassed the ground now occupied by Belfast and Bermuda Avenues, just south of Horseshoe Canyon and Leona Heights Park. One source, the Laundry Farm map, shows something called the Hotel Mine in this area but I have found nothing about that mine, only information about the Crusher Quarry. It was operated by the E. B. & A. L. Stone Company around the turn of the last century.

Here’s where we start, at this fire road just north of Bermuda Avenue off Mountain Boulevard.

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The square object is the upturned west end of a long, arcing concrete ramp. It slopes down to ground level and levels off beneath some oak trees.

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Then it turns up again, as if aiming at the old quarry face.

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Here’s the broken upper end.

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This was once the working middle part of a cable tramway, a set of steel cables that carried huge buckets back and forth between the rock face and the crusher. The ramp therefore describes a roughly catenary curve corresponding to the natural sag in the cables.

Onward to the high working face of the quarry.

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This is highly fractured “Leona rhyolite” that needed little processing because it was already naturally half-crushed. The name of the quarry may be typical 19th-century American irony.

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The stone has acquired a typical blushing orange hue because it releases a lot of iron, which oxidizes and hydrates forming thin crusts on exposed surfaces. I think that the notable deposits of red and yellow ocher in this area, which were widely known among Bay Area native tribes, arose from many thousands of years of uplift, fracture and weathering of this rock.

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Higher on the hillside above the quarry, you can find lots of natural outcrops. They show signs of working in places. I think the stone has a nice presence.

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You’ll see sheltered spots with the glimmer of various shades of ocher; also evidence of wildlife.

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On the way back out, take time to see what’s blooming in the raw land the quarry left behind. These are called red maids.

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And the fine gravel even supports an underground ecosystem that produced this emerging mushroom.

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The locals enjoy and watch this place, so there isn’t any tagging or bottle-smashing to speak of. Climbing is dangerous; stick to Berkeley’s rock parks for that. Visit discreetly and leave the place cleaner than it was when you came.


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