Archive for the ‘oakland rocks’ Category

Butters Canyon serpentinite

11 May 2015

I will continue to focus on serpentine rock—serpentinite—for a while longer because I’ve been visiting it a lot lately in Oakland. This post shows some exposures in the lobe of serpentinite mapped just south of Joaquin Miller Road. This bit of the geologic map shows where we are, and the five numbers are where I took the following photos.

butters-serpmap

The first photo is at the head of Hedge Lane, right off Joaquin Miller. The map says this is a blob of Leona “rhyolite,” but the ground is clearly serpentine here, on the whole street.

butters-serp1

A little farther, on beautiful bucolic Burdeck Drive, is this serpentine stonework. It doesn’t prove that there’s serpentine bedrock here, but it does suggest that some is lying around.

butters-serp2

So here we have a piece of the geologic map that’s mapped as one thing, but is clearly another. That’s the nature of geologic maps. They aren’t like street maps. They’re simplifications of a welter of vague clues in the landscape and bits of rock, surmises based on drilling records and engineering reports, and highly informed imagination. No doubt the mapper’s field notes mention “serp” where I saw it, but the area is still predominantly “rhyolite” so that’s the color it’s assigned.

Along the rest of Burdeck, the ground looks like something other than serpentinite, although no bedrock is exposed: unlike serpentine ground, the soil is thick and the oaks are vigorous. Then we join Butters Drive, just to the right of the “sp” symbol, and serpentine returns in all its blue-green, scaly glory.

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Here are two closeups from this exposure, testimony of mighty squeezings deep underground.

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Oakland’s serpentinite patch

4 May 2015

Oakland’s serpentinite patch is the snaky zone shown in deep purple on the geologic map. I’ve written many posts about individual localities, but this is the first time I’ve shown and talked about the whole thing.

serpzonemap

Here’s the same area in Google Maps terrain view, for reference. The rock doesn’t have a strong expression in the topography, although it tends to support less tree cover.

serpzonetopo

Serpentinite (accent on the “pen”) is a rock composed of the serpentine minerals (accent on the “serp”). Those are three: chrysotile, antigorite and lizardite. I haven’t found chrysotile in Oakland, although I’d be thrilled to: it’s the fibrous mineral used to make asbestos. Antigorite forms at higher temperatures and is often brownish green. That’s not common in Oakland.

Lizardite is the shiny, slippery-looking, soapy-feeling serpentine we have. Here’s a typical example sitting by Redwood Road.

serp-redwoodrd

That color is on the light side of its range in hue. It can sometimes be white, or close to it. On the dark side is this splendid specimen from a Millsmont hillside, what’s sometimes called California jade.

millsmont-jade

Serpentinite is what happens to the deepest rock in the oceanic crust—the dark and heavy material called peridotite—when it reacts with seawater at high temperature and pressure. That is to say, large parts of the deep oceanic crust, miles below the seafloor, consist of this stuff. We would never, ever see this rock unless it somehow got lifted up and put on land. Luckily, plate tectonics enables that to happen sometimes. The result is a body of rock called an ophiolite.

Russ Graymer, author of the geologic map, classifies our serpentinite patch as part of the Coast Range ophiolite. The Coast Range ophiolite is a little older than Oakland’s Franciscan rocks underlying Piedmont and surroundings. It’s not pure serpentinite by a long shot, but the name is a good shorthand. The Coast Range ophiolite is strewn up and down the Coast Range in dribs and drabs. The experts are still working out its story.

If you look on the geologic map you’ll see six areas of serpentinite defined by heavy black lines (plus a thin wisp next to Holy Names that I’m ignoring). I happen to have photographed it in all six, going from north to south:

1: In the Piedmont Pines neighborhood it crops out on Castle Drive and Las Aromas

2: Joaquin Miller Park’s Visionary ridge is serpentinite, and the body extends south to Butters Canyon.

3: Joaquin Miller Park has a small body of serpentinite separated by a thrust fault from body number 2. It’s exposed in the Friends of Sausal Creek nursery.

4: The biggest area of serpentinite encompasses an area from upper Joaquin Miller Road to the Crestmont area and, over the ridgetop to the east, Serpentine Prairie (I and II). Its long tail running past Merritt College is a part I have yet to explore.

5: A long, narrow strip of serpentinite extends down to Lincoln Square.

6: Another long strip extends from the belly of body number 4 across Redwood Road, including Old Redwood Road.

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.

bluschst-oldredwood1

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.

bluschst-oldredwood4

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.

crusher1

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.

crusher2

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.

More of the “Leona rhyolite”

4 April 2015

(Starting now, I will be putting up images 600 pixels wide instead of 450. Hope you like that as much as I do.)

A big chunk of the high hills consists of the “Leona rhyolite,” shown as the pink unit labeled Jsv on the geologic map below.

Jsv-geomap

To help you orient yourself, here’s the same area in Google Maps. The area I’m featuring is around the asterisk near the bottom of the geologic map.

Jsv-Gmap

“Jsv” stands for Jurassic silicic volcanics (or keratophyre), the kind of sticky, explosive lava and ash that island arcs are made of. Similar examples today would probably be the Greek islands. The original rocks have been rather thoroughly altered since their birth around 165 million years ago, but they still stand out among Oakland’s rocks. I described them previously here in the former Leona Quarry and here in Leona Canyon. The experts are still arguing over these rocks, and for now I will spare you the details. But basically, they aren’t really rhyolite so the name isn’t used officially any more.

This is the view north across I-580 to the northern side of the quarry scar. The high valley is the headwaters of Chimes Creek, and I continue to be fascinated by the idea of standing up there in that perched catchment.

chimesCktop

A property on Sunnymere Avenue has a nice boulder of this stone.

Jsv-boulder

And a yard on Columbian Drive uses the stone for landscaping. You may already notice how consistent the color of this stuff is. I think that may be the best way to identify it around town. Some of the Tertiary sandstone has a similar honey color, but it weathers into evenly colored, rounded forms whereas the Leona turns craggy and mottled.

Jsv-yard

And here’s a hand specimen found at the end of Field Street.

Jsv-hand-FieldSt

This appears to be Oakland’s oldest rock.


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