Archive for the ‘oakland rocks’ Category

Serpentine garden

19 October 2013

This house on Melvin Court has a splendid front yard based on serpentinite: serpentinized peridotite on the right, a serpentine-lined walkway with slate in matching colors, and inlays on the path composed of serpentine medallions. The house itself is painted serpentine blue-green. Click the photo for a big version.

serpgarden450

The Oakmore district is uniformly mapped as Franciscan sandstone, but just a little farther east it’s mapped as undivided Franciscan, so we might expect a mixture of possibilities here. The neat lines on the geologic map are as much hypotheses as they are conclusions. Other outcrops nearby look like volcaniclastic rocks, and some of this home’s neighbors use it effectively. I conclude, though, that this home’s landscape was composed with imported stones rather than assembled from what was lying around. But the rocks may well be Oakland natives from just up the way, perhaps even from the Serpentine Prairie quarry.

Baldwin Street boulder

6 October 2013

Out in East Oakland at the corner of 85th Avenue and Baldwin Street is this fine, underappreciated boulder.

85th-ave-chert

To all appearances, it’s good old Oakland chert, hard at work. I assume it was put here to keep vehicles from cutting across the corner, or perhaps to keep a runaway vehicle out of the building behind it. Who knows? I was just glad it was there to break the monotony. It’s free of graffiti, too.

The other end of Baldwin Street is east of the Coliseum, where it serves as a back entrance for staff and athletes, at the edge of Arroyo Viejo. That’s the creek you cross when you’re walking from the BART station to the game. So between stone and water, Baldwin Street pays more homage to geology than most of its peers.

Elverton exposures

25 September 2013

Elverton Drive is a very distinctive place in the high hills, not so much for its houses—though no insult meant to their owners—as for its bedrock. From end to end, it offers the best exposures anywhere of the Claremont chert.

elverton1

If it weren’t for the parking situation, this would be a great spot for a class exercise in field mapping. The strata are clear, the winding road offers a range of orientations to refine measurements, and the rock isn’t totally uncomplicated. Take this spot.

elverton2

What is the nature of the change between neat rows and rumpled layers? What can the student conclude from the evidence, and what should the student look for elsewhere to test those conclusions? I don’t know; I’m just asking and I didn’t inspect this closely. Besides, it might be on someone’s homework.

A few years ago, Elverton was blocked by a landslide. Residents could get in and out from either end, so it wasn’t that bad, but I stayed away until a few weeks ago. I think that this spot must be where it was. (If it’s not obvious, this is sculpted concrete.)

elvertonslide

Near the road’s east end is an old excavation, perhaps a small quarry, where you could examine these rocks at leisure and collect a specimen. But do notice the presence of fallen blocks, and if you feel an earthquake while you’re there, step the hell back.

East of Elverton, the chert crosses the ridgeline into the Huckleberry Preserve and trails into the back hills.

Lakeside Park terrace

1 September 2013

Lakeside Park is one of the most parklike parks I know. Perhaps I feel this way because I imprinted on it at kindergarten age.

Click for larger photo

Youthful feelings aside, I think that geology makes the park this way: it’s set on the late Pleistocene marine terrace, planed and beveled by the sea waves during an interglacial highstand approximately 125,000 years ago. The planar setting, studded with trees to the limits of vision, suggests a vision of infinity, or at least limitlessness.

This spot is in front of Children’s Fairyland, where young children then and now can experience mind-blowing things everywhere they look. Show them these rocks, too: they’re ochre-stone and chert from our own hills.

The route 24 cut, south side

11 August 2013

A few weeks ago I managed to get a very nice view of the roadcut where the end of Gudde Ridge was trimmed to make room for route 24, east of the Caldecott Tunnel.

24southcut

That’s the lava flows of the Moraga Formation on the left. On the right edge is the coarse conglomerate of the underlying Orinda Formation. Click the photo for a 1000-pixel version. (The cut on the north side of 24 is shown here.)

This is not a view straight on; the ridge runs toward the lower right, so the steeply tilted rocks look vertical from this angle. It can often be hard to figure out the exact orientation of rocks, even in a splendid exposure like this.

Middleton hill

26 July 2013

Way down in the Sheffield Village neighborhood is a big hill and a little hill, but most of the place is in a flat little basin. The geologic map shows it as a patch of old alluvial sediment of the same vintage as the Fan:

sheffieldmap

The big hill is on the east side, across the Hayward fault (the black line from the lower right corner). The little hill is on the west side, butting against I-580. The road on its west side is Middleton Street, so I give the hill that name. I didn’t realize it when I poked around there earlier this month, but the hill is mapped as San Leandro Gabbro, of Jurassic age. The rock doesn’t show itself very much, but here’s a small exposure. Next time I’ll bring a hammer.

middletonhill

The highlight of the hill is the little private park inside the ring of houses there, just a microscopic piece of the original oak meadow (although this is actually a cork oak).

middletonpark

From there you get a nice view of the big hill. The fault runs along the foot of the hills, behind the homes in the center and in front of the lower set of homes on the right.

sheffield450

Click the photo for a big version.

Oakland jasper

15 July 2013

My place lies at the edge of the Fan, downhill from the Piedmont block, and the soil keeps producing these gorgeous pieces of lightly polished high-grade chert. I decided that this one should sit for a formal portrait. Click the image for a larger version.

chertportrait450

It’s really more of a jasper than a chert. Jasper is a glassier, more refined chert that gets its appearance from the redistribution of silica fluids. This one mixes the red of oxidized iron with the green of reduced iron—at least that is my inexpert explanation—reflecting the invasion of oxidizing fluids at some point during its rough-and-tumble history as part of the Franciscan complex.

I believe that upstream, in Moraga Canyon, there were chert quarries in the earliest days, but I need to check the records to be sure one way or the other. This is not a quarried stone, though, because it has the burnished surface of an object that has been carried a short distance by streams and natural mass movements. I can’t look at it enough.


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