Archive for July, 2013

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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