Archive for the ‘oakland rocks’ Category

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.

Trees and serpentine

29 March 2015

There’s a stretch of Castle Drive, up in the Piedmont Pines neighborhood, lined with huge trees. On the Walk Oakland map, it’s even labeled “Colonnade of Eucalyptus.”

CastleDrTrees

These give me mixed feelings, as tree removal projects have aroused lately elsewhere in Oakland.

First, there’s the experience the trees provide. For one thing, you basically can’t walk here, so the colonnade is not a realistic attraction for walkers. Its main effect is a momentary diversion for drivers, who really don’t need one at this location.

Second, there’s the effect on the surroundings. As you climb up in this valley, the trees emerge as a very tall fence that blocks the view of the hills and the city and the bay.

Third, there’s the geologic setting. This part of the roadway runs along a very steep 40-degree slope through pure serpentinite, visible in the small landslide scar on the right side of the photo. Serpentine rock is poor footing for these massive trees. The trees may seem like they’re buttressing the roadway, but when they inevitably tip over in a storm or earthquake, they’ll uproot it instead, forcing the locals to drive up and down Ascot Drive for many months.

But how about that rock? Here’s a hunk of it that spilled across the road.

castleserpfoot

And here’s a hand specimen. I love this stone, but roadbuilders don’t.

castleserphand

It’s not my problem, since I don’t live there, but I think the best thing to do is to turn this colonnade into a line of ground-level stumps. The root systems would bolster the soil for another decade or so, giving the city time to plan and execute a properly engineered roadway. And bollards set in the stumps would preserve the trees’ most useful current function of keeping cars out of the canyon.

Trees are supposed to be wonderful stockpiles of carbon, sequestering it from the atmosphere. For me, that argument shouldn’t apply to individual trees or even individual groves of trees. What do we do, in the long run, with the carbon in trees—pile the trunks in pyramids? Carbon is best stored in the soil, where it provides excellent tilth and maintains a thriving ecosystem that resists fire and drought. It’s like circulating money in an economy: do you hoard it in vaults or spread it around among people ready to use it as a medium of exchange? Humans have spent thousands of years degrading the world’s soils, and I’d rather we begin to restore them.


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