8 September 2013
A comment to one of my posts talked about the sulfur springs of Bushy Dell Creek, in Piedmont Park. I said I couldn’t detect any and the commenter said where to look. So a few weeks ago I looked and found this small example.
It’s just a trickle, but it offers a whiff of sulfur gas. More tellingly, it supports gray filaments of sulfur bacteria, seen here in closeup.
These look like pollution, and I guess in our context that’s what they are. But whole microbial ecosystems center on a molecular economy of sulfur, oxygen, carbon and hydrogen. They’re mostly hidden underground near sulfur-bearing minerals, but here and there they get flushed out into the light.
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.
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.
23 August 2013
Years ago, a homeowner installed this yard facing Humboldt Avenue. I think of it as a California zen garden.
These days, landscapers are obliged to buy rocks that come from out of town, produced in anonymous quarries by the big-rig load. This yard could have been furnished with stone from just a couple miles away. One of my many pipe-dreams is to set up a stoneyard where I’d salvage and sell recycled rock from local sources. The market would be vanishingly small, but if just a few people cared that might be enough.
19 August 2013
Piedmont’s Witter Field occupies the valley of Bushy Dell Creek, in an opening in the topography where a large formal garden once sat. The creek is culverted here, but once in a while it needs attention, as it did last month. Out of sight is not out of trouble.
It reminds me of the city streets: it seems like as soon as a nice new pavement is put down, a crew comes along to cut it open, as if the asphalt were there just to make a clean work surface.
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.
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.
6 August 2013
Foothill Boulevard descends to Lake Merritt in this gentle slope down from the old marine terrace (Clinton terrace).
For a mile and half east from here, Foothill runs along the terrace. East of 14th Avenue, where it’s a two-way street, it skirts the steep edge of the Fan at San Antonio hill where the contrast between the two geologic units is most dramatic.
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:
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.
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).
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.
Click the photo for a big version.