Archive for the ‘Oakland rocks’ Category

Upper Knowland Park and the Chabot fault

22 February 2016

The upper part of Knowland Park is quite different from the lower part. I made a reconnaissance visit last week. Here’s the geologic map, along with white numerals indicating the localities I took the photos from or at.

upper-knowland-geomap

Whereas the lower part of the park (west of Golf Links Road) is dominated by Franciscan rocks and the Leona rhyolite, the upper part is mapped as completely sedimentary. My main destination was the saddle between the areas mapped as Joaquin Miller Formation (Kjm) and Knoxville Formation (KJk), where the obscure Chabot fault runs. Here’s a view of the saddle and the bare knob of Knoxville beyond it.

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The Joaquin Miller is a fine-grained sandstone here, sometimes with a slightly slickensided texture that makes it almost glossy.

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The roadbed displays it nicely. The saddle doesn’t display any obvious signs of a fault.

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But as you approach it, the honey-colored rock in the roadbed . . .

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. . . gives way to a deep sandy soil with chunks of strange rock floating on it. Not what I expected at all. I thought I’d see a hard, dark shale/conglomerate like what’s in the streambed of Arroyo Viejo. Instead it looked for all the world like a Franciscan assemblage. Here are a few of the stones.

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This last specimen, and the first, appear to be bits of the Leona rhyolite. So there is some complexity here that’s not recorded on the map, perhaps a splinter of Franciscan that got mixed up in here.

I didn’t learn much about the Chabot fault, except that the abrupt change in lithology is a sure sign of a fault contact. I’ll have to do more poking around before I can write something coherent about it. (In fact, please ignore site 4 on the geologic map; I’m not showing that this week.) This is the view south from point 3 along the valley that marks the fault trace (1000 px).

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And here’s your weekly cheesecake shot looking north from point 5 (1000 px). Rabid fans will note Sugarloaf Hill on the skyline.
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This is a great time to visit, but do watch for the newly sprouting poison oak.

Bike trails, right wrong and ambiguous

15 February 2016

One day a year ago, as I set out to investigate the old Crusher Quarry, I was standing off the fire trail looking at something when a rattling sound came from up the hill. Two mountain bikers burst out of the woods in a spray of dust and gravel and jerked to a halt nearby, rear wheels tipping off the ground. “That was intense!” I heard one say. Then they rode a few yards down the fire trail to Mountain Boulevard, where someone in a pickup had just arrived to meet them, and a minute later they were gone, with no one the wiser except for me.

Where they’d come from, tracks ran up a delta of dirt to a narrow trail.

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It was really steep, and slippery too (that was a dry winter, you’ll recall). A false move might get a rider impaled on young acacia stumps. And if another biker had hurtled down upon me I had precious little space to duck. I climbed the trail a bit nervously. None of us really belonged there.

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Bikers get an intense ride here. An intense ride is a legitimate thing. But isn’t all they see here just a blur of trees during their minute of white knuckles? What I saw was untreated erosion on the ground, an unpermitted trail in the city’s Leona Heights Park, and a community of scofflaws for whom it’s their personal secret.

After a while I struck off the trail to continue my own intense hike, and that was that. Then the other week I found the trail’s top end, near the Merritt College parking lots.

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The trail is pretty nice up there. Not exactly legal, but not in the city park either. Useful. I even approve of it. I saw access to some nice countryside with splendid views, a well-built trail that suits the slope, and a community of avid cyclists for whom it’s their personal secret.

Once again I got sidetracked, so I haven’t traveled the whole trail yet. Clearly, though, the top and bottom segments, Jekyll and Hyde, meet at the fire road in Leona Heights Park. It would be really nice if the lower part were converted to foot traffic only, with erosion control and occasional steps. Mountain bikers could take the fire road down to McDonell Avenue.

At Leona Canyon Open Space Preserve, less than a mile south, “the park is an ideal place for hiking, running, biking, dog walking, and similar activities.” The intense, pellmell riding experience can be had on either the Artemisia or Pyrite trails there — just not the thrill of living outside the law.

Chimes Creek headwaters

1 February 2016

I’ve mentioned how tempting the uppermost catchment of Chimes Creek looks, perched above the Leona Quarry scar:

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Finally got up there last week. Access is difficult and not for casual visits.

Here’s the valley in Google Earth, looking obliquely at it. At first glance it looks natural, but it’s heavily engineered.

Chimeshead-view

I’ll show photos going from top to bottom, between the two dots on the above image. The valley above the upper dot was filled in by the Ridgemont developers, using material cut from the ridge to its left. This is the view downhill from the position of the upper dot.

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The floor of the valley here is crisscrossed with concrete ditches, which converge about a third of the way down the transect at a culvert. Along the way you pass a large outcrop. All the rock here consists of the Leona “rhyolite.” The outcrops are tempting, but the slope is steep and treacherous.

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Here’s the culvert, possibly the only one in Oakland without graffiti, joined from the right side by the concrete ditch. I think it must carry runoff from the Ridgemont streets. But what’s that ugly orange?

Chimeshead-outlet

Why, it’s acid drainage from one particular part of the subsurface here. I would love to see the geotechnical reports from the time they built this development.

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This is one of many places where I’ve seen “yellowboy” in the Leona; the most notorious is the old sulfur mine, of course, and I noted another last week. By now I think that every excavation in this rock unit, old or new, should be treated as a potential hazard.

The next couple hundred feet downhill from here is a lovely tree-shaded, undisturbed steep cascade over large boulders. Here are just two of them.

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This beautiful rock is extensively slickensided (polished by underground movements) and coated with the iron hydroxide minerals that result from natural weathering. It is not stained orange by the pollution from upstream; in fact the water at this point is only slightly milky. It’s very much like the rocks in the Redwood Road boulder pile.

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And what to say about this one? It’s an unusual lithology within the Leona. My impression just from eyeballing it was that it’s an autobreccia — a ground-up body of lava or tuff consisting of lumps of the original rock in a matrix of pulverized (and relithified) rock. But that’s not the only possibility.

Another outcrop up on the valley wall exposes a slickenside that covers a good square meter.

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At the base of the cascade is more engineered land, a small catchment housing a screened culvert entrance. Chimes Creek is trapped here and conveyed beneath the old quarry and across the freeway to emerge in the Millsmont neighborhood. As I say, it’s engineered land, but it’s planted with trees and rather pleasant. It also catches runaway rocks before they can take out a townhome down below.

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The view up from the lip of the catchment shows the two outcrops and the shape of the land.

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I can’t wait to return for a closer look, though with so many other places to see it might be a while.


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