Archive for February, 2008

Basalt in Claremont Canyon

23 February 2008

claremontbasalt.jpgThe other week I took a hike up Claremont Canyon, one of those parts of Oakland that everyone thinks is in Berkeley. Walking there is slightly dicey because the road is narrow and cars don’t expect pedestrians, but I plan to do it again. The bedrock shifts from nondescript sandstones of the Great Valley Formation to vertical stripes of Claremont Chert, to coarse conglomerate of the Orinda Formation, then basalt of the Moraga Formation near the top. That’s what this is. According to the map, this must be float—loose stones carried downhill in the soil—rather than actual bedrock. The fresh rock is black and the weathered rock is brown. This is the stuff exposed throughout Sibley Volcanic Regional Preserve, but you can’t collect it there. You probably can’t collect this either, because Claremont Canyon is also a reserve, but nobody cares about roadcuts. Still, I try to observe these things because I talk to the public and really need to set the right example. You can collect it in the fantastic exposures along Route 24.

As you continue south on Grizzly Peak Boulevard from the top of Claremont Canyon road, you can look back and follow the basalt along the hillside, over the crest, across Route 24 and up toward Round Top. This is a hard region to map, but the Moraga basalt is easy.

The dangerous hills

15 February 2008


The people who build in the hills are wealthy and determined. Wealth is a good thing, and it’s usually the reward for determination and risk-taking. The benevolent-looking slopes of the Oakland Hills are held so high in the air by tectonic compression and carved to their angle of repose by all forms of erosion. The long-term compression across the Hayward fault, in fact, puts the hillslopes in a chronically oversteepened state, with highly fractured bedrock and a continuous risk of slope failure.

Homebuilding in this hostile setting pits these wealthy, determined, risk-taking people in an arousing contest against geology. It’s also a contest against engineers, builders, planners, insurers and other taxpayers. Those people share that risk to various degrees. This morning’s paper reported that a guy hauling lumber to a construction site in the hills was crushed to death by his load. He couldn’t find a level spot to offload his truck, and the wood slipped onto him as he untied it.

That reminds me: Last week I passed the landslide I mentioned earlier. Another sizable chunk of it had fallen in the scarp’s implacable retreat. Farther along, Skyline Boulevard was cut off by a fresh slide, and another one had narrowed upper Tunnel Road to a single lane. That’s the infrastructure for these highly demanding neighborhoods.


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