Basalt in Claremont Canyon

The 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

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.

The fault at Roble Road

From College Avenue you can take Chabot Road east, parallel to Route 24 toward the Hayward fault. It passes some exquisite and interesting houses, also the East Bay Municipal Utility District’s main water line, which cuts through the Oakland Hills in a tunnel that has recently been reinforced against fault movement. You also pass right underneath the BART line, on its way to Orinda. But when you get quite close to the fault, Roble Road exits to the north, a one-lane road with a few empty lots from the 1991 fire and a creek running alongside it. Just over the Berkeley line you start to see boulders. This one is at 57 Roble. It’s most likely a large specimen of Leona Rhyolite, but I’m not yet familiar enough with the two rhyolites, Leona and Northbrae, to tell them apart.

Roble Road debouches onto Tunnel Road very near the fault. If you head uphill on Tunnel, you cross the fault at the point in this photo. The house in the woods is at the corner of Vicente and Tunnel roads; its entrance has a stone block engraved “The Rocks.” Berkeley Rocks says that the house is perched on outcrops of the Leona Rhyolite. The wall in front divides the two opposite lanes of Tunnel Road. The chip on the right side, the only one on the whole wall, is probably due to movement on the Hayward fault.

The 9th Avenue palms

This row of palms has a history, but I don’t know it yet. The Oakland Heritage Alliance has a guided walk through this area, which was part of the Francis “Borax” Smith estate. Smith made his fortune developing the borax industry in the playas of Nevada and the Mojave, and nothing spoke success in turn-of-the-century California like an allee of palms. (Click it for an 800×500 version)

The ground here is a large, dissected alluvial fan that spans much of central Oakland. Pill Hill marks its west end, the rise of Evergreen Cemetery the east end. Foothill Boulevard runs along its base. The soil is a firm silt of excellent quality for building or growing. Many of the stream valleys that dissect it are now major streets: Park Boulevard, 14th and 23d Avenues, Fruitvale Avenue, High Street, Seminary Avenue.

These palms are widely visible around Oakland, if you know where to look. Yesterday I spotted them from the Kaiser Hospital parking structure; they’re also easily seen from upper Mountain View Cemetery and the Chetwood Street bridge over I-580. Other sighting reports are welcome.