Archive for June, 2008

Knocker 7

30 June 2008


This is the last knocker I’ve documented in Mountain View Cemetery. It’s by the second road down from the one leading to the top, on the uphill side. It looks like the gray sandstone that makes up much of the Franciscan Complex.

The bulk of the upper cemetery is underlain by Franciscan mélange, a mix of rock types in a weak, shaly matrix. You never see the matrix except temporarily in roadcuts; it quickly crumbles into soil and becomes covered with vegetation. The chunks of other rock types emerge from this soil, and that’s what knockers are.

There are other exposures of bedrock along some of the cemetery roads, but I don’t count them as knockers. And there are several knockers in the off-limits land at the very top of the cemetery. Maybe I’ll document some of them next. And I really should do one more concerted search to see if I’ve missed any.

Siesta Valley

26 June 2008

siesta valley

Last week I took a nine-mile ramble up Claremont Canyon, then along the East Bay Skyline Trail as far as Lomas Cantadas, then down to Orinda and the BART station. The trail goes across the head of Siesta Valley, an interesting geologic feature and a wonderful view (click full size). Route 24 cuts across the valley right in front of the construction, which will be Orinda’s newest neighborhood some day.

This valley is not a streamcut valley, but rather is formed by the folding of the rocks beneath it in a syncline. That’s a shape with a trough in the middle and upturned sides. (The opposite is an anticline, a ridge with downturned sides.) The notch that route 24 goes through is cut by a stream. My guess is that it’s a water gap, cut by the stream at the same time as the rocks were being folded. The rocks of the Siesta Valley are sandstones and mudstones belonging to the Siesta Formation, the next youngest set of rocks after the basalt flows of the Moraga Formation. Speaking of which, I also took this shot of Round Top from the north, with the old basalt quarry grounds in front of it.

round top

Points south, north and meta

22 June 2008

hayward fault

The Hayward fault is not hard to see if you have practice seeing it and if you have a good map that you’ve studied well. But even so, in Oakland there are not many spots like this, where the evidence is unmistakable. This set of echelon cracks is in the Sheffield Village neighborhood on Revere Avenue, just above its intersection with Marlow Drive. Where my previous post showed Oakland’s northernmost point on the fault, this is the southernmost spot in Oakland where the fault is clear. A little farther along is Chabot Park, a corner of Oakland so remote that you have to get the triple-A map of San Leandro to see it and drive through San Leandro to reach it. But there the fault is apparent only as a break in slope.

Anyway, Oakland is as plain as a textbook compared to Berkeley. There the fault runs through rugged land covered with rocks, woods and homes. Near the University its location is well known and evidence is good, but to the north it wanders a bit and has vaguer signs. Keep that in mind when you visit the Walking the Fault blog, an occasional project by Berkeleyan Andy Datlen. Relying on the new USGS “helicopter tour”, he is quick to identify specific homes and other features as straddling the fault or otherwise direly threatened. I don’t blame him. I think that any citizen using the USGS tool is likely to reach the same conclusions. But I don’t, and I don’t point out specific homes as threatened, for several reasons.

First, I take a scientist’s more cautious approach to the maps. The red line is an inference, a hypothesis except in the specific points where trenches, measured offsets and cracks point precisely to a fault. I use the 1992 paper version of the online map, on which every piece of evidence is given specific degrees of certainty and quality. Scientists are in love with uncertainty as well as precision; where evidence presents a blurred picture they avoid oversharpening their vision, and so do I.

Second, the fault is not obliged to rupture exactly where it did the last time. Yes, deep underground it is safe to say that the fault is a clean surface, but our best evidence is that strike-slip faults like the Hayward are a tangle of cracks, a skein of fractures. If you were to cut across the fault and pull the cut apart to see a cross section, those fractures would gradually coalesce at depth. Looked at the other way, the deep fault flowers upward from a single crack into a fan of them, among which only one or two is currently active. This “flower structure” is seen commonly in geophysical studies of the San Andreas system, of which the Hayward fault is a part.

Third, whether someone’s home is in danger is not my place to say. Only a licensed professional geologist or geotechnical engineer can determine that responsibly.

Fourth, I hold that people should expect a degree of privacy, and identifying their homes on a website is not something I want to do.

(PS: This post has become a spam magnet, so commenting is turned off.)


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