Archive for June, 2008

Knocker 7

30 June 2008

knocker

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.

The fault at Stonewall

19 June 2008

stonewall road

Stonewall Road is a cute little curving street, cozy at the bottom and overbuilt at the top, just across Claremont Boulevard from the Claremont Resort. The street sits at the break in slope where the Hayward fault passes through, its northernmost appearance in Oakland. The nice new red-painted curbs hide the evidence, as so often happens in this town, but I have a photo from 2001 (the second on this page) showing how it used to look. A stairway named Tanglewood Path runs west from the north end of the street, and it’s disrupted by aseismic creep near its top, although tree roots and landslip do their part as well. This view looks southward pretty straight down the fault, which runs along the hillside above the road and to the left of the hotel.

If you come out Stonewall onto Claremont Boulevard and look for fault evidence, you don’t see it amid the cracking and wear-and-tear that normally accompanies a heavily trafficked asphalt road in a stream valley near steep soil-covered slopes. But this wall, on the north side of the road right where the fault should be, makes me think that it must be some sort of homage to the fault. (click it large)

fault wall

There is some more-organized cracking in the pavement behind the hotel. The fault goes onward along Alvarado Road, ducking briefly into a salient of Berkeley then crossing Tunnel Road straight toward Lake Temescal.

Oakland Paving

16 June 2008

oakland paving company

Oakland’s sidewalks are full of old mason’s marks. They’re like fossils in the city’s hardscape. Since I started paying attention to them a few years back, the oldest have reached a century’s age. There’s a mark on 49th Street that dates from 1906, another at John and Gilbert streets from 1907, and several around town from 1908. Some of the makers operated from addresses that don’t exist any more, such as one from a street number that’s now underneath the Kaiser complex, or from streets that have changed their names. If someone has a blog about them (and why not?), let us know.

Just last week I learned that the operator of the Rockridge Shopping Center quarry was called the Oakland Paving Company. The very next day, I spotted this mark on Claremont Boulevard near The Uplands. Presumably, if we broke this pavement open we’d see gravel made of the quarry’s basalt inside. That is, the quarry produced the aggregate, while the cement came from elsewhere, possibly the giant plant in Davenport, which has produced cement since 1903 (and which I’ll be visiting this weekend). My hypothesis is that the Oakland Paving Co didn’t do much of this labor-intensive retail-type work making sidewalks, which is why its maker’s marks are so rare. But they say the best geologist is the one who’s seen the most rocks, and maybe I just need to see more marks.

Fire on the mountain

13 June 2008

hiller highlands fire

Yesterday there was a fairly small fire in a treacherous place, the Hiller Highlands neighborhood. There was confusion in the media accounts I saw, but here is the correct version, as you can see in this view from across the freeway this morning. The streets, from top to bottom, are Charing Cross Road, Tunnel Road, Caldecott Lane and Route 24. (A typo in the Tribune, “Charring Cross Road,” may give you grim amusement.) The blaze began on Tunnel where street work took place a few months ago, and nearly reached Charing Cross. As I shot this photo, fire crews were still combing the burn area in search of embers.

The hills love fire, and the ecosystem is adapted to it, but civilization here is not. Given that we have irreversibly encroached on the hills by permitting residential construction there, we’re stuck with the price in dollars and lives in perpetuity. Not even the next major earthquake on the Hayward fault, less than a kilometer west, will change this even though the whole neighborhood would likely burn down again, just like 1991, if it happened today.

Yesterday not an hour before the fire started, I was standing on Grizzly Peak Boulevard looking down at this part of town and sensing just how dry everything is. Instead of coming down through here, as I have before, I walked down through the Grandview neighborhood to its north. Upper Grandview is an uncanny place, having been wiped out in the 1991 fire and disneyfied since. Today I was going to visit the fire site, but I got this shot because instead I took the opportunity to try the fire road above Broadway that ends overlooking the North Oakland Regional Sports Center. If you visit the park, have a look at the fire-resistant garden there. The rocks are mapped as undifferentiated Great Valley Sequence and are mostly an undistinguished gray sandstone.

Huckleberry

11 June 2008

huckleberry chert

Huckleberry Botanic Regional Preserve is a little-visited piece of wildland just over the Oakland Hills crest south of Sibley Volcanic Regional Preserve. It owes its existence to this rock, the Claremont Chert. The brochure lavishes attention, and rightly so, on the plants of the botanic preserve, but it’s also a good place to see the chert in many settings. Cliffy here or buried there, shaly or rugged, the chert asserts itself amid the soils and growth like the bass player in a jazz combo.

Owing to its history, the Claremont Chert is high in silica and low in nutrients. It drains quickly and breaks down slowly, and for the purposes of today’s vegetation it slows down the natural process of faunal succession—the series of plants that goes from pioneer species to climax forest. Thus where much of the hills is a uniform oak/madrone woodland, the Huckleberry Preserve is a variegated assemblage of everything from gravelly manzanita balds to soft seeps populated with irises, plus huckleberry thickets of course. Hike the nature trail and meet some of the natives. The self-guiding brochure carefully states the role of fire in maintaining the hill ecosystems, mainly to show how the Huckleberry is an exception. But these days, everywhere you look in the hills is a fire long overdue. Some day we will have to catch up with the Ohlone tribes, who managed these lands with regular burnings.

mount diablo

The trail provides several fine views eastward. You can pretend that white settlement never happened and imagine Mount Diablo pristine, as it was when Cabrillo forced his men through this land 250 years ago. (click for larger version) And you can enjoy Round Top’s symmetry from the rare southern vantage:

round top


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