The labyrinths of Sibley

Doing urban geology in a place like Oakland adds a new question to the mental checklist that cannot be bypassed: “Is this truly a natural feature?” A boulder may be imported. A terrace may be an old railroad bed. Sibley Volcanic Reserve is a former quarry, therefore it’s safe to assume that this huge pit is not natural and that the labyrinth, one of several in the park, is of even later vintage. But park staff and other visitors have told me that some people insist, against all persuasion, that the labyrinths were made by cosmic visitors.

There is something about human beings, isn’t there? I used to trouble myself over our ability to believe nonsense, but now I realize that banging my head against that wall just hurts my head, and the wall likes it. The fact is, the general run of people love to be amazed. The trouble is, they aren’t particular about what amazes them.

Don’t get me wrong about labyrinths—they are good to experience, they do things to your head, they help pull you out of tedium. That’s cool. I think that crediting them to space aliens is a failure of imagination and a poor reflection on human ingenuity. What amazes me about labyrinths is that we invented them.

But what amazes me more satisfyingly is that people could examine this ground and figure out that it used to be the insides of a small basaltic volcano, now tilted onto its side. It takes imagination first, then the perseverance to test your imagination against the rocks again and again until every question you can think of has been met with a reasonable answer. I haven’t done that at Sibley, but having been to geology school I know how to do it if I set my mind to it. The people who did do that amaze me. They used directed imagination and rigorous skepticism instead of listening for voices and watching for signs, unhuman as that is.

Horseshoe Canyon tramway

Horseshoe Canyon is a small, but dramatic gorge cut into the Oakland hills between Holy Names College, Merritt College and Mills College. The waterway in it is Horseshoe Creek, a branch of Lion (Leona) Creek. Today the canyon is preserved within Leona Heights Park, but in Oakland’s oldest days it was first logged, then mined and quarried. Great stumps remain from the aboriginal San Antonio redwood grove, the mine tailings stain the creek below the old sulfur mine, and the quarry scar sits in the undeveloped scrub at the north end of Merritt College.

This stout concrete structure once braced one of several aerial tramways, whose steel cables carried large buckets of ore and rock from a rail line coming down from the heights to another railhead near the mouth of Horseshoe Creek, in Laundry Farm Canyon. You can visit it by walking down from the Merritt College campus or up from the end of McDonell Avenue near the sulfur mine. This photo was taken in 2003, and the art has undoubtedly been painted over since then with something more contemporary. (click full size)

See more detail about this area’s history in Steven Mix’s History of Laundry Farm Canyon page.

Points south, north and meta

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.

Oakland Paving

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.

The Big-Enough One

The theme of this month’s Accretionary Wedge blog carnival is, “a geological event you consider most significant to you.” I know what that one is. It didn’t awaken my sense of awe and turn me toward science. It didn’t injure me or make me rich or poor. No famous historical figure was involved. But the very month my wife and I moved to Oakland, the Loma Prieta earthquake changed the city irreversibly.

Those first couple weeks of October 1989 were fun. We loved having a proper downtown with fine old buildings, great weather, a lively cultural scene and a city with real geography to it. We had moved our stuff from the house in Concord and were readying it for the next owner, so at 5:04 pm on 17 October we were out of town, cleaning the old house for the last time. It was totally empty. The shaking went on for a long time, but out there it wasn’t very strong. Driving back to Oakland a little later, we felt worse and worse as the news rolled in and the signs of damage appeared.

There was a pall over Oakland for a long time. I felt it for years, not just on the anniversaries but every time I went downtown; every time a new friend or neighbor told their earthquake stories and listened to ours; every time I saw the news from other places and people struck by earthquake. Even now the effects linger in the scars left on the map, buildings left empty and the new Bay Bridge yet unfinished. And while the downtown, the weather, the scene and the land have endured, I now have a deep-seated relationship with earthquakes, Oakland geology and the Hayward fault that gives me a pang every time I feel the little shakers from beneath our side of the bay and think of the Big One to come.

The US Geological Survey has unveiled three new publications on the Hayward fault, all of them well worthwhile. They are a four-page fact sheet on the current hazard, a 96-page field guide with tons of information and photos covering the whole length of the fault, and a Google-Earth virtual tour for deep background and visualization. Get learning!

Oakland Earth Art

In front of a building on Jean Street near Grand Avenue is an installation of stone-based artworks by Richard Botto. In my essay on Earth art over on my About.com site, I distinguish several varieties. This kind ranks low on the scale of sublimity or artistic rigor, but it is a lot of fun to look at and quite a commitment to beautification of the public space. And, of course, it gets people to see rocks.