Civilization moves on, and history gets obliterated, especially in the Route 24 corridor. If you walk up Broadway from Lake Temescal, first you pass the back entrance with its spikes, then you see a bit of wasteland and a broken viaduct. Just beyond is a precious Deco structure built in 1922 that still serves as a PG&E electrical substation. This driveway with its curious chalk inscription leads onto the viaduct, which turns out to be the last remnant of Landvale Road. A 1955 photo at the Oakland Museum shows it intact. The structure looks just like the Golden Gate Avenue undercrossing on Broadway, which is dated 1934. No road is shown there in a 1912 street map, so until someone comes up with more information, those are the constraints we have on its lifespan. This reminds me of certain problems in geology: a feature was absent at date 1, present at date 2 and gone by date 3. It is like talking about some poorly attested ancient Greek thinker: “Landvale Road, fl. 1930s-1950s.”

Walking along Acacia Avenue, you may feast your eyes on the homes and grounds of the core street of the Upper Rockridge neighborhood, but one exception stands out at 6240 Acacia: this rock peeking over the scene. (click it for full size) This appears to be Cactus Rock, attested to in old postcards about the ongoing development of this streetcar suburb (still served by the improbable bus route 59A). I can’t get a good look at it, but it appears to be a standard Franciscan knocker that is much smaller than the mysterious Rockridge Rock. To judge from the prospect at Alpine Terrace, the next street uphill from here, the views from the rock are fantastic.

Adams Point isn’t really named Adams Point—the name refers to the neighborhood overlooking Lake Merritt from the north. (And it’s named for Edson Adams, who once owned it all.) But I like to think of the low peninsula of Lakeside Park, between the lake’s two arms, as being Adams Point proper. One day in early 2003 I strolled along the water at its base, looking for outcrops. It appears to be the only spot on the entire Oakland shoreline that is nearly in its original condition.

The rocks in this view are not original; they’re pieces of the old wall that lines the rest of the lake. And the large boulder is a decorative one that fell or was pushed down here from the park lawn up above. What truly belongs here is the sand and gravel, which makes up the lake shore and the hills around it, Haddon Hill on the east and Adams Point hill on the north. Both are part of a large alluvial fan of Pleistocene age.

Under the roots of a tree, I found the original sediments exposed:

This being a city park, I left the material untouched, but the pebbles of local red chert and bluish basalt are unmistakable. These particles eroded from the hills and were carried here by vigorous streamflow, which also rounded their sharp corners somewhat. Outside of building excavations, exposures of this material are rare in Oakland.

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 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.

The Rockridge neighborhood has migrated over the years. Today Rockridge centers around the Rockridge BART station, in the valley of Temescal Creek. But a hundred years ago, Rock Ridge referred to the highlands between that valley and the one to the southeast, through which the Rockridge branch of Temescal Creek flows (in the Claremont Country Club golf course and upstream along Broadway Terrace). The ridge was supposedly named by the Livermores, who put their mansion on it amid their 600 acres of land making up today’s Upper Rockridge neighborhood.

There was a famous rock somewhere up there that became a popular picnic site in the late 1800s. Photos show it as being blocky, massive stone, maybe 10 meters in height and greater in width, big enough for dozens of people to stand on for their portrait. Lately I’ve been trying to find it. Old maps don’t show it, but Jonathan Chester’s excellent book Berkeley Rocks has some information. He claims that the developers of the area named the rock itself Rock Ridge and encouraged the public to visit it. Today I found an article from the Call of March 12, 1910 about the area:

“Hundreds of people went out to Rock Ridge park, during the week, to enjoy the inspiration found in the budding trees and flowers awakened by the first touch of spring. The visitors found that the opening of streets, avenues, walks and drives through the famous old Rock Ridge picnic grounds has facilitated access to the points of interest. Those who have known best and enjoyed most the peculiar charm of the place were delighted to find that the plans of the owners of the Rock Ridge property in laying out the tract with wide streets and avenues and ample lots have preserved the distinguishing features unimpaired. The army of admirers of Rock Ridge property has received new recruits every day. A number of sales of lots, were made during the week to people who will build immediately fine homes on their grounds. The street work is being rushed. The planting of flowers and shrubs along the cement curb line is also receiving attention from a force of [men].”

The story suggests that while the “famous old” picnic grounds were wiped out by subdivision, “the plans of the owners . . . preserved the distinguishing features.” From this evidence and that cited by Chester, the rock was widely known and hard to miss. Therefore I can’t understand why Chester thinks that the site of the rock is on Glenbrook Drive at the corner of Bowling Drive:

It’s a beautiful outcrop, nicely integrated into the two properties that own it, but no way is that the Rock of Rockridge. It’s the wrong size, the wrong shape and the wrong rock type. And the Call story at least hints that the Rock was “unimpaired” as of 1910.

I think that Chester may not have looked hard enough. I have found at least two other candidates for the Rock, but there is still some ground to cover and many places for rocks, even huge ones, to hide.

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.

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.

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.

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)

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.