Archive for July, 2008

Shutter ridges

31 July 2008

A reader was unclear on the concept of shutter ridges, so I thought I’d try to show it as well as tell it. Look closely at this excerpt from the Oakland geologic map covering Lake Temescal.

shutter ridge map

The lake is the blue blob near the top. The Hayward fault slashes through it and across the map from top to bottom. The left (west) side moves north with every major earthquake on the fault. The blue area labeled KJfm (Cretaceous-Jurassic Franciscan mélange) is part of what I refer to as the Piedmont block; it makes up the ridge you see across the lake:

lake temescal

As that ridge moves north, it cuts off the course of Temescal Creek and forces it to flow north to get around it. That’s where the “shutter” term comes from—the ridge barrier moves like the shutter of an old-fashioned box camera. You can see on the map how Temescal Creek flows today, after tens of thousands of years of this process: it comes downhill on the right edge of the map, jogs to its right for almost a mile, goes through Lake Temescal, then turns left around the curve of Route 24 (the double purple line) in a culvert to resume its course to the bay. See the ridge from another perspective in this post.

Another excellent example is on the fault just north of the Oakland Zoo, where Arroyo Viejo comes down Golf Links Road and makes a similar jog around the hill of Toler Heights before resuming its bayward course under 82d Avenue:

Rocks of Upper Rockridge

29 July 2008

During my search for Rockridge Rock, I’ve been finding some fine Franciscan knockers doing their understated bit to add character to the neighborhood. Call this part 1, because I’ll surely find more as I pursue my search. These five rocks are shown in descending order of elevation. The first is at Buena Vista Avenue and Hill Road, a red chert.

buenavistahill

This gray chert is on Bowling Westminster Drive, near the purported Rockridge Rock site on Glenbrook.

bowling drive

This greenschist or blueschist is next to Romany Road.

romany road

This rugged outcrop is in a private yard.

broadway terrace yard

This greenstone boulder is at Cross Road and Broadway Terrace.

cross road

Landvale Road

23 July 2008

landvale road

Civilization moves on, and history gets obliterated, especially in the Route 24 corridor. If you walk up Broadway from the park at 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’s it in the trees) that still serves as a PG&E electrical substation. This driveway with its curious 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.”

Cactus Rock

18 July 2008

cactus rock

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 alluvium

15 July 2008

adams point

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:

adams point

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.

The labyrinths of Sibley

11 July 2008

sibley maze

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

8 July 2008

leona graffiti

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.


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