‘Mount Ararat’, another Rockridge rock
27 August 2008 by AndrewIn my search for Rockridge Rock1912 Oakland street map I rely on in my research labels the Wilding Lane hill “Mt Ararat.” Here’s the view from Wilding and Canyon View Lanes (click full size). If you strip away the trees and homes, the appeal of the place is obvious. I’ve found some nice places. I consider the hillock embraced by Wilding Lane to be one, possibly even the Rock itself. One piece of lore that guides me is the fact that in the late 1800s, the Rock was easily reached from town and picnickers enjoyed an expansive view of the Bay and hills. The
Mount Ararat, unlike Cactus Rock near Acacia Avenue, is right next to the main drag of Broadway Terrace. Here’s a map to help you find the place.
The Crestmont serpentine patch
20 August 2008 by Andrew
Every neighborhood in Oakland with a “mont” in its name has bedrock exposed. Crestmont is a good example. Most of the Crestmont neighborhood, north of Redwood Road and east of Holy Names University, sits on serpentine rock—more precisely, serpentinite. This is the biggest exposure, at the intersection of Crestmont Drive and Westfield Way. Below is an exposure just to the north (click full size), which is somewhat bluer than the blue-green of the main patch. I’ve shown pieces of this rock to local geologists, asking them “Is this serpentinite or blueschist?” They usually say “Beats me” or, even better, “Yes.” These rocks are not easily assessed by just eyeballing them.
The open lot on the east side of Crestmont near Samaria Lane displays some splendid outcrops of this “lizard-skin” stone:
Eucalyptus Road Rock
12 August 2008 by Andrew
While walking the sidewalks of Oakland for my Oakland Sidewalk Stamps blog last week, I made a special trip to Eucalyptus Road, a short loop with each end in Berkeley south of Claremont Avenue. It’s the northernmost place in Oakland on the Piedmont block, better known as our lump of the Novato Quarry terrane. At the heart of the road, edged with eccentric brickwork (and some old sidewalk stamps, natch), is this sprawling property with lush grounds, old eucalyptus trees and a fine rounded knocker reigning over it all. (click full size)
Elsewhere on the street you’ll see a little more bedrock, too. Also, if you have an aversion to Berkeley you can get there via an unmarked, unmapped spur off uppermost Harwood Avenue.
The Novato Quarry terrane
8 August 2008 by Andrew
Here’s a deeper look at the local bedrock. The hill on which Piedmont and its surrounding neighborhoods sit is held up by a body of Franciscan rocks, roughly 100-150 million years old. It’s mostly sandstone, as shown in the Dracena Park quarry pit, and mélange, as shown in the Mountain View Cemetery grounds. But the huge Franciscan Complex consists of a subset of nine different bedrock packages or terranes. Oakland’s Franciscan inventory is part of the Novato Quarry terrane, named for (of course) its exposure in a quarry in Novato. But here in Oakland is its southern end (I’m pretty sure). The terrane stretches from here to Bodega Head in a discontinuous string across the structural grain of the land. This view (click full size), from Dwight Canyon, shows other outposts of the Novato Quarry terrane, with Albany Hill (”el cerrito”) in front, then Point Richmond, and at rear right the rocks of Novato itself and Big Rock Ridge beyond. Another locality is Point San Pablo. To the west of it is the Marin Headlands terrane, which is self-explanatory.
What strung out the rocks of these terranes was movements on the various parts of the San Andreas fault system, which has sliced and spread out the pre-existing geology that was compiled during hundreds of millions of years of straightforward subduction.
Suiseki and gongshi—in Oakland?
4 August 2008 by Andrew
Sunday I visited the Garden Center in Lakeside Park, including the bonsai garden, an attraction not to be missed. This large gongshi stone ushers you toward the entrance. The photo is from 2005 (click full size), because at the moment the stone has a wire around its waist holding up a small tree. It reminded me too much of a leashed animal, and I hope they remove it soon. Gongshi stones generally come from China, where the art form arose many centuries ago. This one is a white, deformed crystalline limestone that weathered and eroded into its arresting shape. Inside the bonsai garden there are several interesting California stones in various places. They are not quite suiseki, merely decorative rocks. This is a suiseki:
It was collected in California and mounted in its wooden daiza by Lance Plaza for the 2006 suiseki show, in the Garden Center. I’ve enjoyed several of these annual events, but for some reason missed this year’s. Had I gone to San Francisco instead, that very afternoon I could have seen the annual show of the San Francisco suiseki society. But Oakland has a goodly share of suiseki artists, including the bloggers Mas Nakajima and Janet Roth.
These art forms are a braintwister for geologists. We tend to dwell on what the stone says rather than what it is—or is that the other way around?
Shutter ridges
31 July 2008 by AndrewA 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.
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:
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:
