This is a block of Swainland Road, at the high end of the Glen Highlands neighborhood hard under the powerline ridge just south of Route 24. The bedrock is mapped as Great Valley Sequence sandstone. It’s a lot harder and stronger than much of the rock in the hills, but it still doesn’t like being exposed and oversteepened. It’s steadily encroaching on the sidewalk, trying to reach a more comfortable angle. Vegetation would probably help.
I mentioned a few posts back that I would be doing a field trip related to the new bore of the Caldecott Tunnel. Here’s how it went.
We (the members of the Northern California Geological Society) started with a talk at the Caltrans headquarters in Lafayette. They showed us historical photos and engineering diagrams, explained the New Austrian boring technique, and ascertained that we weren’t interested in a “photo opp” near the tunnel entrance. Apparently that last is a prime demand of many tour groups, but we didn’t care.
First stop was at the old tunnel entrance, which I’ve shown here before. The old plaque has been refurbished and looks garish now.
The tour leader unfurled a huge geologic map of the tunnel and talked a lot about all the rocks (click the image to see the 1000-pixel version). I studied it closely, because it’s the best we have for this part of town.
The blue stripes on the west side of the map are the Sobrante Formation, the green, purple and tan stripes are the Clarement Shale, and the buff stripe on the right is the Orinda Formation. These are stacked up in order of age, old on the east, and tilted nearly straight upright.
Next we wound our way around to the extreme top of Broadway, next to the tunnel’s west entrance. Here’s the view from there looking north.
The rocks in the immediate area are in the putative Sobrante Formation, but the Claremont formation takes over just to the east (right). Here’s the Sobrante exposed at that spot.
It’s pretty crummy rock, and the tunneling was quite slow there. Every couple of meters, the tunnelers drilled a fan of holes over the roof of the upcoming dig segment, each one reinforced and grouted. That reinforcement allowed the tunnel roof to stay up long enough to spray it with shotcrete, which in turn held up the roof until the real concrete tunnel lining could be emplaced. The tunnel’s final shaft is round, as strong as an eggshell in its resistance to the earth’s weight. (To see what I mean, try gripping a raw egg firmly in one hand, fingers on all sides, and attempt to crush it. You can’t do it.)
Next we visited the type locality of the Claremont Shale, where I learned something new about it.
The unit here is mostly thin beds of chert and thinner interbeds of shale, but there are also lumpy beds in it—see one running down the center and another one near the right edge. Those are dolomite. It turns out that carbonate and silicate minerals dislike each other enough that they tend to segregate themselves: in chalks, you get flint lumps, and in cherts (as here) you get limerock lumps.
Our last stop was on Fish Ranch Road just east of the tunnel, where we looked at the Orinda Formation in the surrounding hillsides. The Caltrans people brought along a box of the fossils recovered from the fourth-bore project, so I got to lay my hands on this Miocene horse tooth . . .
. . . and this slab of fossil leaves.
The Caltrans public-information official was at obvious pains to say how happy the paleontology contractors were and how well the fossil collecting had gone. Obvious to me, anyway. I won’t forget the complaints I heard from the other party, but I will keep in mind that most stories have at least two sides, and that every fossil preserved is a victory over the universal decay of the world.
The Hayward fault is mapped as crossing Broadway Terrace here, just west of the route 13 overcrossing.
The pavement is a wreck and has been repeatedly patched. You may see a dip in the concrete berm in the road’s centerline, too.
On the other side is the old concrete supports for the railroad line that used to go through here.
I’ll be taking a tour of the new Caldecott Tunnel bore on Friday. This is the view of Route 24 from the ridge south of it, across from Hiller Highlands. Everything here is east of the Hayward fault.
On the right is uppermost Broadway, where a line of locals and advanced commuters chronically hope for a few seconds’ advantage by merging at the last second before the tunnel. I’ve decided that it’s faster to get in the left lane of 24 as soon as possible and stay there. Anyway, all the land from here to the far curve is underlain by rocks of the Great Valley Complex, of Late Cretaceous age. A fault separates it from the Sobrante Formation behind it, which is much younger. The lower part of the skyline ridge is Sobrante, but the high part is chert of the Claremont Shale. The tunnel penetrates both of those units, and I hope for a good look at it.
I was standing by the power line; you get to it by hiking down the road from the sports complex, on Broadway at the overcrossing, or by ducking around the gate at the top of Pali Court. The Great Valley here is a mix of fine-grained sedimentary rocks. Exposures are poor and the fabric is disrupted. Here’s an exposure on Pali Court.
And here’s a closeup from nearby.
It’s very shaly. The lenses of more siliceous stuff don’t add to its strength.
October 21, not October 17, is Oakland’s real Earthquake Day. While many of us remember the 17th vividly, the day of the Loma Prieta quake in 1989, the morning of October 21, 1868 was when the original Great San Francisco Earthquake struck. It was on this side of the bay, on our own earthquake fault, and the ground cracked from Fremont all the way to the edge of this view from the end of Pali Court, in the neighborhood just across route 24 from Lake Temescal.
The Hayward fault runs from the notch on the skyline, which is in Montclair, to the right edge behind the house in the middle distance. This part of the fault, between Mills College and Lake Temescal, is the only place that has large areas of bedrock on both sides. What that means for the purposes of today’s post is that it’s the most rugged and picturesque part of the fault and has probably the greatest concentration of million-dollar homes.
Today I invite you to review this blog’s category “the hayward fault.” I seem to have rattled on at great length on this subject. Some day the fault will rattle all of us at great length.
Today’s Oakland Tribune has an article about newly mapped faults in the Hayward fault zone; I won’t link to it because the Trib’s links die quickly. But the new Alquist-Priolo zone map can be accessed here.
I’ve been exploring the surprisingly intricate topography between Lakeshore Avenue and Park Boulevard, where I count four separate ridges separated by three valleys. From the ground, it’s a challenge to visualize and photograph. This is the view across Mandana valley from the side of Calmar ridge, on Santa Ray Avenue, to Longridge.
I haven’t forgotten the blog, I’m just real busy. The weather lately has been superb for walking.
On this day in 2007, I launched Oakland Geology with a photo of what happens when Oaklanders build without geological awareness. Since then I’ve made plenty of posts along the same lines, but geology is not a one-note subject. Most of the time I’ve celebrated what intrigues and tantalizes me about this remarkable city’s natural underpinnings, always with a photo to share.
In 2010 Oakland Geology was named Best Blog about East Bay Rocks. (I really should get myself over there and pick up my plaque.) This week I learned that one of our celebrated local authors, Michael Chabon, cited Oakland Geology in the acknowledgments of his newest book Telegraph Avenue. These are signs of what I always hoped to achieve with the blog: to extend popular awareness of this city’s place in deep time and its deep present, to include our rocks and soils and landforms and geologic forces in the everyday conversation that is constantly weaving our future.
This blog, more than all my other writing, has brought me face to face with interested Oaklanders, who appear to be roused enough by these snippets and snaps to come see me wave my arms, with Powerpoints or vistas behind me, and dump more data on their heads in person. I appreciate your audience and your readership. Long may we continue to wave.
