Archive for the ‘Oakland rocks’ Category

Oakland building stones: Serpentinite

26 June 2017

In a modest West Oakland neighborhood on Market Street is the modest West Grand Shopping Center. Its ordinary building is clad in rough stone, an exterior treatment similar to the Kaiser Building and many other examples.

But at the West Grand Shopping Center, the cladding consists of fist-sized pieces of beautiful serpentine rock.

The front side of the building is pristine. The rear side, on Myrtle Street, is a full block long and completely faced with serpentinite. Unfortunately the bottom seven feet or so has been painted over.

The mutable color of this stone, blue-green in the shade and olive-green in the sun, gives the building a real Oakland look. I don’t know where the stone came from. Our own serpentinite is usually bluish and not of this quality, except maybe in small outcrops in the Franciscan melange. Perhaps it’s from a quarry in the Mother Lode country. It must have taken a few carloads of rock and a crew of skilled artisans to put this together.

A few months back, when I was presenting the building stone verd antique, serpentinite’s dressed-up cousin, I said “You can’t do much with California serpentine except admire it.” Makes me happy to be proved partly wrong — you can always admire it, and sometimes build with it.

The Hayward fault at Warm Springs

17 April 2017

Every extension of BART opens up a new region accessible to geologizers using public transit. So the other week I paid a visit to the far end of the Hayward fault, less than a mile from the new Warm Springs station in south Fremont. The station has nice views of the San Mateo Peninsula mountains to the west and Mission Peak to the east.

It appears, too, that the Irvington Gravels site to the north is accessible for determined walkers who bring provisions — that is, hikers.

To get to the fault just walk east on South Grimmer Boulevard toward the place marked “Weibel” on Google Maps.

Here’s the same area in Jim Lienkaemper’s detailed 1992 map of the fault. The map has a key to all the annotations. Note that both images are tilted to make the fault vertical; north is at about 1:30.

The fault runs through the “D” in “Blvd.”

Look back at the Google Maps image. See the line of green along the fault trace? That’s because of the 1972 Alquist-Priolo Act, which forbids new construction within 50 feet of an active fault. The area in the middle must have been built up before the act took effect. That’s where I went.

South Grimmer reveals the offset from fault creep well. This view is looking east toward Mission Peak. On the fault map, the locality (just below the horizontal dashed line) is circled and labeled “C1,rc,rf” signifying “strongly pronounced” evidence of creep in the form of right-offset curbs and a right-offset fence line.

And this is the other side of the road, looking west. Notice that the sidewalk is offset as well as the curb.

There’s another, much smaller offset higher up the slope that I didn’t get a good picture of. Repeated measurements show that together these offsets add up to about 6 millimeters per year. The slope itself is a sign of the fault, too.

To the north across little Arroyo Agua Caliente Park on Gardenia Way, this nice set of echelon cracks marks the fault trace. That’s what the “ec” in the circle labeled “C1,ec,rc,cc” stands for.

The fault nips the corner of Gardenia and Ivy Way, bending this curb (the “rc” in the label).

The city or the homeowner copes with the sidewalk by patching it as needed. You’ll see stuff like this everywhere on the Hayward fault.

Walking north through the park to Parkmeadow Drive on its north edge, you can look west down the street and see both an offset curb and the change in slope that marks the fault.

You can do this yourself all along the fault. The map has all the evidence (and the USGS has an updated version as of 2008).

A week later I hosted two French journalists — a writer and a photographer — for an afternoon, showing them fault offset features like these up in Hayward and Oakland. The writer went and spoke to a resident whose home was on the fault, and his fatalistic response took her aback a bit. She said “we don’t have attitudes like this in France.” I told her we Californians have been this way since the Gold Rush.

East Bay diatomite

3 April 2017

The geologic map I rely on for this blog — U.S. Geological Survey map MF-2342 — extends north to Pinole, where it shows this little pod of rocks labeled “Tsa” and “Tdi” between Pinole, El Sobrante and Richmond.

Both units are of early Miocene age: Tsa stands for sandstone and Tdi stands for diatomite. The T stands for Tertiary, the catch-all term for Cenozoic rocks older than Quaternary, which — OK, you don’t need the whole lecture just now. The point is, I had to go see this diatomite because I didn’t know it existed in the East Bay. I’ve seen it in the Central Valley, but never around here.

Going north on I-80 you take the Appian Way exit right and immediately turn left on Sarah Drive. Down at the bottom of a valley is Pinole’s little, undeveloped Sarah Drive Park.

On the way to the hilltop, you start seeing this odd rock in the road. Pick up a piece and you’ll find it’s very light. That’s the diatomite.

The trail becomes very steep, exposing the bedrock. The hilltop affords nice views. I was especially taken with the view north.

And the view east looks up Pinole Valley toward Mount Diablo on the horizon. If you’re riding toward Sacramento on the Capitol Corridor Amtrak train, there’s a moment just east of Point Pinole where you can catch this same view of the mountain.

And there were butterflies.

So that’s all great. But here’s what’s cool about the diatomite.

Diatomite is composed of diatoms, the microscopic algae that make shells of silica. As an industrial commodity it’s also called diatomaceous earth, or DE, or kieselguhr if you’re feeling smart. As the stabilizing agent for nitroglycerin in dynamite, it made Alfred Nobel’s fortune, and that’s why we have the Nobel Prize.

As its silica content slowly turns into the crystalline mineral quartz, diatomite becomes the rock called chert. As it happens, the Pinole diatomite is about the same age as the chert in Oakland’s Claremont Shale. By some tectonic accident, it avoided being converted, and you can enjoy its lightweight charm without a trip to Los Banos.