Every now and then, a stone really stands out. This one isn’t from anywhere near Oakland, but somehow it wound up in a low wall in the front yard of a house on Pleasant Valley Court. My fellow geobloggers and geotwitterers think it must be tourmaline crystals. Click it for a larger version.
“Exotic blocks” is a phrase used in the research community for bodies of land that seem to have traveled from elsewhere to their present location.
As far as I know, the late Miocene volcano preserved at Sibley Volcanic Reserve doesn’t have a name, so no one will mind if I dub it Sibley Volcano. This is what it’s mostly made of, a dark basaltic lava as exposed in the rock face behind the water tank.
The bullseye pattern is the result of spheroidal weathering. A thick body of fresh, solid rock generally is quickly cracked by sets of joints (a joint is fault that has not yet moved, or a fault is a joint with displacement on it). Joints let groundwater, among other things, into the rock. As the groundwater does its thing on the rock, it works its way into the jointed blocks and, to paraphrase an old Grateful Deadhead saying, the smaller they grow the rounder they get. Spheroidal weathering.
The wonderful rounded granite forms of Joshua Tree National Park result from the same process. They were gradually exposed by erosion and, being coarse-grained granite, they don’t display a dramatic onion-skin structure. This was abruptly exposed by quarrying. It’s a miniature version of the exfoliation domes in the Sierra Nevada granites.
Shephard Creek flows down Shephard Canyon, on the south edge of Montclair. At least that’s what the U.S. Geological Survey says on the Oakland topographic map. The city calls it Shepherd Canyon and Creek, and the road running up the canyon near the old railroad grade is called Shepherd Canyon Road. Anyway, Shephard Creek is culverted for most of its length, and only a little of it runs free at its bottom end, near its junction with Palo Seco Creek to form Sausal Creek.
When I was scouting out the Oakland Conglomerate, I went down to the creek bed behind the firehouse. It’s full of nondescript boulders of shale and sandstone like those in the foreground. But a tiny bit of bedrock crops out. The geologic map shows this area as the Joaquin Miller Formation, a shale/sandstone unit just beneath the Oakland Conglomerate that is of Late Cretaceous (Cenomanian, about 94 Ma) age.
Next time I’ll make sure to have my Brunton compass—that’s a heavy-duty compass and clinometer that geologists use to observe the orientation of rock beds, their strike (azimuth) and dip (inclination). The Oakland geologic map doesn’t show a strike-and-dip symbol here, so maybe I can add a data point to the big picture. Geologists prefer to make many observations at a locality and average them before drawing that symbol, though. Maybe this single small outcrop, rare though it is, isn’t significant enough. A little farther downstream, the boulders disappear and the streambank is serpentinite. The big patterns on a geologic map may cover up a great deal of variation.
Here are some of the samples I collected from the Oakland Conglomerate last month.
This rock is utterly and thoroughly brown, deeply brown with iron oxides. You can’t scrub it off—it’s brown all the way down. That reflects two facts: the matrix is iron-rich volcanic material and it’s been pervasively shattered and flushed with fluids under oxidizing conditions to let the iron out. The clasts—the bigger stones in the conglomerate—are beautifully rounded, presumably in a vigorous river or beach. But the setting of Oakland at this time, in the Cretaceous, was way offshore in a shallow sea. It appears that all this coarse material was carried offshore in underwater landslides and laid down in a deep series of sandstones and conglomerate beds.
Long after the conglomerate was laid down and lithified, the plate boundaries changed and the San Andreas fault system (including the Hayward fault) splintered coastal California and wrenched it northward, one earthquake at a time, for tens of millions of years. The forces of that time have crunched nearly every stone in the Oakland Conglomerate. Yet at the time these rocks were still deeply buried, and chemical action and pressure cemented the stones back together. The clasts are delicate, but intact. It’s impressive to imagine the force that shattered these hard stones like so many soda crackers. The next thing I want to do is open some of them with my hammer and see what the original lithologies are.
For several months, the quarry pond at the Rockridge Shopping Center at Broadway and Pleasant Valley was mostly drained while a crew built a new drainage system. The water grew very clear over the summer, but I never managed to get back to the cliff overlooking it to see what was visible. Too bad—the work is done and the pond is full again, its bottom probably never to be seen in my lifetime. This habitat must be pretty sterile at the moment, but the geese sound happy to be there. The creek has been noisy, too.
At the 40th Street Cut exhibit, I was speaking with one of the artists, either Keith Evans or Dylan Bolles, and I pointed to these stones and blurted a dumb geologist question that was totally irrelevant to the artwork: “Those aren’t local rocks. Where are they from?”
They are a fairly high-grade schist, something you won’t see anywhere closer than the Sierra foothills, and maybe not even there. But that isn’t important to the art, dammit. Obviously the rocks were picked for their shape and color, to arrange in the symbolic pad under the rising and falling key evoking the Key System and contrasting the mute ground of all life with the mechanical motions of civilization and the metal rails whose relation to the rugged road metal beneath them recapitulates the pounding of hammer on rock that epitomizes Man’s Attitude To Earth——
But Keith (or Dylan) lit up instead. “I got those rocks in Maine,” he said. “They’re musical rocks. You strike them and they ring, like xylophone keys.” So these specific stones allude to the sound dimension of this work, and now you know about it too. You don’t particularly notice at first, but there are soundmaking elements that fill the room with humming tones rather like the lost electric whine of the old streetcar motors.
These guys are on their game.
