Archive for the ‘oakland rocks’ Category

Elverton exposures

25 September 2013

Elverton Drive is a very distinctive place in the high hills, not so much for its houses—though no insult meant to their owners—as for its bedrock. From end to end, it offers the best exposures anywhere of the Claremont chert.

elverton1

If it weren’t for the parking situation, this would be a great spot for a class exercise in field mapping. The strata are clear, the winding road offers a range of orientations to refine measurements, and the rock isn’t totally uncomplicated. Take this spot.

elverton2

What is the nature of the change between neat rows and rumpled layers? What can the student conclude from the evidence, and what should the student look for elsewhere to test those conclusions? I don’t know; I’m just asking and I didn’t inspect this closely. Besides, it might be on someone’s homework.

A few years ago, Elverton was blocked by a landslide. Residents could get in and out from either end, so it wasn’t that bad, but I stayed away until a few weeks ago. I think that this spot must be where it was. (If it’s not obvious, this is sculpted concrete.)

elvertonslide

Near the road’s east end is an old excavation, perhaps a small quarry, where you could examine these rocks at leisure and collect a specimen. But do notice the presence of fallen blocks, and if you feel an earthquake while you’re there, step the hell back.

East of Elverton, the chert crosses the ridgeline into the Huckleberry Preserve and trails into the back hills.

Lakeside Park terrace

1 September 2013

Lakeside Park is one of the most parklike parks I know. Perhaps I feel this way because I imprinted on it at kindergarten age.

Click for larger photo

Youthful feelings aside, I think that geology makes the park this way: it’s set on the late Pleistocene marine terrace, planed and beveled by the sea waves during an interglacial highstand approximately 125,000 years ago. The planar setting, studded with trees to the limits of vision, suggests a vision of infinity, or at least limitlessness.

This spot is in front of Children’s Fairyland, where young children then and now can experience mind-blowing things everywhere they look. Show them these rocks, too: they’re ochre-stone and chert from our own hills.

The route 24 cut, south side

11 August 2013

A few weeks ago I managed to get a very nice view of the roadcut where the end of Gudde Ridge was trimmed to make room for route 24, east of the Caldecott Tunnel.

24southcut

That’s the lava flows of the Moraga Formation on the left. On the right edge is the coarse conglomerate of the underlying Orinda Formation. Click the photo for a 1000-pixel version. (The cut on the north side of 24 is shown here.)

This is not a view straight on; the ridge runs toward the lower right, so the steeply tilted rocks look vertical from this angle. It can often be hard to figure out the exact orientation of rocks, even in a splendid exposure like this.

Middleton hill

26 July 2013

Way down in the Sheffield Village neighborhood is a big hill and a little hill, but most of the place is in a flat little basin. The geologic map shows it as a patch of old alluvial sediment of the same vintage as the Fan:

sheffieldmap

The big hill is on the east side, across the Hayward fault (the black line from the lower right corner). The little hill is on the west side, butting against I-580. The road on its west side is Middleton Street, so I give the hill that name. I didn’t realize it when I poked around there earlier this month, but the hill is mapped as San Leandro Gabbro, of Jurassic age. The rock doesn’t show itself very much, but here’s a small exposure. Next time I’ll bring a hammer.

middletonhill

The highlight of the hill is the little private park inside the ring of houses there, just a microscopic piece of the original oak meadow (although this is actually a cork oak).

middletonpark

From there you get a nice view of the big hill. The fault runs along the foot of the hills, behind the homes in the center and in front of the lower set of homes on the right.

sheffield450

Click the photo for a big version.

Oakland jasper

15 July 2013

My place lies at the edge of the Fan, downhill from the Piedmont block, and the soil keeps producing these gorgeous pieces of lightly polished high-grade chert. I decided that this one should sit for a formal portrait. Click the image for a larger version.

chertportrait450

It’s really more of a jasper than a chert. Jasper is a glassier, more refined chert that gets its appearance from the redistribution of silica fluids. This one mixes the red of oxidized iron with the green of reduced iron—at least that is my inexpert explanation—reflecting the invasion of oxidizing fluids at some point during its rough-and-tumble history as part of the Franciscan complex.

I believe that upstream, in Moraga Canyon, there were chert quarries in the earliest days, but I need to check the records to be sure one way or the other. This is not a quarried stone, though, because it has the burnished surface of an object that has been carried a short distance by streams and natural mass movements. I can’t look at it enough.

Pinehaven canyon

4 July 2013

The headwaters of Temescal Creek lie east of route 13 in a steep canyon that has no name on the USGS topo map, so I will feel free to name it Thornhill canyon. The canyon splits at the site of Thornhill Nursery, with Pinehaven Road heading left up its own canyon and Thornhill heading right.

pinehavencynterrain

Pinehaven canyon is heavily wooded with a lot of eucalyptus and is almost entirely underlain by the crumbly Sobrante Formation. It’s a beautiful place, with a nice running stream that helps keep Lake Temescal full.

pinehavencrop

Whenever I visit the high hills I can’t help but think of its hazards, so different from those down below. The risks of landslide and fire, even in the absence of earthquakes, are compounded by the narrow, winding roads as we all know from the 1991 hills fire. Pinehaven canyon has not burned since it was settled, although the 1937 fire came close. Its firefighters are served by a couple of large water tanks, the Swainland tank at the top of Fairlane Drive and another tank above Skyline at the top of Broadway Terrace. If these run dry, a pumping truck is supposed to go halfway up Pinehaven to a spot where the next lower water system can be tapped to replenish the high system.

pinehavenfiresign

Oakland ochre

24 May 2013

oaklandochre

Here’s the story behind this photo. I just got back from five days in Fresno at the annual meeting of my regional section of the Geological Society of America, the Cordilleran Section. The first order of business was a field trip to see the Pleistocene fossils of the Fairmead Landfill, near Chowchilla. The guy whose hand this is is Blake Bufford, director of the Fossil Discovery Center just across the road from the site. His job is to follow around the giant scrapers at the landfill when they dig new pits and watch for fossils, so he’s the most important pair of eyes in the entire project. Blake showed our group the latest pit and then accompanied us to the Center for a tour and a reception by the FDC’s owners, the San Joaquin Valley Paleontology Foundation. Some nice Madera County wine, local cheese, “mammoth” meat balls and so on.

Blake and I got to talking, and the topic of Oakland came up. He asked if I knew anything about the traditional ochre diggings he had visited there. I told him about the Holy Names site, but that’s not the one he meant. No, he said, this was another place that was in the process of being wiped out to build a supermarket, where both red and yellow ochre were produced. “Let me show it to you.”

The Center is full of fossils, but it also has a little display cabinet dedicated to the people who once had the area to themselves, the Chowchilla tribe. There was an antique woven reed basket, of course, but everything else was a modern replica made with traditional techniques: arrows with interchangeable arrowheads, various kinds of twine, deerbone trowels and scrapers, a tiny wooden flute, necklaces, a soapstone bowl and trade beads, and balls of red and yellow ochre. Blake made all of it. He unlocked the case and showed me everything in it. He made the ochre balls, the size of a small egg, by grinding the stone to powder, then mixing it with boiled soaproot to hold it together. “We don’t know exactly what they used, but this worked.” There were a couple of raw ochre specimens lying next to them. “I collected this one from Oakland,” he said, and I said, “Please let me photograph that.”


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