Archive for the ‘Oakland boulders’ Category

Oakland ocher

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 ocher 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 ocher 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 ocher. Blake made all of it. He unlocked the case and showed me everything in it. He made the ocher 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 ocher specimens lying next to them. “I collected this one from Oakland,” he said, and I said, “Please let me photograph that.”

Sandstone concretion, Joaquin Miller Park

11 January 2012

This odd tumorous-looking thing, on a sandstone boulder in the Oakland Conglomerate in Joaquin Miller Park, is a concretion.

concretion

I’ve documented concretions in Oakland before, in rocks of the Great Valley Sequence and in the nameless unit of Eocene mudstone above Shephard Canyon. This concretion is unlike the other two in (I assume) not having a siliceous matrix like the first and not being finely layered like the second. I assume that this is a typical featureless ball of extra-strong mineralization that formed slightly before the rest of the rock lithified. (And on KQED Quest Science Blogs this week, I talk about other concretions in the Bay area and California.)

By the way, I visited the lower end of Joaquin Miller Park the other day, below the Woodminster area where the Miller cottage is, and finally saw my sign about the rocks of the park. I hope that people have gotten some benefit from it.

Big Rock at Lake Temescal

27 September 2010

I was at Temescal Regional Recreation Area yesterday—you probably call it Lake Temescal—and was happy to spot one of my favorite places.

big rock

This big hunk of Franciscan something-or-other sits near the head of the lake, displaying the ugly attractiveness that the French call jolie laide. Not a hundred feet away runs the Hayward fault, and there’s a free-running stream here too. Some classic oak trees shade picnic tables, and you can even swim where fish can nibble your toes.

Big Rock is the place I used to illustrate my post “Leave the Stone Alone.” Because for some reason, people seem to respect Big Rock.

Breccia

17 November 2008

breccia

Here is a nice example from the Oakland hills of breccia, a kind of rock that consists of broken pieces of rock. There are several different ways to make a breccia, and not having a lab and a petrographic microscope I can’t specify what made this. But considering that the hills have been raised by vigorous tectonic action accompanied by earthquakes, it’s easy to imagine some seismic event opening a fracture, shattering the rock, and allowing mineral-charged fluids into the space. Other ways involve dissolving the rock until it collapses, slumping while the rock is young and poorly consolidated, and simply cementing together a pile of landslide debris or volcanic deposits.

Because breccia really signifies an activity rather than a material, geologists would rather think about brecciation when they see a breccia.

UPDATE: I should mention that I have lots more breccia pictures on About.com: start here and try a search on “breccia” for more.