Archive for the ‘Oakland chert’ Category

Mountain View Cemetery rocks: The back forty

7 December 2015

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Mountain View Cemetery is a never-failing source of interest. If you tire of graves, then why not collect the knockers exposed in this splendid preserve of Franciscan melange. I haven’t featured them here in several years, but recently the weather up there was especially photogenic. The one above, near the north edge west of the Cogswell monument, is my favorite, but they’re hard to choose among. Let’s say that knocker 1, my “secret chert,” is my favorite in the civilized part of the cemetery.

As you climb the hills, it’s natural to look around, away from where you are. Lately the cemetery managers have eliminated the overbearing fringe of eucalyptus along the rear, and the views north are enticing.

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If you look carefully, maybe you can spot Cactus Rock, a leading candidate for the mysterious Rockridge Rock. It’s at the bottom of this shot, in the middle.

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But the view south hasn’t changed. This is the cemetery’s back forty, looking nearly unchanged after 150 years.

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Here’s a look at it head on, over the uppermost of the three ponds that occupy the headwaters of Glen Echo Creek. It’s not fenced off, so it’s open to exploration.

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Up the course of the creek is another small basin, above which the creek briefly emerges from a culvert. This is its current birthplace.

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The hillside gravel here — what geologists call the float — samples several different rock types that occur in the melange. Red chert, graywacke and some sort of serpentinized thingy is visible just in this small footprint. There’s also green chert, greenstone and basalt around.

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Here’s the graywacke — a dirty sandstone — close up.

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And this is my favorite knocker (one of several) in the UNcivilized part of the cemetery.

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It’s made of the high-grade green chert, whereas the first knocker I showed is the classic red ribbon chert. You could brave the traffic and see a huge expanse of it in the Marin Headlands, which is a nice field trip. Or you could stroll here and have it all to yourself, as long as I’m not hanging around.

Claremont chert closeup; or, Oakland hills are falling down

23 November 2015

On Grizzly Peak Boulevard, pretty much right above the Caldecott Tunnel, there’s a little old fire road that heads downhill to the west. I poked my nose down it the other day. The whole area has excellent exposures of the Claremont chert, starting with the roadside.

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It’s real nice right now. The ground is moist and makes for quiet walking. Pine needles smell great. The rock is pretty.

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There’s a spot where a lot of loose rock has tumbled down. The Claremont can be crumbly, because it’s so brittle, even though the stone itself is rather hard. The loose stuff is good for collecting a specimen if you’re into that. Unlike the bleached stone exposed along the ridgetop, there’s some variety here, including the black, kerogen-rich stuff that has made this formation, like its larger cousin the Monterey Shale, good petroleum source rock.

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During the Caldecott Tunnel dig, this formation leaked significant amounts of oil and gas into the working space. Precautions had to be taken. The same black Claremont crops out at Alum Rock, as I showed you a few years ago, as well as at the Calaveras Dam site.

That’s all fun. But the road’s cut off by a washout ripped into the hillside, a twisted galvanized drainpipe sprawled along its path. At some risk, I scrambled across it and noted that at its floor lies the Claremont chert, which has its bedding planes oriented only slightly steeper than the gully. Treacherous ground. I don’t recommend that you follow me.

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And just beyond it is another gully, somewhat bigger but not eroding as actively. Giving up on the fire road, I scrambled up to Grizzly Peak Boulevard and this is what’s at the top of that gully.

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At the top of the active washout is this innocuous-looking street drain.

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As far as I can tell, every one of these cute drains is carving gouges into the hillside. This one points toward the Parkwood condos.

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Can’t we do better than this?

Perhaps our children can revise the old playground song to “Oakland hills are falling down.”

A lot of my outings are like this — mixtures of pleasure and concern.

Knowland Park knockers I: High-grade chert

12 October 2015

As I review my discoveries at Knowland Park since that day in July when I first set foot there, it makes me want to linger over its rocks. The large exposure of the Franciscan Complex is probably the park’s most significant geologic feature.

Oakland has two exposures of Franciscan rocks. The big one underlies Piedmont and the adjoining neighborhoods of Oakland, which means it’s covered with houses, roads and landscaping. The small one underlies Knowland Park and the uplands of the Chabot Park and Chabot Park Highlands neighborhoods. The portion in Knowland Park is the largest open exposure of this rock unit in the city and perhaps the whole East Bay.

It’s Franciscan melange, but distinctly different from the Piedmont block. Melange is a mixture of rock types, largely mudstone of various types, with big chunks of harder stuff floating in it. In the landscape, the chunks emerge as the surroundings erode away, and generations of California geologists have called them knockers.

Knowland Park’s knockers include several different rock types, and the largest number of them are high-grade chert, or metachert. Unlike the red chert that’s typical of the Piedmont block (and San Francisco and the Marin Headlands’ great exposures), Knowland’s chert is green and hard and recrystallized. It has undergone deep burial, perhaps more than once, yet it appears to retain its original layering.

Here’s one of a number of very large boulders exposed down by Arroyo Viejo in the woods. (All images are large and should be clicked for the best detail.)

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Most of the chert knockers aren’t this well organized. Here’s one that I showed to the group on the October 4 geology walk.

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And here’s one I didn’t. The chert fabric is pretty much gone. Still other knockers are probably totaly chewed up, and without hammering them to get a fresh exposure, which is forbidden in the park (and against my practice anyway) it may be hard to identify.

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This is the park’s most prominent chert knocker. Both the October 4 walkers and the September 20 mappers enjoyed the stone and the views.

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Close up, the stone shows tantalizing details.

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But elsewhere in the park I found this beautiful fragment that displays the clean color of the stone and the etched surface of its layers. I left it behind for you to find.

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Perhaps some amateur with a hammer made this fragment. If people feel free to take pretty things away from the park, after a while there won’t be any more pretty things in it. On the other hand, maybe it was the graders who created this fragment as they maintained the roads in the park. Those are lucky breaks for the rest of us.