Archive for the ‘Oakland rocks’ 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.

Rocks of the Chabot Reservoir northside

30 November 2015

The hike on the Goldenrod Trail from the Grass Valley staging area, where Grass Valley Road meets Skyline Boulevard, down to Chabot Reservoir is a lovely walk. On the geologic map below, it’s the dirt road between the two O’s on the right side.

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When I walked here the first time, a few weeks ago, the idea was to check out the Franciscan Complex — shown as blue in the geologic map — where it crops out along the lake. On the east side is the Joaquin Miller Formation, which is a straightforward sandstone here.

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And on the west side is the Knoxville Formation, which is a straightforward shale here.

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Nice rocks: brown, crumbly. Trees like the soils they make. They don’t stand out. In between is something completely different: blue and green metamorphic stuff. You’ll see it in boulder piles.

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You’ll see it in knockers.

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And you’ll see it along the lakeshore. The other two formations leave plain old sand, which the birds seem to prefer, but the Franciscan gravel is worth looking at close up.

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The cool weather is a great time to explore this part of town, even if you don’t care about rocks.

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The hill here is Fairmont Ridge — its forested back side. If you’re used to seeing it from anywhere else in Oakland, you won’t recognize it. And that’s what makes this a getaway.

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.


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