Archive for the ‘oakland rocks’ Category

Grizzly Peak

20 December 2014

Grizzly Peak is the highest point in Oakland, at 1754 feet elevation (sources differ). As you approach it on Grizzly Peak Boulevard, it seems to loom quite high.

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That’s an illusion caused by the eucalyptus forest. As you get closer, you start to see through the trees.

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And from the bay side, the peak has a mohawk look because the trees are stripped off its northern half.

There’s a vague trail up the south side. Even in its true contours, Grizzly Peak is a steep little climb, and the thick layer of leaves is slippery. I’d rather the eucalyptus trees weren’t here, but they do offer a lovely privacy.

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And underfoot are rocks! The peak is mapped as the Moraga Formation, a set of lava flows from 9 to 10 million years old. This is the stuff connected to the volcano at Round Top.

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The top of the peak has a broadcasting tower of some sort, with a fenced-in support building at its foot. There used to be a lookout tower here, and a benchmark nearby attested to its elevation. Mount Diablo is almost exactly due east—not that you can see it through the damn eucalyptus.

You could walk up the access road instead. Either way, you can’t get any closer to the peak per se than this.

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That route offers nice views, and it takes you past a lot of broken rock, if you have your heart set on a specimen.

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Most of the rock is like this—weathered and fractured. There’s no easy way to tell what causes the strong layering, as this rock has been tilted almost vertical and then eroded by the fog, rain and earthquakes of the Berkeley Hills.

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It’s hard to expose unaltered bits. What’s there is a medium-gray, featureless stone that geologists typically call andesite until they can study it in the lab.

Grizzly Peak is not a place to stay long, but it seems that there are those who love it.

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Another hematite boulder

29 November 2014

Some of Oakland’s busiest places are little visited and little noticed. This set of boulders sits at the intersection of MacArthur and Foothill Boulevards, up by the Eastmont Transit Center.

macarthur-foothill

The stuff of these boulders appears to be the hematite ore of upper Leona Creek. It almost looks like they would fit together in a larger rock.

Who knows how they got here. Somewhere in the city’s maintenance yards there must be a stockpile of Oakland boulders, kept handy for deployments like this where you want to keep cars off the dirt and on the road.

Claremont chert, and other Oakland rocks, in Sunol

16 November 2014

The other day I took a field trip to see the construction site of the new dam for the Calaveras Reservoir. (The old dam is being replaced because it’s not strong enough to endure a big earthquake on the Calaveras fault, which runs essentially right through it.) It was a cool time, with fossils and big construction vehicles and engineers and grout. As the dam people were orienting us that morning, they passed around specimens of the major rock types in the area, and here was a fine chunk of chert from the Claremont Formation.

Claremont-in-Sunol

The stuff in our Oakland hills is white, because its carbon content was been leached out. This deep specimen retains the organic remains of ancient plankton that make the Claremont, like its larger sibling the Monterey Formation, source rocks for petroleum. (I showed you an exposure of similar stuff down at Alum Rock Park a while back.)

What the heck, I’m not planning to publish these anywhere else: here’s a cobble from a conglomerate down there that’s been stretched and fractured by activity on the Calaveras fault. It was exactly like the examples in our own Oakland Conglomerate.

calaverascobble

I think this one was from the Berryessa Formation (which is also found up at Alum Rock). Considering the looseness of the definitions of these formations, I feel safe in correlating them, although Crittenden defined the Berryessa as lying above the Oakland. Anyway, the phenomenon is the same.

One major feature of the construction project is an enormous cut made into the hill on the west side of the dam site. Basically, they discovered an active landslide there and decided to excavate the whole damn thing. We got to wander out along this exposure and hunt for fossils in the Temblor Formation, or at least rocks mapped as the Temblor.

calaverascut

There were a fair number of big ol’ scallop shells to be found, usually in pieces. Also some coaly bits of fossil wood. This is a rare exposure of an actual bedding plane, as marked by a pavement of scallop shells in what looks like their growth position.

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We also learned a lot about the recent project that rebuilt part of the Hetch Hetchy Aqueduct that runs through here. Once the work was done, this tunnel-digging machine became surplus.

calaverasdigger

The guy asked us if we wanted to bid on it. I thought it had the makings of a good Burning Man stunt.

The Las Aromas serpentinite

26 September 2014

The southern edge of the Piedmont Pines neighborhood butts against Joaquin Miller Park, and there’s no reason to visit if you don’t happen to live there—except if you like serpentinite. The street named Las Aromas dips into the belt of serpentinite that extends up and over the ridge to Serpentine Prairie, and the rock type is well exposed. One resident has done a lot of landscaping in this inhospitable rock type.

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The scaly structure is typical of serpentinite, and here it’s fairly close to the Hayward fault. Farther uphill is this exposure, where the grain of the rock is parallel to the street.

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Its blue-green color is striking. And there are a few spots where friction in the recent geologic past has turned it glossy. The rocks aren’t as spectacular as Serpentine Prairie itself, but they’re still noteworthy.

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I never tire of our state rock. Speaking of Serpentine Prairie, today on KQED Science there’s a story on the ongoing effort to propagate the rare Presidio Clarkia. (I photographed it a few years ago before they fenced off the area.)

Local boulders in the Calaveras triangle

8 August 2014

Where the Warren Freeway ends in its merge into I-580, most people drive south onto 580 east. The handful of locals or lost drivers who instead take the last exit to get onto 580 west will go through perhaps 580’s most deserted interchange. That’s where the highway builders installed this humble triangle, splitting the freeway exit for the even smaller handful of drivers going east on Calaveras Avenue to Mountain Boulevard and those going west, briefly, on Calaveras and onto the upramp to 580 west. The triangle is paved with river cobbles and populated with natives.

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The oak trees are easy enough to see in this view from across Calaveras. But what’s that in their shade? Why it’s one of several large local boulders.

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I didn’t take notes, but I think these are Leona volcanics, the same stuff that was quarried nearby for decades at the Leona Quarry. Pay them a visit next time you’re walking or biking through that godforsaken area. Sit on them in the shade; they like that.

More local serpentine

1 August 2014

Every now and then I come across a wall or a garden boulder that’s so beautiful I have to take a picture. Such was the case with this serpentinite specimen on or around Huntington Street.

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This is near Oakland’s great serpentine patch, and it likely came from there.

Geranium Place rocks and runoff

27 July 2014

Geranium Place occupies a sloping bit of land just north of Horseshoe Creek below Redwood Road. This map shows the location plus the sites of the photos in this post.

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The bedrock here is mapped as Franciscan melange on the west and the Leona volcanics on the east, the same stuff exposed in the huge Leona Quarry just down the Warren Freeway. The rocks I saw were clearly the latter, but you have to take the lines on the geologic map as approximations. If I’d been there fifty years ago when the homes here were being built I might have been able to tell better, while the foundations were exposed. BUT! there is bedrock to look at in any case.

The first thing that caught my eye, though, was the gutters. What’s going on here?

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Iron-rich runoff like this is not expected from undisturbed land. Perhaps the area was once a borrow pit or small quarry. Across Horseshoe Creek is the big quarrying operation above Laundry Canyon, and just beyond is the notorious McDonell sulfur mine site, so naturally the resemblance to the nasty-looking streambed down there is striking. What etched away the cement in the gutter? Probably not acid drainage.

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The white crust is another clue. In our deathly dry conditions it might conceivably have been salt, but it had no taste when I nibbled a fragment of it. It’s probably either gypsum or carbonate; without any chemicals handy I couldn’t learn anything more. But a buildup of crystals like this could gradually disintegrate the cement in the gutter. It may also be seepage from the serpentine exposed along and above Redwood Road here. In sum, very hard water here, but probably not nasty water.

The bedrock is heavily iron-stained and chewed up. Being so near the Hayward fault (it grazes the lower left corner of the map area) surely accounts for that.

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Some of the households here have worked with it to good effect.

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A closeup is impressive.

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This is breccia—pervasively shattered rock—that has been abraded by tectonic shearing so the pieces are rounded, as if you took crushed rock and rubbed it between your hands with a giant’s strength. It’s fairly well cemented, not crumbling apart, so this process happened at some depth.

At the northernmost bend of Geranium, up against the highly cantilevered Redwood Road, the ground is empty and there are monitoring wells of some sort. We have deeply disturbed this area.


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