Archive for the ‘oakland rocks’ Category

More Crestmont serpentinite

25 May 2015

I’ve done some more poking around in the fat part of the serpentinite patch. Today I’ll show photos from three localities, as marked below on the geologic map (where “sp” means serpentinite).

crestmont-geomap

The first locality is a truly spectacular exposure at the intersection of Crestmont and Kimberlin Heights Drives.

crestmont-kimberlin

The neighbors have made an effort to love this inhospitable stone, as you can see in this closeup.

kimheights-closeup

But serpentinite is not only very low in calcium and extremely rich in magnesium; it also tends to have high levels of toxic metals such as nickel, cadmium and chromium. Serpentine soils tend to be thin and poor (see this helpful Forest Service page). You can see that the little palms are almost dead, and on the upper slope even the pampas grass, one of California’s toughest invasives, is looking peaked.

I didn’t climb on this exposure—there wasn’t time and the rock has treacherous footing. It’s full of texture, though, and this exposure higher up on Kimberlin Heights suggests that it’s pretty chunky.

kimheights-exposure

Locality 2 is on the next street down, Colgett Drive, which is too new to be shown on the base map above. It exposes serpentinite all the way to the end, where I collected this fragment to take home.

crestmont-blue-specimen

I wanted to inspect it as closely as I know how. I have several hand lenses as well as a teeny pocket microscope that gets me up to 45X. With those tools, I perceive that the rock consists of blue bits the color of glaucophane in a matrix of light-colored serpentine. I continue to think of it as blueschist, but I’ll save that discussion for a later post. The point is, just because this area is mapped as serpentinite doesn’t mean it’s 100 percent one thing or another. But for all practical purposes this area is serpentine rock.

Locality 3 is the spot on Crestmont Drive I showed in my very first post, back in 2007. It looks a lot better now. You could creep around the fence and give it a good examination.

crestmont-ur-exposure

There are a couple of interesting things going on here. First is that just around the corner on the left side, where Butters Drive intersects Crestmont, the rock abruptly changes to the Knoxville Formation (KJk), a nice brown mudstone. The geologic map puts a thrust fault there separating the two rocks.

Second is that on the righthand side you can see signs of the pavement being disrupted by some sort of ground movement. Cracking related to it extends across the road. That has taken place in the 7-1/2 years since I last came here. I went to inspect it more closely, but my attention was drawn instead to an unusual sight: a swarm of bees gathered on open ground.

crestmont-bees

I lingered long enough to take two shots, but after the first one I sensed dozens of bees zipping past both my ears, so I let the poor critters be. It was a raw day, and I hope they found a new space to set up housekeeping.

Butters Canyon serpentinite

11 May 2015

I will continue to focus on serpentine rock—serpentinite—for a while longer because I’ve been visiting it a lot lately in Oakland. This post shows some exposures in the lobe of serpentinite mapped just south of Joaquin Miller Road. This bit of the geologic map shows where we are, and the five numbers are where I took the following photos.

butters-serpmap

The first photo is at the head of Hedge Lane, right off Joaquin Miller. The map says this is a blob of Leona “rhyolite,” but the ground is clearly serpentine here, on the whole street.

butters-serp1

A little farther, on beautiful bucolic Burdeck Drive, is this serpentine stonework. It doesn’t prove that there’s serpentine bedrock here, but it does suggest that some is lying around.

butters-serp2

So here we have a piece of the geologic map that’s mapped as one thing, but is clearly another. That’s the nature of geologic maps. They aren’t like street maps. They’re simplifications of a welter of vague clues in the landscape and bits of rock, surmises based on drilling records and engineering reports, and highly informed imagination. No doubt the mapper’s field notes mention “serp” where I saw it, but the area is still predominantly “rhyolite” so that’s the color it’s assigned.

Along the rest of Burdeck, the ground looks like something other than serpentinite, although no bedrock is exposed: unlike serpentine ground, the soil is thick and the oaks are vigorous. Then we join Butters Drive, just to the right of the “sp” symbol, and serpentine returns in all its blue-green, scaly glory.

butters-serp3

Here are two closeups from this exposure, testimony of mighty squeezings deep underground.

butters-serp4

butters-serp5


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