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.

The fossil treasury of UCMP

18 May 2015

Last month, as part of Cal Day, they were giving backstage tours at the UC Museum of Paleontology. The day is a mob scene, but on the last tour of the afternoon I had a fine time being shown around by two of the museum’s scientists.

The collection is vast, the world’s largest university collection. Only part of it is in the Valley Life Science building. There’s also a building in Richmond, and there are great accumulations of tar-seep fossils (from La Brea and McKittrick) housed in Sather Tower—the Campanile—because they smelled too much to keep with the others.

The collection at Valley Life Sciences is housed in huge cabinets that slide along tracks to open enough space to pull out the drawers. Many of those drawers are full of microfossils.

UCMPmicros

These are a huge part of California’s heritage. It was a thorough knowledge of microfossils in the rocks of the Central Valley and Southern California that informed and guided our petroleum industry. California remains America’s fourth-largest oil and gas producing state thanks to this scientific-industrial heritage.

But naturally, the museum had some of their photogenic specimens out. This Triceratops horn, with slices sawn off to expose its interior, was a privilege to hold. Unfortunately, the Bay area has no dinosaur fossils at all, although a few marine vertebrates from Mesozoic times have been found.

triceratop-horn

This ammonoid shell was interesting because it has a beautiful set of tooth marks on it, courtesy of a mosasaur. In the corner of the photo is a drawing of the specimen, as published in a scholarly paper.

mosa-chomp

Mosasaur fossils, to my knowledge, are not known from California either. Not yet, anyway.

And this thing sitting on the floor gave me joy. It’s a stump from the world’s first forest, dug up in rocks of Middle Devonian age in upstate New York.

psaronius-erianus

The inscription reads, “Psaronius Erianus Dawson / [Hamilton Group] Tree Fern / GILBOA, Schoharie Co. NY.” The Gilboa forest was first excavated in the 1870s. When I was on About.com, I featured Gilboa in my list of geo-attractions of New York state, and I hope you’ll seek it out if you’re ever there.

This specimen was undoubtedly part of an exchange of fossils between UC Berkeley and the State Museum of New York, a traditional way for paleontologists to enrich each other’s collections. It was surely a source of pride for Joseph Le Conte, the geologist who was the University of California’s first president. His fossil collection was the nucleus of today’s museum.

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

Oakland’s serpentinite patch

4 May 2015

Oakland’s serpentinite patch is the snaky zone shown in deep purple on the geologic map. I’ve written many posts about individual localities, but this is the first time I’ve shown and talked about the whole thing.

serpzonemap

Here’s the same area in Google Maps terrain view, for reference. The rock doesn’t have a strong expression in the topography, although it tends to support less tree cover.

serpzonetopo

Serpentinite (accent on the “pen”) is a rock composed of the serpentine minerals (accent on the “serp”). Those are three: chrysotile, antigorite and lizardite. I haven’t found chrysotile in Oakland, although I’d be thrilled to: it’s the fibrous mineral used to make asbestos. Antigorite forms at higher temperatures and is often brownish green. That’s not common in Oakland.

Lizardite is the shiny, slippery-looking, soapy-feeling serpentine we have. Here’s a typical example sitting by Redwood Road.

serp-redwoodrd

That color is on the light side of its range in hue. It can sometimes be white, or close to it. On the dark side is this splendid specimen from a Millsmont hillside, what’s sometimes called California jade.

millsmont-jade

Serpentinite is what happens to the deepest rock in the oceanic crust—the dark and heavy material called peridotite—when it reacts with seawater at high temperature and pressure. That is to say, large parts of the deep oceanic crust, miles below the seafloor, consist of this stuff. We would never, ever see this rock unless it somehow got lifted up and put on land. Luckily, plate tectonics enables that to happen sometimes. The result is a body of rock called an ophiolite.

Russ Graymer, author of the geologic map, classifies our serpentinite patch as part of the Coast Range ophiolite. The Coast Range ophiolite is a little older than Oakland’s Franciscan rocks underlying Piedmont and surroundings. It’s not pure serpentinite by a long shot, but the name is a good shorthand. The Coast Range ophiolite is strewn up and down the Coast Range in dribs and drabs. The experts are still working out its story.

If you look on the geologic map you’ll see six areas of serpentinite defined by heavy black lines (plus a thin wisp next to Holy Names that I’m ignoring). I happen to have photographed it in all six, going from north to south:

1: In the Piedmont Pines neighborhood it crops out on Castle Drive and Las Aromas

2: Joaquin Miller Park’s Visionary ridge is serpentinite, and the body extends south to Butters Canyon.

3: Joaquin Miller Park has a small body of serpentinite separated by a thrust fault from body number 2. It’s exposed in the Friends of Sausal Creek nursery.

4: The biggest area of serpentinite encompasses an area from upper Joaquin Miller Road to the Crestmont area and, over the ridgetop to the east, Serpentine Prairie (I and II). Its long tail running past Merritt College is a part I have yet to explore.

5: A long, narrow strip of serpentinite extends down to Lincoln Square.

6: Another long strip extends from the belly of body number 4 across Redwood Road, including Old Redwood Road.

Blueschist on Old Redwood Road

27 April 2015

How many people are going to visit a quiet dead-end up a steep hill just to see if any rocks are there? Not too many, but I’m one of them. The street is Old Redwood Road, a short arc overlooking the Munck Elementary School that doesn’t quite return to Redwood Road. It has a handful of homes and all of this blueschist.

bluschst-oldredwood1

The exposure is about 100 feet long and in good condition. I didn’t inspect it with minute care, but it shows evidence of lots of shear, which you’d expect in this rock and this setting.

bluschst-oldredwood2

It tapers off at each end and rises to about 10 feet in the middle. This view of the bottom end shows an interesting feature: there appears to be an irregular pod of serpentinite in it, on the right-hand side.

bluschst-oldredwood3

A closer look at it reveals some typical serpentinite features: a gleaming surface, a relatively green hue, and balls of partially remineralized stuff that are polished and shaped like balls of clay rolled between the hands.

bluschst-oldredwood4

I didn’t find any reference to it online, but perhaps the real experts know all about it. If so, they haven’t published anything. Or maybe they don’t know it’s there—the geologic map says that this is a sliver of “Leona rhyolite” at a contact with the Knoxville Formation (which is indeed exposed downhill). No way.

While I’m talking about this area, I have to say that the Munck School grounds, and Pinto Playground next to it, look like yet another former quarry, although they may have been excavated between 1959 and 1968 when the new Redwood Road was built, isolating Old Redwood Road (and the loop around the Hebrew Day School farther uphill).


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