Archive for the ‘oakland rocks’ Category

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.)

Know-Fran-chert-arroyo

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.

Know-Fran-chert-high

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.

Know-Fran-chert-rough

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.

Know-Fran-chert-white

Close up, the stone shows tantalizing details.

Know-Fran-chert-white-close

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.

Know-Fran-chert-close

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.

Watershed wilderness

5 October 2015

The lower end of Skyline Boulevard offers a tantalizing glimpse of the wilderness right next door. This post has large images, so I encourage you to click on them.

When you look east from most of Oakland’s highest hills, the center of attention is Mount Diablo. You can drive there and drive up and it’s a wonderful place. From the southernmost end of the hills, though, Diablo is hidden by Rocky Ridge.

rockyridge

Rocky Ridge reaches just over 2000 feet elevation and forms the west side of Bollinger Canyon, in Las Trampas Regional Wilderness. Between here, at the city stables just north of Keller Road, and Rocky Ridge lies Grass Valley, with its patches of grass and a power line running up it; then a darker, more distant ridge on the other side of Upper San Leandro Reservoir. From there to Rocky Ridge is an untrammeled area of mixed woods and fields and chapparal that’s East Bay MUD watershed land.

The ridge is about 5 miles away in a straight line but more like 8 miles on foot. With a permit, you can walk there but you can’t bike and you can’t camp. In a word, reaching that land from here is a pretty extreme challenge. You could reach it from the other side if you’re up for an 800-foot-plus climb out of Bollinger Canyon.

It’s so near, yet so remote.

Here’s the geology: fairly young sedimentary rocks, of late Miocene age (roughly 10 to 5 million years old), deeply folded to create dramatic exposures on Rocky Ridge’s flank. Don’t worry about all the symbols and labels, they’re significant only to a few specialists.

rockyridgegeomap

The photo is taken from the lower left corner where it says “Ko” (for the Oakland Conglomerate) and points toward the upper right corner. In the upper right quadrant, that set of stripes with the heavy line on its right edge represents the package of rocks making up the ridge, and the heavy line marks a thrust fault along which those rocks have been uplifted.

To help you visualize what the map is showing, the map includes a cross section of these rocks, drawn along that straight diagonal line near the top left. The point labeled B’ corresponds to the same point on the cross section, below.

rockyridgeprofile

Is your brain stretched to breaking yet? No? You may have the makings of a geologist.

For a much easier experience, hike in Grass Valley instead. That’s not watershed land, because it drains into Chabot Reservoir; instead it’s in the northern part of Anthony Chabot Regional Park, where only the locals go, and is as peaceful as can be. Some day I’ll post about it.

Rocks and land of south Mountain Boulevard

7 September 2015

First things first: I’m leading a “fieldwork-style outing” at Knowland Park on Sunday the 20th, from 2 to 4 pm. Details and tickets at Wild Oakland. This will be an experiment in having people learn about how geologists do their jobs and experience the landscape.

The southern end of Mountain Boulevard, between the zoo and the former Leona Quarry, is a little-traveled piece of road. I walked it the other day simply because I’m walking every road in Oakland, but it gave me a Eureka moment to share in this post. As usual, here’s the topography, with asterisks at the localities featured.

southMtnBvd-topomap

And here’s the corresponding geology. Jsv is the Leona “rhyolite,” which is actually a metamorphosed body of erupted volcanic material with a rhyolitic composition. This will come up again later. Jpb stands for Jurassic pillow basalt. They’re the two rock units I’ll be showing.

southMtnBvd-geomap

But first, a longing look through the fence at the empty piece of property where the Oak Knoll Naval Hospital once stood.

OakKnollvalley

It looks like a peaceful valley, but the aerial view shows that it’s full of old concrete. When it gets redeveloped, which will happen some day, a portion will surely be kept woodsy. I hope the friends of nature are being vigilant about retaining as much habitat as they can here.

Also visible is the clubhouse of the short-lived Oak Knoll Golf Course that preceded the naval hospital.

OakKnollClub

There is little significant bedrock geology on the property, which is almost all Leona “rhyolite,” although how it managed to get so deeply eroded here is a puzzle. It would be interesting to get a look at the ground as it’s being excavated. The valley here is drained by Rifle Range Branch.

riflerangebranch

As is common in our relatively arid climate, the stream runs in a deep-cut bed or arroyo in a wider floodplain.

The second rock unit crops out along Mountain Boulevard at the western edge of the Oak Knoll property. It’s very different from the Leona and not really like the other brown rocks of Oakland either.

J-basalt-MtnBlvd-at-Fontaine-overcross

It’s mapped as pillow basalt, which is not at all evident right here. Basalt I can buy, although it’s pretty shattered by exposure and multiple tectonic insults since its eruption about 145 million years ago. Will definitely visit again for a closer look.

Farther north in the residential areas, you start seeing a lot of Leona rock in the landscaping, including some big boulders.

Leonarhy-decoboulder

And I couldn’t resist a close look at this nice actinolite boulder across the street.

actinboulder

Farther north, Rifle Lane strikes up into the hills next to the Leona Quarry development. It’s secluded and rustic and full of rocks.

Alturasboulder

I also noticed a fair number of stones with greenish bits, which I’ve seen in many places in Oakland. This time I realized that they must originate in the Leona. Here are two examples. The left-hand one is from Dunsmuir Ridge, and the right-hand one is from a hillside on Outlook Avenue.

Leonarhy-celadonite

Clifford Hopson, one of the greats of California geology and a close student of the Coast Range Ophiolite of which this rock unit is a part, wrote in 2008 about these rocks, “Devitrification of once-glassy tuffaceous and fragmental siliceous rocks, including silicification that accompanied devitrification, accounts for their hard flinty character. Local turquoise-green beds mark pervasive celadonite, a typical low-temperature devitrification product of rhyolitic/dacitic tuff and pumice.”

And at the top of Altura Place is a colorful boulder of this stuff (1000px).

Leonarhy-celadonite-boulder

This rock superficially resembles the greenish metachert of the Franciscan Complex, shown here and here and here and here. But it forms in a fundamentally different way, and once you’re familiar with both rocks they’re easy to tell apart.


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