Knowland Park: On geologic maps

21 September 2015

I enjoyed yesterday’s Wild Oakland outing in Knowland Park. It was hot, but a gradually freshening breeze made it a dry heat, and we spent a good bit of time in the shade for discussions.

My goal was to explore the relationship between the ground of the actual Earth, in its strange and opaque reality, and the maps that geologists make of it. Here’s the set of places we visited, in Google Earth — we started at the end of Snowdown Avenue.


There were obvious things, like big outcrops, and subtle things, like the texture of the dirt in the roads, that yielded information to us about the rocks underlying various places as we walked. But that collection of observations wasn’t always easy to reconcile with the bold, definitive-looking patterns on the geologic map (800 pixels; see the same five localities on it).


I tried to explain that a geologic map, especially one of Oakland, is as much an act of imagination as it is of observation. The rocks aren’t very well exposed, the different rock units are hard to describe and each one includes a lot of variety.

The Leona “rhyolite” (Jsv) and the Knoxville Formation (KJk), to the extent we could see them, were easy to distinguish after a bit of exposure to them. But the major rock unit we encountered, the melange of the Franciscan Complex (KJfm), is really a meta-rock unit, a mixture of blocks (“knockers” in the local geo-parlance) of very different lithology. It’s a meta-rock unit in the same way you might call a package of frozen mixed vegetables a meta-vegetable. So that’s not an easy concept to grasp, but I think the group enjoyed getting a taste of the subject.

Notice that almost all of the contacts between different rock units on the map are shown as bold, dashed lines. These all mark faults — fractures where the rocks on either side have been displaced — and none of them are visible on the ground. They are inferred. We’re sure they’re there because our knowledge of rocks in general, and these rocks in the Bay area, leads us to that conclusion.

That may seem like arm-waving, and it is. Geologists have a joke that the way to make us shut up is to tie our arms down. Geology, more than most branches of science, is a tentative discipline. Geologists hold that tentativeness close. Consider how this area was first mapped 100 years ago by Andrew Lawson, professor at Berkeley and highly regarded then and now. The excerpt below is from USGS Geologic Atlas Folio 193, published in 1914. (800 px)


It’s barely recognizable. We’ve learned a lot since then, but there’s no guarantee we know it all, and geologists of 2115 may have a sympathetic chuckle at our mixture of certainty and puzzlement today. Someone asked me what has changed in California geology since John McPhee wrote about it in Assembling California forty years ago, and I said the basics are still sound, but in some important topics our ideas have changed greatly. In the progress of geology there is no prospect of an end.

Working rocks

14 September 2015

Last week I made another visit to Knowland Park in preparation for my two upcoming Wild Oakland walks on September 20 (tickets still available) and October 4 (no tickets needed). I’m holding off on posting about the park’s geology until after these events, but it’s hard to wait.

Meanwhile, here’s something different. Oakland has plenty of excellent native rocks, but the days are long past when Oaklanders could shop for Oakland stone at an Oakland quarry. Today newer places must make do with more anonymous stones from distant sources. They do their jobs with stolid competence and the occasional dash of flair.

This roadside lot on Cameron Drive uses guard rocks to discourage parking and keep runaway vehicles from breaking the wall. They’re picturesque, but kind of brusque.


Sobrante Park Elementary School has this splendid green boulder by its front entrance. It’s not serpentinite but, most likely, a beautifully chloritized basalt from the Franciscan Complex. The two neighboring boulders are sandstone. I think their job is keep runaway vehicles from the building, although neither Topanga Drive nor El Paseo Drive is a high-speed thoroughfare.


And down by Lake Merritt, the latest Measure DD improvement, the Sailboat House Shoreline Project, has made the shoreline more wildlife-friendly. The marsh vegetation will have to wait for the rainy season, but the infrastructure is in place.


These boulders are undistinguished sandstone, but they’re laid with care. I predict that the gulls will be cracking mussels on them, if they aren’t already. The rocks will also allow people to step into the marsh without kicking up the reeds and mud. And they’ll keep runaway vehicles out of the lake. Hmm, there seems to be a common thread here.

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.


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.


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


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.


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.


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.


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.


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


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


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.


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).


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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