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.

The Hayward fault by the Oakland Zoo

31 August 2015

Most people don’t know this, I think, but the Hayward fault runs right through the Oakland Zoo. I won’t take you there this week; instead let’s look at the residential area just south of the zoo. Here’s the topography from Google Maps, tilted 40 degrees from north to bring out the grain of the land.


The fault runs from the intersection of Golf Links Road and I-580 at the top, between the words “Oakland” and “Zoo,” through the S in “Hood St,” and across the base of the truncated hillside at the bottom. The USGS map of the fault trace shows the specific features of the fault here.


You’ll see things marked in the area just south of the zoo, a length of the fault labeled “G1, sl” and a dotted oval marked “G1, df.” G1 means a geomorphic feature (a landform) of “strongly pronounced” character, the most clear-cut kind of evidence. The first item is a linear scarp and the second is a fault-related depression. They’re somewhat visible in Google Earth if you tilt the view and squint (800 pixels).


If you go there, all this is more apparent. Here’s a view west, toward the bay, down Hood Street across the odd level spot marking the fault. The cross street is Mark Street.


This is looking north up Mark Street toward the zoo. The depression is behind these houses, in their back yards (800 px).


And this is the view toward the Knowland Park hills back up Hood Street from the bend at its west end (800 px). It’s very odd for the steep slope of our foothills to be interrupted this way. Normally a valley like this would be carved by a stream, but none is evident.


In all of Oakland, there are only four places where geomorphic evidence of the fault is ranked “strongly pronounced.” Two of them are now obscured: one was a line of vegetation in Redwood Heights that quarrying has removed, and the other was at Sausal Creek where Park Boulevard crosses the Warren Freeway. The remaining high-grade feature is the valley south of the LDS Temple, which is inaccessible and highly disturbed by landsliding.

So this is Oakland’s clearest trace of what the fault has done to our landscape. To me it looks quite similar to the Jordan Road stretch of the fault.

Even with all that build-up and explanation, you don’t really see much here unless you know what you’re looking at. But this shot I took in 2005 looking south from the zoo parking lot shows the depression pretty well.


Next time you visit the zoo, take a peek. You might also see if the zoo parking lot is showing cracks from fault creep. the last time I looked, they had just repaved everything.


Get every new post delivered to your Inbox.

Join 9,469 other followers