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.

Encounter with Sugarloaf Hill

24 August 2015

As promised, here’s a look at Sugarloaf (a/k/a 1175) Hill. But first two announcements:

  1. The 46L bus line goes from the Coliseum BART station past the zoo to Oakland’s remote Grass Valley neighborhood. It’s the only public transit providing access to the area that includes all of Knowland Park. AC Transit funded a year-long trial of this route to restore service that was cut in 2010, when a transbay line served the area. It’s still a bit of a trek to reach the park, but for carless people it’s essentially impossible without the 46L. (Yes, the bus has a bike rack.) There will be a hearing on September 2 at an awkward time, 5 pm, to consider keeping the 46L going. That’s what staff recommends, but I will lobby the board to continue service because it’s up to them, not the staff. I hope you’ll speak up too. [Update: The AC Transit Board voted to make the route permanent.]
  2. Speaking of Knowland Park, I’m discussing the possibility of leading some geology walks up there next month, for Wild Oakland. Updates here and there.

Thank you.


Sugarloaf Hill is on the rear side of the Merritt College campus and is in the Leona Canyon Preserve. The East Bay Regional Parks District has plans to enable access to Leona Canyon when the college is closed, though it seems like a low priority for them.

The trail up the hill is not marked or mapped, but it’s not hard to find. As you go up through the woods, you’ll pass exposures of the Leona “rhyolite” that underlies this whole area.


Once above the trees, the campus unfolds below you. This shot also shows the range of habitats on the hill. (It’s a 1000-pixel shot; there are three more later in this post.)


The upper part of the hill is largely grassland with ferns and some bedrock. The soil is very thin. This land used to be grazed. The grassland, says the EBPRD, is dominantly non-native species. It doesn’t mention the ferns, considering them part of the forest biome.


Besides grassland, the hill comprises shrubland and oak/bay woodland. Some parts are pleasingly mixed.


The view west over Leona Canyon juxtaposes grassland and forest. The forest is typical coast live oak and bay laurel, along with buckeye and hazelnut and a whole bunch of different native shrubs. The houses are on Campus Drive.


And this view south over Leona Canyon shows the shrubland, consisting of coyote brush, sagebrush and poison oak. It’s quite overgrown. Absent grazing or fires, this tends to turn into oak/bay forest.


The rocks don’t form many outcrops, per se. There are abundant boulders like this one. They’re naturally covered with lichens, so you have to search to see any details of this metamorphosed volcanic sandstone. Please don’t take a hammer to this stone—besides being protected by EBRPD rules, it deserves to look the way it wants. (1000px)


The point of a hill, so to speak, is its top and the views it makes available. When I climbed Sugarloaf Hill it was a cool and hazy day, so the next two photos are just versions of what you might see from there. Here’s looking north toward the ridge of Redwood Peak, over the ballfields and solar array of Merritt College. (1000px)


And here’s looking south over Leona Canyon from the summit. The rectangle of boulders is I think the work of idlers rather than the remnants of an old foundation. On the horizon, from left to right, are the dimly seen Knife, the dark wooded ridge behind Lake Chabot, and tree-topped Fairmont Ridge behind the hills of Knowland Park. (1000px)


Your viewing may vary.


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