Archive for the ‘Oakland geology walks’ Category

A hunt for silica-carbonate

12 December 2016

The geologic map of the northern East Bay that I rely on has a few rock units that are very small and hard to notice. One of them is the ultra-purple unit designated “silica-carbonate rock.” The map shows only three small exposures — one in Oakland and two in Berkeley — but they’re close enough to each other to visit in an afternoon.

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So that’s what I did back in August, hiking from the lower-right corner to the upper-left through all three areas.

“Silica-carbonate rock” is what happens when serpentine rock is invaded by superhot carbonated fluids, which replace the serpentine minerals with quartz and magnesium carbonates (dolomite and magnesite). The spectacular mercury deposits of the New Almaden and McLaughlin mines are of this type.

I wasn’t too sure what to look for, except that a rock made of hydrothermal quartz and Mg carbonates would be white and messy. Fortunately, the U.S. Geological Survey library, in Menlo Park, has a boulder-size specimen of mercury ore sitting around.

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Unfortunately, that’s not a very informative specimen; moreover it’s labeled “calc-silicate rock,” which would be quite different (it’s what happens when lime rocks are invaded by silica-rich fluids). So who knows.

To traverse the first locality, I started at the end of Chabot Road.

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It’s a highly disturbed place where railroads, streets, culverts and freeways have come through over the years, and it’s hard upon the Hayward fault. There is little promise of bedrock here, but I kept a close eye out anyway. There was some float, or loose rock, that was likely local: some brecciated stuff from the Leona “rhyolite,” tumbled down from its exposures above Tunnel Road.

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Some more of the “rhyolite” plus gray sandstone from the Franciscan melange mapped to the west.

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And at the north end, during a strenuous climb, some Franciscan chert from the melange.

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The next locality is at the top of the UC Berkeley campus. That was hopeless, given all the buildings and landscaping. Except for Founders Rock.

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This excellent knocker has a plaque on the back that reads, “Founders Rock / College of California / April 16, 1860 / Inscribed May 9, 1896” but there are no geological notes. Close up, the rock is enigmatic.

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Not much to do here but scratch your head, and feel sorry for any geology students assigned to write a report on this rock.

Onward through Berkeley’s steep hills to Keith Street, the third locality. That’s a residential street with all of its bedrock hidden, but I scrutinized the stone walls, in case the builders had used local rocks. You never know.

Some of those were could-bes. (A reminder: all of my photos click through to a 600- or 800-pixel image.)

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What I see in these is a uniform light color, suggesting pervasive alteration to siliceous material; brecciation and deformation, typical of an active hydrothermal environment; hints of channels and fractures such as you’d expect from hydrothermal replacement; and bits of iron staining from weathering sulfides. Without chemical tests and petrographic thin sections to examine, none of that is definitive. I did drop acid on them, but there was no reaction, nor would you expect one.

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That’s OK, I still had fun. And North Berkeley neighborhoods are famous for their integration of stone with stylish dwellings of all vintages.

Because it was a one-way walk, from the Rockridge BART station to the 67 bus line, this qualifies as a ramble.

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Oakland geo-walks for out-of-towners

5 December 2016

Next week will be the 2016 Fall Meeting of the American Geophysical Union, held in San Francisco since time immemorial — actually, since the hippie days of 1968 — and I’ve attended every year since the early 1980s. Back then it was held in Bill Graham Auditorium; next week some 24,000 people from all over the world will overfill the entire Moscone Center to swap geoscience presentations.

Oddly for a worldwide geoscience organization, the AGU doesn’t schedule any field trips in the days before and after the meeting. If you come, you’re on your own. So, cross the bay and visit beautiful, geologically interesting Oakland.

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Geologists, it’s easy to show yourself a good time here. You know your way around a geologic map: put USGS MF-2342 on your tablet or my Oakland-only excerpt. If you’re a Zipcar subscriber, ride the BART to the 19th or 12th Street stations and take your pick of cars.

No car? No problem — BART and bus are what I usually rely on. For the AC Transit bus lines, the secret to an easy experience is to buy a day pass (5$ cash, half if you’re 65+) the first time you climb aboard. The free CityMapper app will keep you oriented and informed.

Let’s talk about a typical afternoon day trip, because that’s what I know best — set out during early lunchtime and finish by early dinnertime or beer time (BeerByBART lists the best craft beer places, organized by BART station). You can travel light and cover lots of ground. There are three main starting/ending points: downtown, Rockridge and Fruitvale.

Downtown: Infinite number of lunch places on weekdays, you can’t lose. Goodly number of dinner places, from Jack London Square up to Grand Avenue (served by a free shuttle on Broadway). Many brewpubs and beer gardens on Telegraph Avenue and Broadway.

Rockridge: Delis, grocers for takeout on College Avenue. Plenty of restaurants. Ben & Nick’s for beers.

Fruitvale: Taquerias and carnicerias galore for food, Ale Industries for beer.

Your destinations are in our beautiful hills, because that’s where the rocks and the views are.

From Rockridge you can:

From downtown you can:

From Fruitvale you can:

If you have a car, you can:

And if you’re a maniac hiker, why not contemplate my hard-core one-way Oakland geology rambles:

Come on over.

A geologist’s reconnoiter of Piedmont

14 November 2016

The city of Piedmont sits surrounded by Oakland, like an organelle in a cell. I’m undecided on which organelle it might be — a mitochondrion (powerhouse)? a nucleus (brain)? Maybe just a vacuole of well-controlled living. Whatever the case, it’s a good walking town if you happen to be fit. And geologically it’s fully part of Oakland, occupying the best part of the Franciscan block. Lately I’ve been checking out the geology Piedmont has to offer. It varies.

The terrain is a rocky hillside dissected by small streams. Streets and homes tame this into an attractive rolling landscape.

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Once it was grassland, but today the land is heavily planted, and vistas are few.

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Although homeowners enjoy good views from their second floors, it is as if the public rights-of-way are deliberately shrouded, not at all like Oakland.

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But the hardscapes are robust and the plantings are gracious. Piedmont Park is the largest of the city’s public spaces and well worth exploring (here and here); so is Dracena Park. Don’t miss smaller features like the Hall Fernway.

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A little farther east is Crocker Park, where Oaklanders will find a familiar face.

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It’s a dark version of the statue “Bear and Cubs” by Beniamino Bufano that sits by the 10th Street entrance to the Oakland Museum of California.

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Whereas that one is cast in a concrete resembling granite, this is something else, an artificial stone of great character. The large polished clasts are a greenish black, probably serpentine (“black marble”), in a black cement matrix. It’s an understated tour de force.

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The official bike route through eastern Piedmont runs along St. James Drive, and there you’ll come upon bedrock at the head of Indian Gulch.

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The sandstone here is pretty sound, although no rock is immune to erosion. Occasional rockfalls expose the sandstone’s true golden color.

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The best exposure is around the powerline tower.

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Apparently someone there is who doesn’t love bare rock, who wants it green.

Oakland geology ramble 3: Knowland to Dunsmuir Ridge

7 November 2016

This ramble is special for its surprising woodsiness and remoteness. I’ve seen enough deer bones along the way to guess that if any mountain lions live on this side of the hills, this is where they are.

The route is a bit less than 4 miles long and has 500 feet of elevation gain in the middle. You go up the valley of Arroyo Viejo starting at the Oakland Zoo entrance, over the drainage divide in Knowland Park, then down through the twin canyons of Upper Elmhurst Creek, a tributary of San Leandro Creek. So, there’s a lot of woods:

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A lot of hills:

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And a lot of rocks:

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This post has been a long time in the making because I wanted to try all the options. The photos are from this summer, when I explored it in earnest.

Take the 46 or 46L bus line to the zoo entrance. From there, walk in through the gate and turn left. There’s an unsung trail, dotted with seating and signage, that shows off the creek here.

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The trail crosses the creek and connects with a fire road. Soon after that, the stream valley splits and you have two routes — the northern one is the fire road, and the other is a tiny trail along the spine of the odd, narrow ridge that runs for about half a mile between the two creeks. I prefer the northern route for its rock exposures and the southern route for its seclusion and its views, like this one of the new zoo facility under construction.

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The two routes rejoin where Golf Links Road and Elysian Fields Drive meet. If you like, rummage in the creek bed and examine the fresh-scrubbed rocks of the Knoxville Formation.

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Walk carefully up Golf Links and take the fire road into Knowland Park. This part is steep, but the views open up nicely.

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And the rocks suddenly change to Franciscan melange. Knowland Park’s rocks are a destination in themselves, as I’ve pointed out here a few times.

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At the top of the park’s broad, flat crest you have three route choices. The western route slots through Snowdown Avenue, the central route through Cameron Avenue, and the eastern route through Lochard Street. I’ll show them in that order. Please note that off of paved streets, the routes are just rough lines, not precise plots.

For the western route, take Malcolm Avenue and then Montwood Way down into the headwaters of the north branch of Upper Elmhurst Creek. The fire road at the end of Montwood takes you into real wildlands.

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Bear left and head down into the valley. A fresh landslide blocks the fire road and exposes the Leona volcanics.

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Although I call these fire roads, I think they’re really sewer roads because they all seem to serve sewer lines for the Chabot Park Highlands neighborhood.

After you cross the creek and head downsteam, the route gradually climbs out of the stream valley and takes a sharp left at the fence at the top of the Dunsmuir House property. Here it intersects with the middle route.

The middle route is picturesque because it follows Kerrigan Drive along the ridge between the two branches of Upper Elmhurst Creek. The ridge ends in a knob that widely visible, as seen here from Lochard Street.

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At the end of Kerrigan is a fire road leading to the right that goes down to the edge of the Dunsmuir House land and meets the western route.

The combined route runs level across a well-formed faceted spur, which reflects the action of the Hayward fault just a hundred meters or so to the west. Then it turns up the valley of the south branch of Upper Elmhurst Creek.

This branch should be considered the main stream of the creek, as it runs at a little lower elevation than the north branch. I suspect that the two branches may have drained in separate directions at times in the past. It would be too easy for a landslide along the fault to divert the northern stream to the right — or to the left as it runs today, for that matter. This is dynamic terrain.

Both streams are distinctly incised into their beds by up to several meters.

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This catches my attention because the hillsides are very steep, steep enough that a strong earthquake would send landslides down and clog long stretches of these streams. The last one of those was in 1868. This is an observation that applies generally to streams in the Oakland hills.

The route eventually crosses this creek and turns back west. As it climbs away from the creek it becomes a steep and narrow footpath. In due course you join the eastern route in Dunsmuir Ridge Open Space. I would tell bicyclists not to bother with this particular creek and try the eastern route instead.

The eastern route goes down Lochard Street almost to its end, where a fire road cuts off to the left. This is a very pleasant stroll through our oak-laurel forest that includes more exposures of Leona volcanics.

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The road leaves the woods in Dunsmuir Ridge Open Space, where the view south overlooking Sheffield Village is completely different from the view west in Knowland Park.

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From here you head downhill to civilization, taking either the fire road or the footpath through the woods. They rejoin where the combined west and central route arrives, right at the trace of the Hayward fault.

To meet your public-transit needs, the 75 bus comes through Marlow Drive every hour. If you miss it, and you usually will, it’s a level walk of 2 miles to the San Leandro BART station or a shorter one to Foothill Square.