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.

pdmnt-1

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

pdmnt-2

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.

pdmnt-3

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.

pdmnt-4

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

pdmnt-5

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.

pdmnt-6

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.

bufanoclose

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.

pdmnt-7

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

pdmnt-8

The best exposure is around the powerline tower.

pdmnt-9

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:

ramble3satmap

A lot of hills:

ramble3googlemap

And a lot of rocks:

ramble3geomap

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.

ramble3-01

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.

ramble3-1

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.

ramble3-2

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

ramble3-4

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.

ramble3-3

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.

ramble3-5

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

ramble3-6

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.

ramble3-8

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.

ramble3-7

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 rhyolite.

ramble3-9

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.

ramble3-10

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.

Lakeside Park clay

31 October 2016

qmt-1

They’ve been building some new parking in Lakeside Park, near the boathouse, and the work opened up the subsoil to view. You’re looking at a hard, pure marine clay from a time when the sea rode high, late in the Pleistocene Epoch.

Lakeside Park sits on the flat surface of a terrace a little over 20 feet elevation. It matches other surfaces in the Uptown/Valdez area and in old Clinton, all of which are assigned an age of around 125,000 years.

The scenario is this: As the great continental glaciers melted well beyond their extent today, the sea brimmed over and rose about 6 meters above its present level. The advancing waves carved their way into the Oakland landscape, creating a wave-cut platform, and then all was peaceful for a few thousand years. That’s when the nice clay topping was laid down. Only the finest sediment could get this far away from the new shore because all the stream valleys were drowned.

Later the Earth’s orbital cycles proceeded, the climate cooled, the glaciers grew back and the sea retreated. On this side of the bay, far from the Pacific surf, the process was gentle enough to leave behind much of the evidence. Elsewhere in the East Bay, the clay beds from this time were thick enough to support a brick industry.