Archive for July, 2009

Grandview

31 July 2009

grandview

Grandview is a small, isolated Oakland neighborhood on the ridge above the Claremont Resort. I’m being unfair, I know, but this shot of a house on the north side encapsulates the neighborhood for me: a setting of intimate and exclusive comfort with borrowed scenery. It’s like all of Oakland’s neighborhoods on the east side of the Hayward fault, but with that invisible drop of ever-so-much-more-so. The views are grand, absolutely; beyond this house is Claremont Canyon Regional Preserve and behind me the vast panorama centered around the Golden Gate. All the houses are new, even raw, and oversized; sidewalks are absent, and no one would walk them anyway being too steep. There are no services, no amenities, no history. Only four outlet roads, all vulnerable to fire, landslide and earthquake. But it’s quiet and peaceful up here. With good provisioning, you could hold out OK after the next big quake in an incomparable setting, although service wouldn’t be restored for quite a while.

In my walk here last spring I saw only hints of bedrock, all mudstone of the Great Valley Complex.

Slump

28 July 2009

slump

This slump sits at the edge of a level area in Sibley Regional Volcanic Preserve. Notice the well-formed curved headscarp on the right side and the flow of material exiting the slump at its toe on the left. A classic textbook slump has a rotational aspect, because it moves along a scallop-shaped underground surface. This is easier to show than to tell, using this USGS image:

slump diagram

So the Sibley slump has aspects of an earthflow—basically it lacks some of the features of a slump. In a classic slump, that middle part would be tilted slightly backward instead of slightly forward.

But why is it here? First, I’m not sure whether this area is bedrock or a big tailings pile; that would involve walking down there and poking around, or looking for old aerial photos (which I could do from my desk). If it’s bedrock, that would be terrestrial mudstone and conglomerate of the Orinda Formation, the stuff you see flanking Route 24 just east of the Caldecott Tunnel. This doesn’t resemble that at all, so my working hypothesis is that it’s tailings: waste rock and soil from the days when this was a quarry. It has little strength, and its slopes tend to give way.

And why is it a slump, and not just a washout or a slide? That involves water and the consequences of human acts. This tailings pile appears to have been graded without due regard for drainage. Rainwater and runoff would collect on and infiltrate into this flat top surface. Then the groundwater would trickle its way through the pile and exit as seeps and springs near the bottom, where Fish Ranch creek is happy to accept it. But in a wet year the water table would build up and exert pressure around its edges. Water would rise and buoy up the precarious sediments at the base of this slump, easing the gravitational force holding it against the ground beneath. The details are important, and that’s why landslide management requires the services of a geotechnical specialist.

That reminds me: the new California budget plan consolidates the state Board for Geologists and Geophysicists into the State Mining and Geology Board. In itself, this should not threaten the state’s licensing system for geoscience professionals, but it does give the governor an opportunity to eliminate board members whose scientific advice tends to contradict business as usual. I don’t have any scuttlebutt to pass on, but I do know that commerce does not always look kindly on the costs of living in the real world, on the real Earth.

Looking at the fault, Sheffield Village

18 July 2009

hayward fault

Click the picture for a 1000-pixel version. I’m standing next to the Hayward fault on the access road to Dunsmuir Ridge, looking south at the southernmost part of Oakland. The hill on the horizon is in San Leandro, and the fault goes through the notch on its right. Downward and to the left is a bit of the pavement of Revere Avenue, where the fault has split the roadway (as shown in this post). The map below shows this stretch of the fault along with the evidence of movement on it—the dirt road I’m standing on is at the top.

fault map

“G” means geomorphic evidence, “1″ is the most strongly pronounced while “3″ is weak. The other codes are as follows: n, notch; rs, right-offset stream or gully; vl, line of vegetation; sl, linear scarp; hv, linear hillside valley; hb, linear hillside bench.

San Leandro Creek (1)

8 July 2009

san leandro creek

San Leandro Creek isn’t the only stream crossing Oakland’s flatlands, but it stands out more than others. First, it’s the trees. Second, the flats are more extensive in southernmost Oakland. In north and central Oakland, the big Pleistocene alluvial fan sticks out from the hills leaving only a narrow lowland strip.

Arroyo Viejo and Temescal, Sausal and Peralta Creeks have tree trails too. But those streams have been extensively culverted and covered, whereas nearly all of San Leandro Creek is in daylight. You really see the creek as you ride BART south of the Coliseum. But all of the creeks are landmarks once you start to notice them.

This view is from Dunsmuir Ridge. See the creek’s mouth here.

Merritt Slough

3 July 2009

merritt slough

When Oakland was first settled, a large wetland extended inland from the tidal channels east of Alameda Island. Samuel Merritt took it upon himself to improve the marsh with a dam, and the resulting brackish water body was named Lake Merritt. Today the lake is closely regulated with a water gate beneath 7th Street, and this creek runs both ways depending on the tide. Not even the Oakland watershed map gives this stream a name, so I’ll call it Merritt Slough. Click the photo for a 800×700 version.

The bank on the right side is the south bank, or the left bank. It’s nearly in its natural state, whereas the other bank is part of an area of made land a couple hundred meters wide. I’m standing just off E. 10th Street, next to the Civic Auditorium.


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