11 August 2013
A few weeks ago I managed to get a very nice view of the roadcut where the end of Gudde Ridge was trimmed to make room for route 24, east of the Caldecott Tunnel.
That’s the lava flows of the Moraga Formation on the left. On the right edge is the coarse conglomerate of the underlying Orinda Formation. Click the photo for a 1000-pixel version. (The cut on the north side of 24 is shown here.)
This is not a view straight on; the ridge runs toward the lower right, so the steeply tilted rocks look vertical from this angle. It can often be hard to figure out the exact orientation of rocks, even in a splendid exposure like this.
6 August 2013
Foothill Boulevard descends to Lake Merritt in this gentle slope down from the old marine terrace (Clinton terrace).
For a mile and half east from here, Foothill runs along the terrace. East of 14th Avenue, where it’s a two-way street, it skirts the steep edge of the Fan at San Antonio hill where the contrast between the two geologic units is most dramatic.
26 July 2013
Way down in the Sheffield Village neighborhood is a big hill and a little hill, but most of the place is in a flat little basin. The geologic map shows it as a patch of old alluvial sediment of the same vintage as the Fan:
The big hill is on the east side, across the Hayward fault (the black line from the lower right corner). The little hill is on the west side, butting against I-580. The road on its west side is Middleton Street, so I give the hill that name. I didn’t realize it when I poked around there earlier this month, but the hill is mapped as San Leandro Gabbro, of Jurassic age. The rock doesn’t show itself very much, but here’s a small exposure. Next time I’ll bring a hammer.
The highlight of the hill is the little private park inside the ring of houses there, just a microscopic piece of the original oak meadow (although this is actually a cork oak).
From there you get a nice view of the big hill. The fault runs along the foot of the hills, behind the homes in the center and in front of the lower set of homes on the right.
Click the photo for a big version.
15 July 2013
My place lies at the edge of the Fan, downhill from the Piedmont block, and the soil keeps producing these gorgeous pieces of lightly polished high-grade chert. I decided that this one should sit for a formal portrait. Click the image for a larger version.
It’s really more of a jasper than a chert. Jasper is a glassier, more refined chert that gets its appearance from the redistribution of silica fluids. This one mixes the red of oxidized iron with the green of reduced iron—at least that is my inexpert explanation—reflecting the invasion of oxidizing fluids at some point during its rough-and-tumble history as part of the Franciscan complex.
I believe that upstream, in Moraga Canyon, there were chert quarries in the earliest days, but I need to check the records to be sure one way or the other. This is not a quarried stone, though, because it has the burnished surface of an object that has been carried a short distance by streams and natural mass movements. I can’t look at it enough.
12 July 2013
I think it’s safe to say that everyone in town is thrilled with the improvements to Lake Merritt. After seeing the final configuration today, I’m feeling a deep satisfaction.
The new roadway and pedestrian bridge over the lake’s outlet serves vehicular traffic as well as ever, but residents and, most of all, the lake and the land get their due. The lake—actually it’s a tidal marsh—is noticeably healthier now that the tidal flow from the bay is no longer regulated with a dam. The range of the tide is greater now and the water is flushed more thoroughly. We have figured out how to trust nature with our lake. We’ll see in the future how the new lake deals with drought and flood, but I think that the city will not overreact to the occasional inundation as it might have in the past.
The new lake is a triumph for the planners of Measure DD, where the money came from. The funds are still being spent on this and many other projects around Oakland, but I’m starting to wonder what the DD crew could do for an encore. Nature holds us in its hand with the Hayward fault, too. Can we envision better ways to live with it?
4 July 2013
The headwaters of Temescal Creek lie east of route 13 in a steep canyon that has no name on the USGS topo map, so I will feel free to name it Thornhill canyon. The canyon splits at the site of Thornhill Nursery, with Pinehaven Road heading left up its own canyon and Thornhill heading right.
Pinehaven canyon is heavily wooded with a lot of eucalyptus and is almost entirely underlain by the crumbly Sobrante Formation. It’s a beautiful place, with a nice running stream that helps keep Lake Temescal full.
Whenever I visit the high hills I can’t help but think of its hazards, so different from those down below. The risks of landslide and fire, even in the absence of earthquakes, are compounded by the narrow, winding roads as we all know from the 1991 hills fire. Pinehaven canyon has not burned since it was settled, although the 1937 fire came close. Its firefighters are served by a couple of large water tanks, the Swainland tank at the top of Fairlane Drive and another tank above Skyline at the top of Broadway Terrace. If these run dry, a pumping truck is supposed to go halfway up Pinehaven to a spot where the next lower water system can be tapped to replenish the high system.
27 June 2013
The valley of Sausal Creek below Dimond Canyon made a natural site for orchards: a nice flat floodplain with decent soil and a permanent stream off on the western side. Also, the valley is straight to a degree that strikes me as unusual, which is handy for laying out blocks of land. It may or may not have been filled with oaks—I have a copy of an old print titled “Oaks of Oakland” that purports to be from this area. In any case it has a classic shape with a flat floor and steep sides formed by the Oakland alluvial fan (the Fan). I’ve shown the high, landslide-prone western side before; here’s the eastern side. This is the view from the Fruitvale freeway exit looking up Harold Street, where the valley wall is pretty dramatic.
Farther down, the valley wall fades away well before you get to Foothill Boulevard, which everywhere marks the edge of the Fan. Here at Fruitvale Boulevard and Bona Street, the valley wall is already lower and more subdued.
It looks like I’ll name this lobe of the Fan the Patten lobe. The valley of Peralta Creek is just over the hill. It’s interesting to speculate why the Peraltas put their rancho buildings there rather than here.