Oakland jasper

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.

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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.

New Lake Merritt

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.

newlakemerritt

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.

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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?

Pinehaven canyon

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.

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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.

pinehavencrop

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.

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Fruit Vale

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.

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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.

fruit-vale-bona

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.

Thermal Hill and the Broadway lobe

15 June 2013

This is my own neighborhood so I don’t always think of documenting it: the western edge of the Pleistocene-age alluvial fan, labeled Qpaf (for “Quaternary Pleistocene alluvial fan”) on the geologic map below.

broadwaylobe-geomap

The Broadway lobe consists of two separate hills of this old sediment: Pill Hill on the south and Thermal Hill (as labeled on the 1912 map) on the north. (In a post five years ago I called it Montgomery ridge, after the street running up its crest.) Broadway Creek runs west of Thermal Hill and crosses the gap in the lobe to join Glen Echo Creek. The only spot it isn’t culverted is in the backyards of Brook Street, down near its mouth. It runs right under Tech High and Mosswood Park. Its valley is in the foreground of this photo, looking up 42nd Street past Opal, Manila and Emerald streets toward Broadway. Click the photo for a big version.

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The tall trees and associated homes are on the ridge. Behind them is the top of Mountain View Cemetery’s property and its continuation south, and the high hills where Skyline meets Pinehurst. Thermal Hill is modest in comparison, but walk over it, or keep an eye on it as you ride along Broadway, and you’ll know it’s there.

Farther south, 40th Street runs straight over the ridge. You really notice because the rest of 40 Street, all the way to Emeryville, is flat as can be.

40St-hill

Once upon a time the streetcar line punched right through it (the 40th Street cut), and today the original 40th Street, the lefthand one in this view, is named 40th Street Way while the former cut, filled in again, is named 40th Street.

The gallery of Oakland geological science

31 May 2013

OMCAserp

The Oakland Museum of California has finished its three-years-plus shutdown of the Natural Science gallery; the new Gallery of California Natural Sciences had its grand opening today. I got a backstage look at it earlier this week and gave it a thumbs-up for KQED Science yesterday. For me the highlight is the gateway section of the gallery, all about Oakland. All except for Oakland (and California) geology. As far as geology goes, the museum’s great serpentinite boulder on the roof, shown here in 2000, is still as good as it gets.

You’ll see in the new gallery how Oakland looked 300 years ago: its forests and grasslands and marshes. But you won’t see how it looked 30,000 years ago, or 3 million, or why those times are still relevant today. You won’t get a clue to the underlying framework that explains this landscape, or the deep history encoded there. You won’t see a hint of the active faults that shaped, and continue to shape, our region, nor will you see how they link us to California—how all of California is united—in a tight tectonic embrace. You won’t learn where and why the first Oaklanders dug for stone and gravel and water, or why they stopped. You’ll be able to add your backyard oak to a biome database that maps the ghostly living traces of our original forest—a marvelous thing!—but not your backyard outcrop or neighborhood roadcut.

All those things would have been simple to weave into the plan. The geologic subtheme would have enriched the whole exhibit and made it more truly a natural sciences gallery. Instead the new gallery is stuck, geologically speaking, back in 1969 when the old gallery was built. I’ll keep doing what I can here to fill the gap.

Oakland ochre

24 May 2013

oaklandochre

Here’s the story behind this photo. I just got back from five days in Fresno at the annual meeting of my regional section of the Geological Society of America, the Cordilleran Section. The first order of business was a field trip to see the Pleistocene fossils of the Fairmead Landfill, near Chowchilla. The guy whose hand this is is Blake Bufford, director of the Fossil Discovery Center just across the road from the site. His job is to follow around the giant scrapers at the landfill when they dig new pits and watch for fossils, so he’s the most important pair of eyes in the entire project. Blake showed our group the latest pit and then accompanied us to the Center for a tour and a reception by the FDC’s owners, the San Joaquin Valley Paleontology Foundation. Some nice Madera County wine, local cheese, “mammoth” meat balls and so on.

Blake and I got to talking, and the topic of Oakland came up. He asked if I knew anything about the traditional ochre diggings he had visited there. I told him about the Holy Names site, but that’s not the one he meant. No, he said, this was another place that was in the process of being wiped out to build a supermarket, where both red and yellow ochre were produced. “Let me show it to you.”

The Center is full of fossils, but it also has a little display cabinet dedicated to the people who once had the area to themselves, the Chowchilla tribe. There was an antique woven reed basket, of course, but everything else was a modern replica made with traditional techniques: arrows with interchangeable arrowheads, various kinds of twine, deerbone trowels and scrapers, a tiny wooden flute, necklaces, a soapstone bowl and trade beads, and balls of red and yellow ochre. Blake made all of it. He unlocked the case and showed me everything in it. He made the ochre balls, the size of a small egg, by grinding the stone to powder, then mixing it with boiled soaproot to hold it together. “We don’t know exactly what they used, but this worked.” There were a couple of raw ochre specimens lying next to them. “I collected this one from Oakland,” he said, and I said, “Please let me photograph that.”


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