Oakland alluvium

1 August 2016

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When someone opens up the ground in Oakland, no matter where, I think it’s interesting. This construction site on Telegraph Avenue between 29th and 30th Streets exposes alluvium, the stream-laid sediment that once supported productive farmland throughout Oakland’s flats. Mapped as “alluvial fan and fluvial deposits (Holocene)” or unit Qhaf on the geologic map, it covers more area than any other geologic unit.

The uppermost part, the brown stuff that the excavators have turned over in curls, is rich in organic matter and clay. A little deeper it turns tan as the organic matter thins out. It’s dense and firm, good ground for building.

The nearer you get to the bay, the finer grained this material gets — more clay, less sand and gravel. Streams have carried it down from their canyons in the hills over the last few hundred thousand years, pushing back the sea. And by “streams” I mean floods. The clear trickling streams we know are actually asleep. Floods are the one day in a thousand when streams awaken, picking up and carrying alluvium from place to place.

Occasionally the streams themselves jump their tracks. If you visualize the land in super-fast motion over geologically recent time, our streams whip back and forth over the coastal plain like firehoses out of control, winnowing the alluvium again and again. From the hills outward they build up low, cone-shaped piles of sediment called alluvial fans. Downstream, these coalesce into an alluvial plain.

The “h” in “Qhaf” refers to the Holocene time period. The Holocene (“fully new” in scientific Greek) began when the latest pulse of the ice ages ended, about 12,000 years ago. It’s been a mostly pleasant time. Many geologists argue, with good reason, that the Holocene has given way to a new permanent state of wrenching climatic changes. Because the natural balance of climate is strongly influenced by human activities, they argue, the climate system today is a writhing firehose we may be able to control. They propose to call our new era Anthropocene time.

A greenstone boulder in Lakeside Park

25 July 2016

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Lakeside Park holds a scattering of boulders and plaques. The plaques are always interesting, and sometimes so are the boulders. This one sits at the west side of Bandstand Cove by a grove of redwood and oak trees. I can tell at a glance — the greenish color, even texture and lack of sedimentary fabric — that this rock consists of metamorphosed lava, informally called greenstone. There’s a lot of it in the Coast Range. There’s also some in the Sierra foothills, and I suspect that this was quarried over there.

One side of the boulder displays a nice slickenside, a sign that the rock was cracked and wrung underground.

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Emily Brodsky down at UC Santa Cruz studies these fault surfaces and has been finding deep clues in them (see the latest paper from her team).

Elsewhere the boulder shows stretch marks — little extensional fractures filled with quartz. Like a run in a stocking, these are evidence of the stresses that affected this body of material once upon a time. Since the boulder has been ripped out of its original setting, these scrape marks and stretch marks have lost their geological meaning, but they’re still pretty.

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Oh yeah, the boulder has a message on it. The plaque announces that the three fountains in Lake Merritt were installed or renovated by Madeleine and Andrew Wong as a gift to the people of Oakland.

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And not least among its functions, the boulder punctuates the most peaceful view on the whole lake, whether the fountain is running or not.

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Lake Merritt needs a lot of human management to stay clean and pleasant, and the fountains are a key part of that.

Oakland building stones: Kaiser Center’s dolomite

18 July 2016

During yesterday’s OHA walk around some of Oakland’s building stones, I was pleased to bring out an underappreciated aspect of this great city landmark.

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The Kaiser Center, like City Hall, is one of Oakland’s signature buildings. When construction finished in 1960 the curving, T-shaped office tower was so iconic that Ansel Adams is said to have photographed it, presumably giving it the same sheen of grandeur he gave Half Dome. (I couldn’t find that image, although Rondal Partridge also photographed it, which is just as good.)

The photo above shows the butt-end of the 7-acre Kaiser Center complex, at Webster and 21st streets. In the foreground is the parking structure/commercial space that holds up the wonderful rooftop garden.

The Kaiser Center is famous for its extravagant use of glass and metal, specifically aluminum, Kaiser Aluminum’s principal product and a design element dictated by Henry J. Kaiser himself. But I spoke to the group about the third element that embraces the metal and glass in a gracious unity — the white stone cladding.

As you walk around the Center, the cladding appears pleasantly rough yet perfectly homogeneous. It’s not a veneer of solid stone, nor is it some kind of textured concrete. Neither is it painted. It’s an extraordinary material. After 55 years of weather, it still looks white and crisp and fresh.

Nearly all of the cladding is out of reach. Some panels touch the ground along the curving roadway behind the main tower, but that’s too hazardous even for your typical roadside-loving geologist. However, at the left edge of the photo is a little section of wall where the architects felt obliged to extend the cladding to the sidewalk. That’s the only safe place to examine it.

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The backing is concrete, but the aggregate that makes up the face is a pure white stone. Steel scratches it, therefore it is not quartzite, which was my first guess. It has the frosty luster of a carbonate, but it doesn’t fizz under dilute hydrochloric acid (I always carry some). Therefore it is not ordinary limestone or marble. It’s something much less common: coarse-grained dolomite, a stone with its own place in Kaiser’s history.

Whereas limestone and marble consist of calcite (CaCO3), dolomite rock or dolostone consists of the mineral dolomite (CaMg(CO3)2), in which magnesium takes the place of half of the calcium. It has many industrial uses, and the Kaiser conglomerate has mined it in several different quarries.

During World War II, magnesium metal was in great demand, especially for aircraft. The supply of magnesium ore, magnesite (MgCO3), was very tight. (Magnesite was mined at this time south of Livermore, in Del Puerto Canyon.) Kaiser sought a way to produce magnesium metal from dolomite using the Hansgirg process, but the process was plagued with difficulties. The magnesium program was known as one of Kaiser’s biggest headaches.

Henry Kaiser was closely involved in the design of the Center — indeed, it’s said he intended to live there. One of his conceits was that the materials, as much as possible, should be supplied by Kaiser companies. His architect, the Los Angeles modernist Welton Becket, was noted for his use of natural stone cladding.

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I imagine that Becket and Kaiser were pleased, each for his own reasons, to showcase the exceptionally pure dolomite from Permanente Metals’ large Natividad quarry, a few miles north of Salinas. I feel quite sure that’s what this is.

More reading:

“Looking Down on Creation” in The Monthly, Nov 2006

“Five Painterly Vistas from Kaiser Rooftop Garden” in The Urbanist, Mar 2011

The Natividad quarry (bottom of the page), at Quarries and Beyond


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