Archive for the ‘Oakland’s soil’ Category

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.

Clay outcrop in Horseshoe Canyon

25 January 2016

The gorge of Horseshoe Creek, in Leona Heights Park, is unusually grand for its size. Its rugged rocks, mostly Leona “rhyolite,” are pretty homogeneous though.

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So when a spot along the stream caught my eye with its color — reddish red and bluish gray — I went off the trail and checked it out. Notice that the surface is cut into the hillside.

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This streamside lump looked just like concrete. But there was a lot of it, in different states of preservation and age, so I took it as a natural deposit.

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It was hard, but a little higher up I was surprised to find soft material. Not just soft, but pure clay.

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The high-silica lava and volcanic ash that makes up the Leona should weather into kaolinite (white china clay), especially under acidic conditions. We have that combination in parts of the Leona that are rich in pyrite. This mineral, with the formula FeS2, reacts with air to form iron oxyhydroxides and sulfuric acid (here’s a brief treatment).

There may be a pod of rock here with a different texture or composition from its surroundings, which might account for the purity of the clay. But I don’t actually know how pure the clay is. The way to tell would be nibbling it. Maybe on my next visit.

I think that a gradient in pH, plus interactions with air and surface water, explains the transition from gray to white to red clay as you go from depth to the surface.

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Without a lab, there’s not much I can say about it, although geologists with more experience probably know this stuff cold. If so, speak up. There was another piece of evidence at the scene, though: a bit of leaking “yellowboy” from the floor of the streambed.

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It means there’s a little bit of acid drainage here, not up to the level of the ex-sulfur mine just south of here. More like a geologically slow bit of natural acid drainage. It will be interesting to watch this spot during this wet winter.

A peek inside the Fan on Piedmont Avenue

19 February 2015

Construction is going on at the lot formerly occupied by a well-behaved motorcycle club, at 4225 Piedmont Avenue by the Kona Club. What caught my eye is that it offered a clean cut into the stuff that constitutes lobe 2 of the Fan.

I’ve referred to the Fan often over the years, but I haven’t formally introduced it. Here it is on the Oakland geologic map.

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It’s a former alluvial fan that was last active during the Pleistocene, which has been dissected by several younger streams. There’s nothing else quite like it in the East Bay, and I think of it as the Fan with a capital F. I divide it into eight separate lobes. Lobe 2 has two separate parts, Pill Hill and Montgomery/Thermal hill. Anyway, I keep an eye on it because it’s rarely exposed. Only excavations and a few stream banks display it.

Here’s what it looks like from a distance.

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There’s indistinct bedding that slopes down to the left. The material is gravely clayey sand that’s quite firm and well behaved. Here’s a closeup of a gravely layer; the stones are large pebble size (about 50-60 millimeters) and represent the Franciscan rocks just uphill in the Piedmont block.

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Farther over, the wall of the excavation has been carved with a backhoe, and the clayey matrix is so strong that most of the stones have been cut in two, even the tough black argillite.

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This is alluvium—sediment carried and laid down by streams. The rock clasts are rounded, showing that they’ve been carried in a stream for some distance, although most of the rocks are sandstone that doesn’t endure long. The hardest chert pebbles are still pretty rugged.

Down on the ground was this very typical Franciscan chert boulder, shattered by the builders after enduring for more than a hundred million years.

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The lot will become a nice set of dwellings. The builders are blogging about the job, complete with cool drone shots.

Harrington valley and ridge

22 February 2014

Harrington Avenue runs up a small valley cut into the Fan by a branch of Peralta Creek. It has a high ridge on its south side and a slightly lower one on the north side. Here’s the Google terrain map:

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and the geologic map to match, marked with the sites of six photos. This part of the Fan is an odd flat area, which I call the Allendale Flat, crossed by several different streams separated by low ridges. The valley of Harrington Avenue holds the Harrington Branch of Peralta Creek. The watershed map posted by the Oakland Museum of California will help keep the streams straight. As for the topography, I’ll name the ridge on the north side Harrington ridge and the one on the south Jefferson ridge (part of lobe 6 of the Fan).

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Harrington ridge is today’s geology puzzle. Its color stands for “Pleistocene alluvial terrace deposits,” and this is the only locality in all of Oakland, indeed the only one between Point Pinole and Hayward. It’s described as sandy gravel with boulders larger than a foot across (“35 cm intermediate diameter”). Only a very powerful river, or flow anyway, could have put such material here. My preference is to suspect a flooding event, given the dynamics of Oakland’s geologic setting: something like the sudden release of a large body of water. If you’re thinking what I am, Lake Chabot is about 5 miles, make that 8 kilometers, down the fault. At the rate the fault is slipping today (about 10 millimeters a year), the two features would have lined up 800,000 years ago. However, I haven’t seen any of this bouldery gravel because it’s covered with homes and yards. If anyone is excavating in the area, let me know.

This view is looking from Harper Street, on Harrington ridge, across the valley (behind the front row of houses) at its lower end.

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This view is from the top of Harrington ridge a block farther up, on Galindo Street. Looking straight down the left-hand sidewalk, across the creek, you may see the long stairway of Carrington Way climbing Jefferson ridge.

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And this is the view looking back from the top of that stairway (click to see it big).

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This next shot is from a point behind the previous one, zooming in on Galindo to show the crown of Harrington ridge (click for a big version). Note that it’s slightly lower than the rest of the Fan’s high points. The building with the colorful roof is the United for Success Academy on 35th Avenue. The four palm trees behind are on Galindo on the far side of Fruitvale Avenue, in the floodplain of Sausal Creek. The trees in front of them, I believe, are on the grounds of Patten University, across Peralta Creek. Behind them all is the Kaiser Center building, 3 miles away.

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This next view north is from one more block east, looking up Gray Street across the informal park called Jungle Hill. At least four homes used to sit here before landsliding took them out in the 1930s. (All I know about this is in a MacArthur Metro article from 2007.) Harrington ridge is relatively even lower here.

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And here’s a view looking almost due north across Jungle Hill and the upper end of Harrington ridge. The valley fades out of existence pretty rapidly as Harrington Avenue climbs out of it and enters the Allendale Flat.

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And there on the far right, as always, are the two ranks of high hills, one on each side of the Hayward fault. If you pick your spots carefully, Jefferson ridge offers a satisfying set of views around mid-Oakland.