Our slides

26 March 2014

A large, deadly landslide in northern Washington has been making news. Smaller ones around here aren’t deadly, thank goodness, but they are sneaky and expensive and everywhere.

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This isn’t a landslide yet, but these concentric cracks in Skyline Boulevard are typical signs of slumping ground. A city with more money to maintain streets would have dribbled tar over these cracks when they first appeared, a year or two ago. As it is, each crack lets rainwater into the hillside where it promotes more slumping. Keeping the road sealed will buy a few years’ time.

This location is underlain by the same incompetent Sobrante Formation that gave the Caldecott Tunnel builders such trouble. But landslides occur in Oakland on nearly every geologic unit, from the lower streambanks to the highest hills.

Fruitvale Station vista

24 March 2014

Sometimes the weather is clear enough, but just hazy enough, to reveal the details of the landscape for quite some distance. A week ago the view down the line from the Fruitvale BART station looked like this.

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I know it’s a small image, so click on it for an annotated 1000-pixel version. Most of the view extends beyond Oakland city limits. The farthest peaks are in the Ohlone Wilderness east of San Jose, some 30 miles away. Once you become familiar with our skyline, it’s never boring.

The Easton & Wilson Quarry

6 March 2014

The Head-Royce School occupies a lovely secluded site next to Lincoln Avenue. Naturally, it was once a quarry.

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Not just any quarry, although its product was ordinary: crushed rock of indifferent quality. It was the quarry for Easton & Wilson, a paving and construction company founded in the late 1890s by Kimball G. Easton and Arthur R. Wilson. (Wilson also partnered with Easton’s brother Stanley in the Leona Heights Quarry.) In approximately 1905, the business was liquidated by transferring its assets to a new venture by Easton and his brother-in-law Warren Porter named Granite Rock Company. The firm is still in business today as Graniterock, which operates a large quarry on the San Andreas fault in Aromas, near San Juan Bautista.

The rock here was described by the state bureau of mines as “a blue metamorphosed sandstone” mixed with softer sandstone and shale that created a lot of waste. It is right at the edge of the Piedmont block of Franciscan sandstone, a district I described in more detail as the Oakmore block. Near the upper end of the school property, the rock walls are still exposed, as seen from Lincoln Avenue.

headroycequarry2

Stone magazine reported in 1900 that Easton & Wilson was sued to stop from opening a quarry on Fruitvale Avenue, on the grounds that the blasting would “addle the eggs in the chicken ranches, which form the leading industry in the neighborhood.”

Apparently this ground sat for 60 years after the quarry closed. The Head-Royce School relocated to the property in 1964. The MacArthur Metro gave more details about the school’s history in 2013.

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:

harringtonGmap

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).

harringtongeomap

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.

harrington1

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.

Lower Colton

31 January 2014

You may think of Snake Road as the ridge road that climbs the crest between Thornhill and Shepherd canyons, but it’s really Colton Boulevard. For a quiet hike above Montclair with views, you want Colton. Its lower part runs north of Snake across ground underlain by the Redwood Canyon Formation. Here’s a roadside outcrop.

colton-Kr

I didn’t even notice the parallel fractures until I opened the photo at home. They probably represent a bit of extra cementation, thanks to a late pulse of fluids, but they may also be original bedding.

I mentioned views. If you loop around Mendoza and Mazuela Drives you can view some immaculate houses and grounds, but I like the big empty lot on Mazuela that overlooks Pinehaven canyon; click it for the bigger version.

mazuelaview450

Lincoln Square – ochre and serpentine

28 January 2014

The Lincoln Square shopping center is a little neighborhood-scale set of shops on Redwood Road next to Route 13. It’s not very natural but it has some interesting natural features. Here’s the topography in Google Maps.

lincsqmap

The graded area sits across the small valley of uppermost Lion Creek, running due south from top center. (A second branch of Lion Creek is to the west cutting through Holy Names University.) Its east edge is a cut into the hillside, exposing a bunch of serpentine rock. It’s the little strip of purple on the geologic map of the same area.

lincsqgeomap

You can see the rock next to the parking lot . . .

lincsqserpcrop

. . . and in more detail behind the Safeway and the other building full of shops. This exposure is quite spectacular, but I was just doing a reconnaissance and didn’t linger.

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I was actually visiting here to look for signs of the aboriginal hematite workings. This is where the local tribes came to dig Oakland ochre. This is as close as I got to that, a boulder rich in iron oxides along the north driveway entrance.

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I have only the most preliminary ideas about this area. The map classifies this area as Franciscan sandstone, and this boulder doesn’t contradict that. There are other brief nods to the original landscape studding the parking lot, but on whole it’s pretty sorry-looking.

lincsqredrockdisplay

My idea is that in this part of the world the development of ochre requires serpentinite and a suitable host rock for the oxides to grow, and that the process happens underground at the base of the soil. It takes careful excavation by nature to reveal this fragile material without washing it away, and Lion Creek and the Hayward fault (on the left edge of the map) combined to do that here.

The 35th Avenue cut, Jordan swale and the Franciscan spike

18 January 2014

If you’ve read this blog over the years you’ve seen me talk about the Piedmont block, a big hunk of Franciscan rocks riding north along the western side of the Hayward fault. Its easternment end tails off in a narrow wedge of undifferentiated rock, shown here in the geologic map.

35thjordancutmap

The next three photos are taken from the locations marked with numbers. That’s 35th Avenue there, right at the curve in the road where it becomes Redwood Road. The curve is where the fault crosses the road, too, so it’s an apt place for the change of name. Just below the bend is this roadcut in hard bedrock. It’s mapped mostly as the material labeled KJf, undifferentiated Franciscan, on the geologic map plus some of the volcanic rocks (Jsv) exposed in the Leona quarry.

35th-ave-cut

I don’t know how old the roadcut is. The road hasn’t changed course since the 1800s, but I guess it was widened in the 1960s or so, because the map base shows the split roadway in purple, meaning a recent change of the same vintage as I-580’s construction. Perhaps the road had a hump in it as it crossed the ridge. Above the bedrock ridge is a small valley with Jordan Road in it, shown below. The homes on Victor and Herrier Streets are visible on the Franciscan ridge beyond this swale (especially in the big version if you click on it).

jordanswale

A bit to the north, Peralta Creek runs into this swale (mapped as a sag basin related to the fault) and then cuts through the ridge in Rettig canyon. I can see the swale filling with water and emptying over the millennia, perhaps occasionally down Cortland Creek past the south tip of the Franciscan spike, as earthquakes and landslides rearranged the topography. The fault is mapped right at the intersection of Jordan and 35th on the west side, but I’ve never seen any evidence of creep there.

The roadcut, according to the geologic map, should expose two kinds of bedrock. It’s covered with boulders of basalt or greenstone, presumably quarried from the spot.

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Bits of bedrock peek through, so it ought to be possible to trace the contact between the two rock types. That’s on my list of projects.


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