Archive for the ‘oakland soil’ Category

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.

the-Fan

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.

piedmont4225-1

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.

piedmont4225-2

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.

piedmont4225-3

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.

piedmont4225-4

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:

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.

harrington2

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

harrington3

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.

harrington4

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.

harrington5

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.

harrington6

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.

West Oakland topography

10 March 2013

West Oakland has always been flat and easy to build on, whether it was for factories like the old Shredded Wheat plant built in 1915 (still operating as California Cereal Products) . . .

defremeryview

or for the middle-class Victorian homes that are West Oakland’s pride. It takes a lot of walking around to note the subtleties of the landscape. Except around Raimondi Park, the area was never a coastal marsh but was slightly elevated sand dunes, the same Merritt Sand that underlies downtown. In the Ralph Bunche neighborhood, north of 18th Street between Market and Adeline, the homes perch above the street, not by much but consistently.

ralphbunche1

Even century-old homes sit up the same way as the newest places. Presumably the streets were dug down, but maybe the lots were piled up too. Perhaps flooding was a concern, and all the earth-moving created more desirable lots here. Only a historian with intimate local knowledge could say.

ralphbunche2

If you look north along any of these streets (Chestnut, Linden, Filbert, Myrtle), you’ll see the land sink at Grand Avenue where the Merritt Sand leaves off.

The I-980 swath

1 March 2013

980swath

Interstate 980 is a huge convenience for drivers. I appreciate it every time I drive around town. But its construction was a major injury to Oakland’s neighborhood fabric, splitting West Oakland from downtown harshly and irrevocably. Every time I walk over 980, as here on the 14th Street overcrossing, I ask, Did they really need to hack out all this space for the freeway? Farther north, where the road becomes route 24, it’s narrower and they left a fringe of homes on Martin Luther King bordering the highway. But on 980, the excavation took out a full city block between Castro and Brush streets.

Maybe the difference was the sand. I-980 is built in the Merritt Sand, which underlies downtown and West Oakland up as far as Grand Avenue. The ancient dune sands probably can’t sustain a steep slope on the sides of the freeway. And the builders had to dig deep to make room for the overcrossings—most of the other freeways are not below grade. A narrower roadway, with tall vertical soundwalls on either side (like the new part of the Nimitz farther west in Bay mud), would not be as safe during earthquake shaking, and without room for the vegetation it would be a dreary place indeed. Bad as it is, it could have been worse.

Displacement at the Altenheim

15 January 2013

The Altenheim complex is on top of the northern side of the Sausal Creek valley, just across the freeway from the reservoir near the McKillop slide. There seems to be a little ground displacement here, too.

altenheim-slump

This view shows the downhill side of the property, on MacArthur Boulevard where it takes a leftward jog north of upper Fruitvale Avenue. The more I explore the stream valleys cutting through the Fan, the more of this I see.

Longridge loess

21 August 2012

I was walking up Longridge Road and spied an excavation, where a homeowner was replacing some water lines and renewing a driveway. Naturally, I sidled over and took the rare chance to look beneath the skin of Oakland’s Pleistocene fan. The material was massive—unbedded—and clean. I pried off this little piece . . .

longridge

. . . and nibbled on it. It was firm, but crumbled like Necco wafers and turned creamy on the tongue with just a hint of grit. Not sticky or chewy with clay. Not indurated like hardpan. No sand or pebbles to be seen. The more I thought about it, the more peculiar this sediment seemed, until I had a wild surmise.

Alluvial sediment is never very well sorted, because it’s carried short distances and laid down by streams. Longridge Road is, as the name suggests, a ridge road running up the crest of a ridge between parallel stream valleys along Trestle Glen and Mandana roads. The crest of a ridge should not be made of this fine silt. But it’s downwind from downtown, which is Pleistocene sand dunes (the Merritt Sand), cousin to the dunes of San Francisco. Dune sand is very fine sand, and the fraction that blows away from the sand is finer still. So my wild surmise is that the fan, at least this part of it, is dusted with a layer of windblown glacial silt—i.e., loess. It’s remarkable stuff, and something I never expected to see in Oakland.

Pebbles and perils

3 March 2011

pebbles

I was writing a post on Pebble Beach for the KQED Quest site. The pebbles there are special because they represent a huge variety of source rocks, pieces of which ended up offshore in deep-sea rocks back in the Cretaceous. Those rocks were pushed up above the sea and eroded again, freeing the pebbles for several more rounds of polishing and recycling. Then they were sifted in the surf to isolate them in a pure deposit.

Oakland wasn’t lucky that way. The sediments here derive mostly from finer-grained rocks, and the big exception, the Oakland Conglomerate, didn’t include such a variety of gravel clasts. And even then, our sediments don’t seem to have enjoyed a vigorous winnowing and polishing in the Pacific waves. Rather than shiny pebbles, our shores are mostly clay and mud.

But not entirely. The Franciscan block that underlies Piedmont and neighboring parts of Oakland contains the grab-bag of seafloor rocks typical of melange, and the debris it has shed down Oakland’s creeks has made it to the bay. So have bits of the basalt in the high hills. One place that these pebbles appear is in a little spot along the shore of Lake Merritt.

Unfortunately the waves of Lake Merritt will never turn them into a dazzling display. However, neither will Oakland ever be ground into bits like the retreating San Mateo coastline. Our dwellings will never fall down the eroding coastal bluffs like they do in Daly City. I can live with that.


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