Archive for the ‘oakland geology views’ Category

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.

fruit-vale-harold

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.

42dSt-big

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.

Tuxedo terrace

12 May 2013

The Fan, my name for the lower hills in central Oakland, has a lot of subtle topography that I’m getting to know as I ramble over its contours. The little valleys are one feature I enjoy perceiving, but the places between them are interesting too. The San Antonio lobe of the Fan, between 14th Avenue and Fruitvale, has a flat top at about 200 feet elevation. This is in the Tuxedo neighborhood, looking down 21st Avenue toward the bay. 22nd and 23rd are the same way.

tuxedoterrace

There doesn’t seem to be a reason for such a flat stretch on an ordinary alluvial fan. Fans slope; that’s why they’re fans. I have to assume that the ground was not excavated flat but is naturally that way. Is it possible that this is a relict wave-cut platform, similar to the Clinton marine terrace but higher and older?

Arguing against that hypothesis, the height is problematic. On the other hand, the East Bay hills are rising and so may be the land west of the Hayward fault. It may be rising in fits and starts (meaning in episodes measured in thousands of years). The next thing I want, and have wanted for a long time, is a really accurate terrain map of Oakland. It would look like the standard digital elevation model of Oakland but would be compiled from lidar data and be accurate to a centimeter or so. Maybe my eyes are fooling me; after all the street does slope a little.

Leona Canyon

23 April 2013

leonacynsign

Leona Canyon Regional Open Space Reserve is an East Bay Regional Parks District property of some 290 acres that is entirely within the Oakland city boundary. It’s got rocks.

The canyon was cut by Rifle Range Branch, part of the Arroyo Viejo stream network. The branch joins Arroyo Viejo underneath I-580 at the turnoff to the zoo. The topography is rugged. I surmise that the rifle range that gave its name to the creek was here once upon a time, because it’s the sort of place where you could shoot a lot without disturbing the rest of the city.

leonacyntopo

Here’s the geology of the same piece of ground.

leonacyngeo

The pink “Jsv” is the same metavolcanic rock (Leona “rhyolite”) found in the Leona Quarry just to the west. The green units are the familiar sedimentary rocks of the Great Valley Sequence, tilted upward so that they get younger to the east. The units, in order of age, are the Knoxville Formation (KJk), the Joaquin Miller Formation (Kjm), the Oakland Conglomerate (Ko) and the Shephard Creek Formation (Ksc). You can see that the canyon is controlled by the faulted contact between pink and green.

OK! The creek is dammed at the base of the canyon, presumably just for flood or sediment control. Maybe the rifle range used to be here. Anyway, the creek is fairly level throughout the park, creating a nice bit of habitat.

leonacyndam

As you walk up the creek, it wanders along the contact between the two major rock units, so you’ll see a mixture of boulders in the creek bed. The Knoxville is a shale with some sandstone, not very distinguished, but near its base it includes some conglomerate and breccia: rocks made of pebbles and cobbles derived from the Leona keratophyre. This example is from the high end of the trail, in the upper left corner of the geologic map.

KJk-cgl

The reserve has two paths that lead up the canyon’s sides. The Pyrite Trail goes west through the metavolcanics. It’s shady and steep. I should note that I saw no signs of pyrite on it.

leonacynpath

Along this trail you’ll see the Leona metavolcanics, kinda ragged-looking stuff that’s been chewed up and spit out a few times since it was a volcanic island arc during the Late Jurassic.

Jsv-brec

There are nice views of the other side of the canyon, which is more open and chaparral-y.

leonacynslopes

The trail up that side is called the Artemisia Trail. I’m not sure that either trail’s name means much. It passes a lot of this fine-grained sandstone.

Kjm-ss

Higher up, you get a good look at this big knob, which is a prominent part of the hills’ skyline as seen from the north. This view is from the south.

leonacynknob

There seem to be a few informal trails on it, and the view must be fantastic. But the Artemisia Trail offers superb views across the middle and south bay, too. I’ll be back.

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.

Dimond valley

6 February 2013

Sausal Creek is responsible for digging a floodplain that is rather wide just upstream from I-580. Here’s the view across it from Montana Street at MacArthur.

uprdimond1

The squat tower just to the left of the speed limit sign is at the foot of Lincoln Avenue, and it’s at the same elevation as where I’m standing. That’s how wide the valley is. Here’s another view looking right down MacArthur.

uprdimond2

The first cross street is Canon Avenue, the next one is Dimond Avenue leading up to the park, and beyond is the Fruitvale Avenue crossing. Here’s how the geology is mapped.

uprdimondmap

You can see that the stream valley is wide because three streams coalesce here at the edge of the Piedmont bedrock block. Restricted upstream, they enter the big alluvial fan (Qpaf) and have room to move and easy material to erode. It is curious that Sausal Creek hugs the west side of the valley; I suspect that tectonic movements may account for that, but only a decade or so of careful satellite altitude monitoring can answer my suspicion.

Northern Upper Rockridge walk (#30)

27 January 2013

Walk number 30 in Charles Fleming’s Secret Stairs East Bay, which he refers to as upper Rockridge west, goes from the Rockridge BART station over the Franciscan bedrock hills of upper Rockridge. The views are great, and there are a few rocks as well.

Here’s the route map (click it for a larger version).

walk30routemap450

And here’s the route shown on the geologic map. It goes counterclockwise.

walk30geo

The orange Qpaf is old alluvial terrace, KJfm is Franciscan melange, and fg is Franciscan greenstone (you might see a little of that near the end if you’re vigilant). Melange is lumpy stuff, as I’ve said before, mostly shale with knockers of harder rocks here and there.

And here’s the topography, with the sites of the following photos marked on it. The walk basically circles the bowl cradling little Rockridge Park with a couple of forays over its rim.

walk30topo

The new parklet at the BART station is nice. Naturally the boulders were sourced elsewhere.

walk30-1

The first part of the walk is housewatching until you cross Broadway to Rockridge Boulevard, where you face the hills through an allee of big palms. We’re at the 200-foot contour and looking at homes above 400 feet. It’s steep land, but not as bad as the high hills.

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Once you get up into the hills, you get views in all directions. Pick a good clear day to do this walk.

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As you go along Acacia Avenue, keep an eye out for Cactus Rock, reputed to be The Rock that gave Rockridge its name. I’m not fully sure that’s true, but I’m at a dead end in that quest at the moment.

walk30-4

The high point of the walk is on Alpine Terrace, at about 450 feet. It has several empty lots left over from the 1991 Hills Fire. This one always gives me a qualm.

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The top of the elaborate Brookside Steps features this gnarly boulder, which I’ve featured here before. This is what they should have used down at the BART station.

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As you wander over to the north end of the loop, enjoy the views that way. Here we have the chaparral of Claremont Canyon, the homes of the Claremont Hills neighborhood, and in front the solar roof of the College Prep School, which I was pleased to see produced a finalist in the Intel Science Talent Search this year. That is a huge brag for Oakland.

walk30-7

Here’s a view of upper Hiller Highlands, including one of the two big round houses up there. This is the lower one, at the end of Devon Court.

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And here’s the view east toward the eastern, higher crest of upper Rockridge studded with homes. A glimpse of uppermost Broadway Terrace is at left. All the distant points in these last three photos are across the Hayward fault.

walk30-9

The long, sturdy stairway was constructed by Schnoor & Son. By my reckoning, that makes this 100 years old. Other sidewalk stamps up here date from 1913.

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We’re finally going back down in to the bowl of Rockridge Boulevard, so you can see now what those high homes have for views—straight out the Golden Gate. The good burghers who settled this area a century ago would take these steps to catch the streetcar to their jobs across the bay.

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Here’s the view of Claremont Canyon from Broadway and Keith. The white bit by the traffic light is the tower of the Claremont Resort. The nearer ridge is just in Berkeley across the valley of Temescal Creek.

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And from here it’s a straight walk down to the refreshments of College Avenue. There are bits of bedrock along the upper part of Keith, but then you’re back to the lowlands.


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