Archive for the ‘oakland rocks’ Category

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.

lincsqserpcut

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.

lincsqredrock

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.

35th-ave-cutclose

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.

Serpentine at Coolidge hook

7 January 2014

At the top of Coolidge Avenue, the road hooks sharply to the left just below the grounds of the LDS Temple. Right there is a nice exposure of serpentine rock.

coolidgeserpcrop

Its green color and scaly appearance are obvious and unlike the brown sedimentary rocks downhill from here. Serpentinite is not mapped here, so make of that what you will. I tend to assume that serpentinite is likely anywhere along the Hayward fault (which is a about a hundred meters uphill from here, running through the temple parking lot). Take a closer look at the rounded blocks amid the scaly matrix.

coolidgeserpblock2

This is the block at upper right in the photo above. It gets its rounded look from being rotated and scrubbed during lateral shear, like a lump of cold butter in piecrust. Here’s the block to its left.

coolidgeserpblock

The block’s surface is festooned with slickensides:

coolidgeserpslick

And the sheared matrix is also well exposed on its upper side.

coolidgeserpfringe

The views in the other direction are great, too. This is pretty much the view you get from the temple grounds, and unlike the temple it’s always open to the public.

The Oakmore block

7 November 2013

The Oakmore district is quite a distinct part of town. Here’s how its geology makes it so. I’ll call it the Oakmore block, although that’s a bit of a misnomer—it’s the eastern end of the Piedmont block.

oakmore-geomap

The neighborhood is defined by the light blue of Franciscan sandstone of the Novato Quarry terrane, bounded by three canyons. The canyon on the northeast side is occupied by Route 13, of course, which corresponds to the Hayward fault. Here’s the view across it, looking east from the easternmost corner of the Oakmore block (the end of Braemar Road) to Joaquin Miller Park. The top of the grassy slope is the overlook at Lookout Point.

joaquinview

Dimond Canyon on the west is the most dramatic boundary, but the bedrock is the same on both sides; there is some accident of geologic history that has maintained this deep streamcut. Leimert Boulevard defines that side of the neighborhood. Roadcuts about midway up Leimert expose a lot of strong sandstone like this.

leimertcrop

And on the southeast is an unsung stream valley cut along the edge of the Piedmont block. Whittle creek, I guess I’d call it, because Whittle Avenue runs up it, and the Head-Royce School is nestled in it. The valley grows into a nice amphitheater at its head. This is the view from there—the end of Melvin Road—across to the Greek cathedral and Mormon temple. Lincoln Avenue is the ridge road on the other side of this valley.

oakmore-temples

Over here I see more shaly bedrock, like this stuff exposed on uppermost Fruitvale Avenue . . .

oakmore-shale

. . . and bit more structure here where Wrenn Street meets Hoover Avenue.

oakmore-wrenn-hoover

All of this is expected in the Franciscan. The odd bit I haven’t figured out is near the end of Melvin, which looks to the naked eye like a volcaniclastic rock. Perhaps the boundary on the geologic map is a little off.

oakmore-volc

All I know is that when you take your eyes off the ground and look off toward the Bay, Oakmore is a mighty fine place.

oakmore-view

I don’t know exactly where the realtors put the southern line, but the dotted-line contact running from Sausal Creek along Whittle, marking the hidden thrust fault at the edge of the hills and the bedrock alike, works for me.

Actinolite decoration

26 October 2013

I enjoy seeing some of Oakland’s more unusual rock types when they show up in people’s yards. It shows that people like our rocks.

actinbartlett

This boulder of actinolite may not actually have come from Oakland, but it could have. Perhaps a resident spotted it in a streambed and said, “I give this to me.” It’s clearly not part of a professionally assembled landscape package.

Serpentine garden

19 October 2013

This house on Melvin Court has a splendid front yard based on serpentinite: serpentinized peridotite on the right, a serpentine-lined walkway with slate in matching colors, and inlays on the path composed of serpentine medallions. The house itself is painted serpentine blue-green. Click the photo for a big version.

serpgarden450

The Oakmore district is uniformly mapped as Franciscan sandstone, but just a little farther east it’s mapped as undivided Franciscan, so we might expect a mixture of possibilities here. The neat lines on the geologic map are as much hypotheses as they are conclusions. Other outcrops nearby look like volcaniclastic rocks, and some of this home’s neighbors use it effectively. I conclude, though, that this home’s landscape was composed with imported stones rather than assembled from what was lying around. But the rocks may well be Oakland natives from just up the way, perhaps even from the Serpentine Prairie quarry.

Baldwin Street boulder

6 October 2013

Out in East Oakland at the corner of 85th Avenue and Baldwin Street is this fine, underappreciated boulder.

85th-ave-chert

To all appearances, it’s good old Oakland chert, hard at work. I assume it was put here to keep vehicles from cutting across the corner, or perhaps to keep a runaway vehicle out of the building behind it. Who knows? I was just glad it was there to break the monotony. It’s free of graffiti, too.

The other end of Baldwin Street is east of the Coliseum, where it serves as a back entrance for staff and athletes, at the edge of Arroyo Viejo. That’s the creek you cross when you’re walking from the BART station to the game. So between stone and water, Baldwin Street pays more homage to geology than most of its peers.


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