Archive for the ‘oakland rocks’ Category

Geranium Place rocks and runoff

27 July 2014

Geranium Place occupies a sloping bit of land just north of Horseshoe Creek below Redwood Road. This map shows the location plus the sites of the photos in this post.

geraniummap

The bedrock here is mapped as Franciscan melange on the west and the Leona volcanics on the east, the same stuff exposed in the huge Leona Quarry just down the Warren Freeway. The rocks I saw were clearly the latter, but you have to take the lines on the geologic map as approximations. If I’d been there fifty years ago when the homes here were being built I might have been able to tell better, while the foundations were exposed. BUT! there is bedrock to look at in any case.

The first thing that caught my eye, though, was the gutters. What’s going on here?

geranium1

Iron-rich runoff like this is not expected from undisturbed land. Perhaps the area was once a borrow pit or small quarry. Across Horseshoe Creek is the big quarrying operation above Laundry Canyon, and just beyond is the notorious McDonell sulfur mine site, so naturally the resemblance to the nasty-looking streambed down there is striking. What etched away the cement in the gutter? Probably not acid drainage.

geranium2

The white crust is another clue. In our deathly dry conditions it might conceivably have been salt, but it had no taste when I nibbled a fragment of it. It’s probably either gypsum or carbonate; without any chemicals handy I couldn’t learn anything more. But a buildup of crystals like this could gradually disintegrate the cement in the gutter. It may also be seepage from the serpentine exposed along and above Redwood Road here. In sum, very hard water here, but probably not nasty water.

The bedrock is heavily iron-stained and chewed up. Being so near the Hayward fault (it grazes the lower left corner of the map area) surely accounts for that.

geranium3

Some of the households here have worked with it to good effect.

geranium4

A closeup is impressive.

geranium5

This is breccia—pervasively shattered rock—that has been abraded by tectonic shearing so the pieces are rounded, as if you took crushed rock and rubbed it between your hands with a giant’s strength. It’s fairly well cemented, not crumbling apart, so this process happened at some depth.

At the northernmost bend of Geranium, up against the highly cantilevered Redwood Road, the ground is empty and there are monitoring wells of some sort. We have deeply disturbed this area.

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.

headroycequarry1

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.

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.

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.


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