Archive for July, 2014

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.

Courtland Creek cut

19 July 2014

Courtland Creek runs just south of High Street; presumably the valley was a footpath long before High Street was laid out in the 1800s. It has the peculiarity of crossing the old alluvial fan without cutting out a floodplain, as shown here in the geologic map.

courtlandcreekgeo

I visited it a few weeks ago. As you go upstream along Courtland Avenue, this dirt road appears. Dirt roads are always interesting in this city.

courtlandcreekrow

It’s the old right of way for the Key Route line, and it leads to Courtland Creek Park, a cool streamside strip with some understated concrete work meant to evoke the history of the area. At one point there’s some unusually elaborate rockwork leading down to the creek itself.

courtlandcreekrockwork

Farther upsteam, too, is a cut into the side of the Maxwell Park hill; this view is looking back west.

courtlandcreekwall

And at the upper end of the park is one of those excellent mosaic trashcans that make this city so special.

courtlandcreekcan

As I’ve mentioned before, the topography of this part of Oakland, in the Allendale flat, suggests to me that the drainage has switched between streams at various times. It will be fun to poke around here some more.

BTW please see the Q&A page for an announcement.


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