Archive for the ‘Oakland geology puzzles’ Category

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.

The Hayward fault in Redwood Heights

3 May 2014

It happens that a commenter asked about the Hayward fault hazard in an area that I surveyed only yesterday, just south of the part of town I discussed back in January under “The 35th Avenue cut, Jordan swale and the Franciscan spike.” This post looks strictly at the Hayward fault between 35th and 39th Avenues and not, as I usually prefer, at the bedrock (there’s almost none to be seen here anyway). Here’s the fault trace, as mapped in 1992 by the US Geological Survey.

RedwdHightsfaultmap

First, note that the map is tipped clockwise to make the fault run vertically. The fault is mapped here with an uncertainty of less than 40 meters; the ticks on the dashes indicate the downhill side. To help orient you, here’s much the same area in Google Maps, tilted to match.

HF-RedwoodHights-map

The two arrowheads mark where the fault is mapped and the two numbers are the localities I’m showing below. The lower arrowhead coincides with the fiduciary mark on 39th Avenue.

The first locality is the lower end of Dunsmuir Avenue at Victor Avenue, where the street curbs are offset to the right.

HF-dunsmuir-victor

The offset is more subtle on the south side (to the right in this view). The pavement on the corner is cracked, but that can happen when a garbage truck cuts across it, to name just one possibility. But there is an offset there, along with a recent sawcut to help gauge any motion there.

HFmark-dunsmuir-corner

To the north, the next street to cross the fault trace is Atlas Avenue. The curb is offset there, too.

HF-atlas

This is marked on the 1992 fault map with the notation “C2,rc,rs,ec.” That signifies “distinct creep evidence, right-laterally offset curb [and] sidewalk, en echelon left-stepping cracks in pavement.” The cracks are not evident now; presumably they were where the pavement has been patched. Offset concrete is harder to hide. (On 39th Avenue the code also includes “right-laterally offset fence line,” “surveyed offset feature” and the code L91 for a particular report documenting the survey.)

The features marked “G” are geomorphic ones, G2 for “distinct” and G3 for “weakly pronounced.” The codes are as follows: sl, linear scarp; lv, linear valley; ss, swale in saddle; df, fault-related depression.

The land is weird here. Drainage from the reservoir area appears to run north instead of west, perhaps feeding the head of Courtland Creek just south of 35th as I envisioned it happening from north of 35th too. If that’s correct, it would be another example of an offset streamcourse (and a corresponding shutter ridge). As earthquakes and creep affect the landscape here, water could shift from one drainage to another in this area where three different creeks run very close to one another.

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.


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