Archive for May, 2014

Salt weathering (tafoni)

11 May 2014

San Francisco does a few geological things better than Oakland. Here’s one.

tafoni

Salt air, specifically salt spray, causes this dramatic pitting in sandstone of the breakwater out by the Golden Gate Yacht Club, on the way to the Wave Organ. San Francisco simply gets more of that raw sea wind that splashes seawater onto the rocks. As the brine dries out, salt crystals grow in the pore spaces and gradually pry the mineral grains apart. This process, called cavernous or honeycomb weathering, affects inland sites where the rock itself contains some salt (Mount Diablo and Las Trampas Ridge have good examples), but along the coast it acts much faster and more pervasively. The hollows are often called tafoni, however that term is properly used for large hollows. Little ones are called alveoli.

You might see some of this in Oakland rocks, but only down in the port, if there. I can’t think of a good example. The riprap boulders in our port are mostly hard igneous rocks that are much more impervious to water.

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.


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