Hayward fault slickensides

9 June 2014

Here’s an odd curio that I spotted down in Menlo Park at the U.S. Geological Survey campus. During a trenching study of the Hayward fault up at Point Pinole, the active trace was uncovered and this plaster cast was made. It sits by the entrance of the Map Sales Room in Building 3.

HF-slicks

Today we would make a lidar scan of a surface like this to document its slickensides in a precise database. But you have to feel some affection for this low-tech artifact, which presumably has also been scanned to ensure its immortality.

Lion Creek restoration

5 June 2014

Down at the mouth of Lion Creek, at what most of us still think of as Coliseum Gardens, the authorities have undone a bit of historic damage to the habitat. A rehabilitation project dug a new channel next to the existing culvert and installed water gates at both ends to manage the flow—brackish tidal water at the Bay end and floodwater at the hill end. After four years, it’s looking the way it was intended. Here’s the view downstream from the Lion Way overcrossing, with the Coliseum in back and the Lion Creek Crossings community all around.

coli-gardens-creek

Here’s the map view. The airphoto is kinda old, but it shows you the plan.

coli-gardens-map

The ground where I was standing is mapped at about 8 feet elevation. The other end of the park is approximately where the historic coastal marsh started, so they’re doing the right thing for this location. The culvert is still there to handle floods, but a real creek bed evolves to coexist with floods. So what we have now is sort of a zoo creek. I’ll take it over what was there before.

Cost estimates vary from $4 to $5 million to create this acre and a half of habitat. Looked at another way, that’s what it costs to lose a plain old natural creek bed, doing what it does best.

More from the City of Oakland

Alameda County calls it a “natural channel”

Alameda County Flood Control district calls it “a natural bypass creek”

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.

Manzanita ridge

28 April 2014

The ridge traversed by Manzanita Drive is one of Oakland’s highest residential areas, higher than 1400 feet. It’s the home-studded rise in the Oakland skyline to the right of Round Top in the blog’s banner image. Here’s the topographic view, from Google Maps:

manz-ridge-topo

. . . and here’s the matching part of the geologic map:

manz-ridge-geo

The ridge crest is held up by the Claremont Chert (Tcc), while Skyline Boulevard and Arrowhead Drive, running parallel just to its south, go through the crumbly Sobrante Formation (Tsm). (The other formations are the Orinda Formation, Tor, and the unnamed Eocene sandstone of upper Shepard Canyon, Tes.) Here’s the chert exposed at the west end of Manzanita; it and the next two photos click bigger.

Click for 800-pixel version

The views are terrific. Here’s the view north over the Huckleberry saddle to the smaller ridge of Claremont Chert that Elverton Drive skirts.

Click for 1000-pixel version

The middle of Manzanita offers fine views of Round Top.

Click for 800-pixel version

And the other end of Manzanita, past the unexpected Hills Swim & Tennis Club, is another saddle at the top of Shephard Canyon where the road to Canyon and Moraga crosses the hills.

skyline-shep-pinehurst-saddle

More than anywhere else in Oakland, this is an island in the sky.

Arroyo Viejo emerges

13 April 2014

Quietly, at the edge of the Coliseum station parking lot, Arroyo Viejo comes out of hiding from beneath Hegenberger Expressway. It runs under the walkway to the Coliseum and joins Lion Creek just short of the bay.

arroyoviejomouth

Even in its coffinlike culvert, the stream wants to curve, laying a gravelly point bar on its left bank and trying in vain to erode the angle of the culvert’s course into a nice meander. Sorry, old creek.

Local stone

31 March 2014

I always get a kick from old walls around Oakland that are made of local stones. This one is on Loma Vista Avenue, in the upper Laurel.

lomavista-stonewall450

Click the image to see a 1000-pixel shot of the whole thing. The mix of blue Franciscan rocks, golden Tertiary sandstones and the occasional reddish chert is distinctive, and it’s nothing that a local landscaping yard would ever offer. Its charm is homely and understated, but authentic.

We haven’t had a working quarry in Oakland for many years, so a lot of these walls are old, or rebuilt. Lovers of local stone today have to scavenge what they can from recycled rocks or their own cellar holes.


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