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.

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

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.

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.

Tuxedo terrace

12 May 2013

The Fan, my name for the lower hills in central Oakland, has a lot of subtle topography that I’m getting to know as I ramble over its contours. The little valleys are one feature I enjoy perceiving, but the places between them are interesting too. The San Antonio lobe of the Fan, between 14th Avenue and Fruitvale, has a flat top at about 200 feet elevation. This is in the Tuxedo neighborhood, looking down 21st Avenue toward the bay. 22nd and 23rd are the same way.

tuxedoterrace

There doesn’t seem to be a reason for such a flat stretch on an ordinary alluvial fan. Fans slope; that’s why they’re fans. I have to assume that the ground was not excavated flat but is naturally that way. Is it possible that this is a relict wave-cut platform, similar to the Clinton marine terrace but higher and older?

Arguing against that hypothesis, the height is problematic. On the other hand, the East Bay hills are rising and so may be the land west of the Hayward fault. It may be rising in fits and starts (meaning in episodes measured in thousands of years). The next thing I want, and have wanted for a long time, is a really accurate terrain map of Oakland. It would look like the standard digital elevation model of Oakland but would be compiled from lidar data and be accurate to a centimeter or so. Maybe my eyes are fooling me; after all the street does slope a little.

Crestmont hill

20 January 2013

The Crestmont neighborhood is centered around this hill, the one on the left. We’re looking north at it from King Estates Open Space. It’s the highest bit of land in Oakland west of the freeway.

crestmonthill

Crestmont hill stands just over 500 feet high, with Crest Avenue running along its crown. The hill is mapped as the same stone as in the old Leona Quarry, which you see on the right in the background: volcanic rocks of the uppermost part of the Coast Range ophiolite. I say “mapped as” because I haven’t seen a bit of bedrock on it. Perhaps building excavations uncovered it.

The ridge runs south, beneath my feet and beyond past Fontaine Street, where Crest Avenue picks up again. But everything south of Crestmont hill is mapped as sediment instead, an older unit of alluvial-fan roughage. The gravel I’ve seen on its upper surface looks like chert of the Claremont Shale. I guess I’m rambling. This area puzzles me. How much of its shape is due to warpage by the Hayward fault, which runs parallel to this ridge just west of the photo? How old is the alluvium, and what stream delivered it here? How much of the map is real and how much is extrapolation? It is likely that my questions are unanswerable.


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