Archive for the ‘oakland geology puzzles’ Category

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.


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.

Eastmont hill

20 January 2013

The upper Eastmont 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.


Eastmont 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 82nd Avenue 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.

Longridge loess

21 August 2012

I was walking up Longridge Road and spied an excavation, where a homeowner was replacing some water lines and renewing a driveway. Naturally, I sidled over and took the rare chance to look beneath the skin of Oakland’s Pleistocene fan. The material was massive—unbedded—and clean. I pried off this little piece . . .


. . . and nibbled on it. It was firm, but crumbled like Necco wafers and turned creamy on the tongue with just a hint of grit. Not sticky or chewy with clay. Not indurated like hardpan. No sand or pebbles to be seen. The more I thought about it, the more peculiar this sediment seemed, until I had a wild surmise.

Alluvial sediment is never very well sorted, because it’s carried short distances and laid down by streams. Longridge Road is, as the name suggests, a ridge road running up the crest of a ridge between parallel stream valleys along Trestle Glen and Mandana roads. The crest of a ridge should not be made of this fine silt. But it’s downwind from downtown, which is Pleistocene sand dunes (the Merritt Sand), cousin to the dunes of San Francisco. Dune sand is very fine sand, and the fraction that blows away from the sand is finer still. So my wild surmise is that the fan, at least this part of it, is dusted with a layer of windblown glacial silt—i.e., loess. It’s remarkable stuff, and something I never expected to see in Oakland.


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