Archive for September, 2008

Fossil hunting

25 September 2008

There aren’t many fossils to be found in Oakland. Maybe microfossils—there must be lots of those in the young hilltop rock units. But on the geologic map is a little body of rock up in Montclair that is supposed to have “well-preserved coral fossils” of Paleocene age (that’s 65 to 55 million years old). Recently I checked it out.

outcrop

The rock unit has no formal name; it’s mapped simply as “unnamed glauconitic sandstone” and extends in a belt from Shepherd Canyon north over the ridge to Snake Road. This outcrop is at the intersection of Paso Robles Road and Shepherd Canyon Road. I examined it carefully and found nothing but massive fine sandstone, with almost no bedding and no sign of fossils of any kind.

outcrop

I traversed all the roads in the area where this rock unit was mapped, and the same sandstone was everywhere I looked. At this outcrop on Paso Robles Road, weathering and lighting combined to bring out some subtle bedding planes, but again no sign of fossils.

outcrop

At the north end of the belt, across Snake Road, is Armour Drive. Here is a large landslide scar that has broken Armour Drive in two, and there was plenty of loose rock for me to apply my rock hammer and take home a hand sample. This kind of rock is all I could find, period. But the unit is said to be “coarse-grained, green, glauconite-rich, lithic sandstone” interbedded with “hard, fine-grained, mica-bearing quartz sandstone.” That’s this stuff.

Somewhere in this belt is hiding a bunch of green, gravelly rock with coral fossils in it, and I haven’t looked hard enough to see it. On the other hand, geologic mapping is an imperfect art, and much of the hills has been mapped on the basis of aerial photos with limited work on the ground. Because I work exclusively on the ground, I’m in a position to do better. Maybe the fossils are only along one edge; maybe they belong to a separate subunit that needs to be mapped more precisely. Maybe someone made a mistake. So far, it’s a puzzle.

Northside knockers

18 September 2008

northside knockers

I made a brief reconnaissance of the land just north of Mountain View Cemetery last week and captured a few more knockers. Poison oak is abundant here, I should report. It’s a lovely thing to see pristine outcrops, although the geologist in me wished for a handy rock hammer. In its absence, I can only say that this and the next knocker looked like typical sandstone or greenstone from the Franciscan mélange making up the area.

northside knockers

This next knocker is definitely chert of the high-grade type found elsewhere in Upper Rockridge. Notice that it supports less vegetation, being poorer in nutrients.

northside knockers

My reconnaissance was brief because the area is undergoing development, and while I didn’t notice a no-trespassing sign where I passed the fence I felt the need for discretion. The best knocker of the area, unfortunately, is under attack (click full size):

northside knockers

The upper body of red chert is still living bedrock while the nearer pile is loose pieces of it. I don’t know what is being planned for the area, but the chert would look nice in a fairway, or left in shabby gentility like some of its siblings.

Seeing the hills

14 September 2008

hills from jack london square

I’ve always said about Oakland that wherever you are in the city, you can walk less than a hundred feet and catch sight of the hills. But in some places, like Jack London Square, you may have to pay attention. (click for big version)

Cal Stadium and the Hayward fault

10 September 2008

I mentioned UC Berkeley’s Andrew Lawson in connection with the Claremont Shale. Lawson also recognized, mapped and named the Hayward fault. It was very well known that the fault crossed the upper part of the Cal campus, and Lawson was not shy about it in 1921, when the University planned its big new stadium right atop the active trace. Like all geologists, Lawson knew that in a bet against nature, nature tends to win, but as a professor he also knew that in a bet against the university, professors tend to lose. So Cal Memorial Stadium sits where it sits, being slowly pulled apart by aseismic creep. (Lawson’s own home, at 1515 La Loma Avenue in Berkeley, was designed for earthquake resistance by Bernard Maybeck.)

Here’s the canonical view of the offset looking up from the parking lot at its south end.

cal stadium

Immediately east is this chunk of lumber amid the stadium’s reinforced concrete. It’s important to remember that good codes and good designs don’t ensure good construction.

cal stadium

Underneath the structure, there are cracked columns in many places. This one, cracked on its south side, shows that the ground is being carried north while the stadium, being a fairly rigid structure, is stationary.

cal stadium

The stadium is actually built in halves, with the idea that during an earthquake the two sides would gently slip past each other. No one knew about creep at the time. These two columns have been pulled out of parallel over the years.

cal stadium

Lawson may have thought that the stadium design was OK. He was not aware of much we have learned about faults and earthquakes since his time.

The real Claremont chert

7 September 2008

claremont chert

The Claremont chert actually has the official name Claremont Shale, and I should stop calling it Chert-with-a-capital-C. The rock unit was first named by Andrew Lawson, UC Berkeley’s memorable professor of geology who masterfully conducted and wrote up the scientific studies of the 1906 earthquake. (Lawson has the mineral lawsonite named for him, too. I hope to find some in Oakland some day.) Lawson named the Claremont Shale because the unit is predominantly shale, even though in Claremont Canyon, as in the rest of Oakland, it’s quite cherty.

When geologists name a rock unit for the first time, they designate a type locality and take the name from that. They also designate a type section, a specific place where anyone can visit and check the definitive example of the XYZ Formation. They draw a detailed stratigraphic column from the type section—it’s sort of like an engineering drawing, very formal. And they publish a detailed description of the unit and the stratigraphic column in a reputable journal or government-issued publication. The type section is supposed to display the top and the bottom of the formation and, ideally, everything between. The US Geological Survey has a panel of experts who do nothing but keep track of geologic names, both for rock formations and for the Eocene/Jurassic/etc. geologic time units. I used to work across the corridor from one of them, and he was quite the specialist.

So Claremont Canyon is the type locality for the Claremont Shale. Lawson’s original description is in US Geological Survey Geological Atlas Folio 193, published in 1914. The UC Geology Library probably has a copy. And this exposure of the chert along upper Claremont Avenue must make up part of Lawson’s stratigraphic column in that old atlas.

Sheffield soil

3 September 2008

soil profile

At the end of Covington Street, in the Sheffield Village neighborhood, you’ll find a little parking area and a public access to the Dunsmuir Ridge Open Space. (It’s also the back entrance to the Dunsmuir House property.) Take the dirt road up the hill and you’ll pass this textbook exposure of the soil profile: dark topsoil on top, pale subsoil beneath. But keep going; this is one of Oakland’s most secluded public spaces. Sheffield Village is one of Oakland’s prettiest neighborhoods, all built in the late 1930s except for the higher houses on upper Revere Avenue, above the Hayward fault.


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