Archive for the ‘Oakland fossils’ Category

Mammoths in Oakland

16 December 2012

The skull of a Pleistocene mammoth appears on this building at Telegraph and Sycamore.

mammothsycamore

Oakland’s youngest rocks are Miocene in age, around 10 million years. Next in age come the gravels and sands of Pleistocene time, within the last 2 million years or so. (I’m vague about this number because the definition of the Pleistocene just changed, and because I don’t know any firm dates for these sediments.) Any material from the time in between either was erased by erosion or never existed.

Because the great glaciers sucked more than 200 feet of water out of the ocean, Ice Age Oakland was part of an enormous grassland that extended across today’s Bay and far west of today’s coastline. State Parks paleontologist Breck Parkman has called it a California Serengetti (sic), teeming with large herbivores and predators to match, all of them extinct today except the condor.

Remains of these animals have been found in Oakland. When the Alameda Tube was being dug, fossils of ground sloths, short-faced bears, mammoths, camels and bison were recovered. Mammoth remains have been found in the Harris Street Tunnel, Montclair Playground, and 81st Avenue. A Pleistocene horse was found at Oak Knoll Hospital. A ground sloth was found at the Coliseum. When the next large-scale excavations happen in Oakland, the diggers may find these as well as fossils of dire wolves, mastodons, deer, llamas, pronghorns and smaller ground-dwelling mammals.

Rocks of the Caldecott Tunnel

23 November 2012

I mentioned a few posts back that I would be doing a field trip related to the new bore of the Caldecott Tunnel. Here’s how it went.

We (the members of the Northern California Geological Society) started with a talk at the Caltrans headquarters in Lafayette. They showed us historical photos and engineering diagrams, explained the New Austrian boring technique, and ascertained that we weren’t interested in a “photo opp” near the tunnel entrance. Apparently that last is a prime demand of many tour groups, but we didn’t care.

First stop was at the old tunnel entrance, which I’ve shown here before. The old plaque has been refurbished and looks garish now.

old tunnel

The tour leader unfurled a huge geologic map of the tunnel and talked a lot about all the rocks (click the image to see the 1000-pixel version). I studied it closely, because it’s the best we have for this part of town.

tunnel geologic map

The blue stripes on the west side of the map are the Sobrante Formation, the green, purple and tan stripes are the Clarement Shale, and the buff stripe on the right is the Orinda Formation. These are stacked up in order of age, old on the west, and tilted nearly straight upright.

Next we wound our way around to the extreme top of Broadway, next to the tunnel’s west entrance. Here’s the view from there looking north.

The rocks in the immediate area are in the putative Sobrante Formation, but the Claremont formation takes over just to the east (right). Here’s the Sobrante exposed at that spot.

sobrante formation

It’s pretty crummy rock, and the tunneling was quite slow there. Every couple of meters, the tunnelers drilled a fan of holes over the roof of the upcoming dig segment, each one reinforced and grouted. That reinforcement allowed the tunnel roof to stay up long enough to spray it with shotcrete, which in turn held up the roof until the real concrete tunnel lining could be emplaced. The tunnel’s final shaft is round, as strong as an eggshell in its resistance to the earth’s weight. (To see what I mean, try gripping a raw egg firmly in one hand, fingers on all sides, and attempt to crush it. You can’t do it.)

Next we visited the type locality of the Claremont Shale, where I learned something new about it.

claremont shale

The unit here is mostly thin beds of chert and thinner interbeds of shale, but there are also lumpy beds in it—see one running down the center and another one near the right edge. Those are dolomite. It turns out that carbonate and silicate minerals dislike each other enough that they tend to segregate themselves: in chalks, you get flint lumps, and in cherts (as here) you get limerock lumps.

Our last stop was on Fish Ranch Road just east of the tunnel, where we looked at the Orinda Formation in the surrounding hillsides. The Caltrans people brought along a box of the fossils recovered from the fourth-bore project, so I got to lay my hands on this Miocene horse tooth . . .

horse tooth fossil

. . . and this slab of fossil leaves.

leaf fossils

The Caltrans public-information official was at obvious pains to say how happy the paleontology contractors were and how well the fossil collecting had gone. Obvious to me, anyway. I won’t forget the complaints I heard from the other party, but I will keep in mind that most stories have at least two sides, and that every fossil preserved is a victory over the universal decay of the world.

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.

Firm clay

13 April 2008

clay

On two occasions I’ve spotted construction sites on the fringes of Haddon Hill, once at the corner of MacArthur Boulevard and Wesley Avenue and once at the west end of Brookwood Road. I asked what the ground was, and both times the owners said the same thing: “firm clay.” There is no bedrock to speak of west of the Hayward fault, outside the Piedmont block and Toler Heights, up where 98th Avenue ends. It is safe to say that any Oakland neighborhood named “Heights” or “Highlands” has some bedrock under it, and some of the “-monts” do. But the rest of the hilly places that adjoin the flats are firm clay, with maybe a little sand and gravel. If the slope isn’t too steep, this soil is good for building.

All are part of a large alluvial fan dating from late Pleistocene times. It stretches from Pill Hill to Evergreen Cemetery, and its closest approach to the Bay is here at San Antonio Park, overlooking Coast Guard Island.

san antonio

Its sediments are said to contain “extinct late Pleistocene vertebrate fossils.” I haven’t read the literature, but that could mean anything from Ice Age mice to the mammoths, horses, camels, sloths and bison known from other Bay area sites, not to mention some extinct great cats. It’s worth keeping an eye on this stuff.