Archive for March, 2010

Shephard Creek Formation

29 March 2010

Moving up the stratigraphic section along Shepherd Canyon Drive, as seen in the Railroad Walk, we go from the Joaquin Miller Formation to the Oakland Conglomerate. (Because I’ve already posted three pages on the Oakland Conglomerate, I’ve put links to them instead on the Joaquin Miller Formation post.) Then comes the Shephard Creek Formation, separated from the underlying Oakland Conglomerate by a fault. It appears in this sloppy outcrop at the very beginning of the Railroad Walk under a tree.

shephard creek formation

Because the walk arcs into and then out of the underlying Oakland Conglomerate, you see the Shephard Creek Formation again at the far end of the big cut in this better outcrop. But beyond that the rock is hard to find.

shephard creek formation

It’s a unit of mostly soft, fine-grained rock, mudstone and shale. Shale is more or less pure clay, and mudstone is shale with some fine grit (silt) in it. The unit also has some siltstone and a few thin beds of coarser wacke. What stands out about it is that, as the mapper says, it’s “distinctly bedded.” The whole thing is of Campanian age, meaning the stretch of the Late Cretaceous Epoch between 83 and 71 million years ago, presumably early in that time. (The age is named for the well-studied rock sequence around Naples, Italy, in the Campania region.) It’s not really as blue as this hand specimen—blame the light from the sky for that—but it does get dark where thin coatings of iron minerals accumulate.

shephard creek formation

I would say that this rock does not support the steepest slopes. The canyon is a little wider here, along Shepherd Canyon Park, than it is upstream where the rocks change to the Redwood Canyon Formation and the hills grow higher.

Living on Bay mud

27 March 2010

The Tidewater district is the nose of land west of the freeway at the end of the channel between Oakland and Alameda:

tidewater liquefaction map

Right now it’s totally industrial, but landowners there want to open it up to residential uses, like the cozy parts of Alameda right across the channel. (Easy to catch up with the news by googling “oakland tidewater industrial“.)

I’m showing this image to point out a geological aspect to the zoning proposal. The map is a portion of the state’s official map of landslide and liquefaction potential, and it’s clear that this low-lying piece of mostly filled swampland is one of Oakland’s worst places in a large earthquake. Just saying.

Joaquin Miller Formation

18 March 2010

At my talk last night to the Friends of Sausal Creek, I delivered a lot of information about the rock units exposed along Shephard Creek. For a while here I will post what I showed the crowd, starting at the bottom.

The Joaquin Miller Formation is a thick sequence of mostly shale, around 95 million years old (Late Cretaceous Epoch, specifically the Cenomanian Age). It underlies nearly all the east side of the valley of Palo Seco Creek, running into Joaquin Miller Park. It weathers readily there, turning easily back into the clay it once was. This exposure is a roadcut at the intersection of Scout and Ascot drives. The beds are steeply tilted into the hillside, something that’s true of all the rocks in the canyon.

joaquin miller formation

And here’s a closeup.

joaquin miller formation

These rocks mostly crumble in the hand. Toward the top of the unit, it gets more sandy; an example is shown here from the bed of Shephard Creek. Eventually it turns to straight sandstone and gets a new name, the Oakland Conglomerate (here are three pages on that rock unit, 123).

The Joaquin Miller Formation was laid down far from land, but not very far. This is all brown clay that comes from continental sources, and the occasional sandy beds are evidence that underwater landslides could sometimes reach here. Picture it way out in the Gulf of Mexico, where Mississippi River mud can cascade down the continental slope for great distances.

Stone speaks

12 March 2010

Mountain View Cemetery is a celebration of stone even if you ignore the knockers scattered about the grounds. Granite and marble are the two traditional rock types used—or I should say, stone types because commercial granite and commercial marble aren’t always what the geologist would call them.

marble

Marble and granite combine at this lovely grave, translucent marble to evoke life and spirit, stern granite to document fact. (Click the photo for a larger version.) It seems a little odd, doctrinally, to depict an angel being sentimental about a human death. But those who mourned this woman of 37 surely wanted to suggest that even a fleshless heavenly being might give a thought to the injustice of her early death. On the other hand, the angel’s face is impassive, and its gaze is not downcast. The book in the angel’s lap says what it says. Maybe the marble is as stern in its way as the granite.

At first I thought this angel was the same one shown as the banner of the Mountain View People blog. The pose and costume are identical, and perhaps the same maker did them both. That angel looks a little more downcast, probably because Crocker was a more important person than Frances Schmidt. (Deborah Dash, administrator of Findagrave.com, names this as one of her favorite graves.)

The big rock show

7 March 2010

Sometimes I go out of town. Today I visited the annual show of the Mineral and Gem Society of Castro Valley, held down in Newark this year. For me, coming from the geological side of things, it’s dazzling kind of place.

mineral show

The hall was full of families, because there’s something for everybody at a show like this: raw materials and tools for the hobbyist, jewelry for those who wear accessories, fossils for the nerds, shiny pebbles for the young ones, specimens for the mineral collectors. They had a complete cave bear skeleton on display. Boulders and slabs of jade. Crystal spheres. And far-out stuff like the ultraviolet mineral collectors’ hall.

fluorescent minerals

All the light in this photo is fluorescence, including the scorpion on the left. These are from the collection of Lee McIlvaine, of San Leandro. I got a cool keychain blacklight for five bucks. You never know what you’ll find in the field.


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