Marine terrace in downtown Oakland

28 December 2015

mar-terrace-grand

Oakland’s ice age features are subtle and easily overlooked. I’m here to help you see them.

The photo above looks west on Grand Avenue from the tip of Lake Merritt. As you pass Harrison Street heading toward Broadway, you climb a low rise about 20 feet high. That rise is a deposit of sediment that was laid down when the sea was higher than it is today. It’s shown on the geologic map below as Qmt, for Quaternary marine terrace.

mar-terrace-geomap

“Quaternary” refers to the age; the Quaternary Period started 2.6 million years ago, when a long series of ice ages began that extends to today, and encompasses the Pleistocene and Holocene epochs. “Marine” says that the feature formed in the ocean. “Terrace” means that it was built upward from the base — it’s a pile of sediment, as opposed to a wave-cut platform, another flat-topped coastal landform.

Oakland has three areas of Qmt, the largest being under the old town of Clinton and the second being under Lakeside Park. Those are easier to see than this third area, which I would call the Uptown-Valdez terrace.

As you approach the lake on 21st Street, you can see how its elevation matches the terrace across the water in the park. The terrace is a little over 20 feet in elevation here and is very flat.

mar-terrace-21stSt

The western edge of the terrace is invisible. Here’s my best effort to show it, looking up Telegraph Avenue on a clear day with little traffic. View it full size (1000 px) for the best experience.

mar-terrace-Tele

I was standing at 17th Street. The nearest cars are at 18th Street. Behind them the ground slopes down to 20th Street, where the pavement turns fresh. Beyond that, the crosswalk is at West Grand Avenue, and the new pavement ends at 27th Street. Telegraph takes the slightest turn to the right at 30th Street, under the “Qpaf” mark on the geologic map. The freeway crosses just past 35th Street.

In all this territory, the elevation changes hardly at all, except in the foreground. That downramp between 18th and 20th is the edge of the Merritt Sand, the ancient dunefield that underlies downtown proper. The creators of the geologic map presumably mapped the marine terrace in this area with the help of engineering reports, well records and the like, because the landscape gives no sign of its presence.

Shale and chert in the Claremont Shale

21 December 2015

claremont-shale

In preparation for my upcoming talk (billed as “drunken education”) for Nerd Nite East Bay, I took a three-hour tour of the Oakland hills to take fresh photos of my favorite rocks. The tour was a whirlwind one.

This is the classic exposure of the Claremont Shale in the valley of Claremont Creek, which I’ve featured here before. On this day, the light on the freshly rain-washed stone was irresistible.

In these hills, the Claremont is mostly chert — that’s the lighter colored, blocky stuff. But it’s larded with thin, soft layers of brown shale, a rock type defined as a more or less pure claystone that breaks in sheets. The layer at my hammer’s point shows how quickly the shale weathers into flakes. Most exposures don’t offer such a clear view of the shaly portion.

Clay is a sediment that comes only from the land. The sediment that makes chert is the microscopic shells of diatoms; you could say that chert is a pure precipitate of seawater. It’s intriguing to see the two so close, and so separate. Not to mention the force that so carefully tilted all of this rock upright from its original horizontal position.

With ten places to visit that afternoon, I couldn’t linger. An hour here to ponder the details would be more to my liking.

Geology meets art in the Oakland Museum

14 December 2015

suiseki-rivera

The Oakland Museum of California just opened an exhibit called “UNEARTHED: Found + Made,” and this is the first thing you see when you go there. It is a suiseki stone, collected by the late Felix Rivera in the California desert and prepared and mounted by him according to artistic principles codified centuries ago in Japan. When you visit the show, slow way down at the entrance and drink in its form, colors and presence. The exhibit features about twenty more of these rugged individuals, all exquisitely lit.

I took this photo months earlier under fluorescent lights in a back room at the museum. More about that later.

That’s the “found” part. The “made” part is a set of works by Jedediah Caesar, which complement the suiseki in an off-kilter way. He picks up things off the ground, too, mostly things that are not rocks. He may mix them into a vat of liquid plastic, let it set, and then saw the resulting block into slices, like building stones. Those are on the walls. This larger piece is on the floor.

Jed-Caesar-art

If I have this right, Caesar mixed turmeric into the plastic, which caused a vigorous reaction much like volcanic gases might produce in magma. In any case, his works have a certain geological cast and are a feast for the eyes.

I took that shot with my phone at the opening reception, where I was an invited guest. The museum staff had sought my help as they were preparing the exhibit. You know how for every piece of art, they say what it’s made of? “Albumen print.” “Stainless steel.” “Oil on canvas.” They wanted me to help them do that with the suiseki — you know, like “pegmatite on wood stand.”

Now suiseki collectors don’t know this stuff. Mostly they ignore geology, as they should. To talk about a suiseki’s rock type is to miss the point of the art. And the whole point of geology is to observe rocks, not appreciate them. But I made my best effort to “identify” the stones, relishing the absurdity as I did so. And if you disagree with the names I chose, I’m sure you’re as correct as I was.


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