Archive for the ‘Oakland stone’ Category

Peridotite-basalt lampposts

10 July 2017

The Lakeside Regency Plaza, 1555 Lakeside Drive, is the 15-story condo building next to the Scottish Rite Temple. Built in 1968, it’s definitely of its time yet of enduring taste. I paid it no mind until a few days ago, when I noticed the four artisanal lampposts that flank its driveway.

In their own way, these are even cooler than the serpentinite cladding I wrote about the other week. The large cobbles lining the shaft are peridotite, a stone that’s rare in the first place and not often usable in the second place. Peridotite (accent on the “rid”) is what the Earth’s upper mantle is made of. Because its minerals are more stable at depth than they are on the Earth’s surface, peridotite characteristically acquires an orange weathering rind as the iron content is released from its olivine and pyroxene crystals. It’s usually shot with veins of serpentine, which degrade its strength.

These cobbles are field stone, not quarried but gathered from the ground in their natural state. They probably came from a riverbed or talus slope in the Klamath Mountains.

The capstones, on the other hand, are lava — specifically, vesiculated (bubbly) basalt like that found in the volcanic Cascade Range. Lava is the opposite of peridotite, to put it briefly, the stuff of the Earth’s crust. These hand-dressed blocks may well have come from northernmost California too, in the vicinity of the Modoc Plateau.

Here’s a look at both rock types in closeup.

Or, of course, inspect the lampposts yourself next time you’re at the lake. I think of them as examples of deeply understated, perhaps even inadvertent, geologic wit.

Oakland building stones: Serpentinite

26 June 2017

In a modest West Oakland neighborhood on Market Street is the modest West Grand Shopping Center. Its ordinary building is clad in rough stone, an exterior treatment similar to the Kaiser Building and many other examples.

But at the West Grand Shopping Center, the cladding consists of fist-sized pieces of beautiful serpentine rock.

The front side of the building is pristine. The rear side, on Myrtle Street, is a full block long and completely faced with serpentinite. Unfortunately the bottom seven feet or so has been painted over.

The mutable color of this stone, blue-green in the shade and olive-green in the sun, gives the building a real Oakland look. I don’t know where the stone came from. Our own serpentinite is usually bluish and not of this quality, except maybe in small outcrops in the Franciscan melange. Perhaps it’s from a quarry in the Mother Lode country. It must have taken a few carloads of rock and a crew of skilled artisans to put this together.

A few months back, when I was presenting the building stone verd antique, serpentinite’s dressed-up cousin, I said “You can’t do much with California serpentine except admire it.” Makes me happy to be proved partly wrong — you can always admire it, and sometimes build with it.

Oakland building stones: Gneiss

19 December 2016

This is the last of my set of posts on Oakland building stones, although I reserve the right to come up with more. What you’re looking at below is gneiss on the wall of the Lionel Wilson Building, in City Hall Plaza.

gneiss

Gneiss is a fun rock, for me anyway, because when I see it I think, “Nice.” And that’s exactly how the word is pronounced.

Gneiss is made of the same kinds of minerals that granite is — quartz, feldspar, garnet, hornblende — and stone dealers call it granite. A large share of your granite countertops are actually gneiss.

The difference is that strongly directional fabric, in which the mineral grains are stretched and aligned and separated into bands and stripes. Gneiss, you see, is actually a metamorphic rock, squeezed like taffy under high heat and temperature.

That fabric, and nothing else, is what defines gneiss. The fabric is called gneissosity (and geologists will skunk you in Scrabble because they know words like this).

Here’s a gneiss boulder I used to keep as a pet. I gave it a new home by slipping it into the front yard of a home with lots of other cool rocks.

gneiss-pet

And here are two gneisses I photographed in New York. The first one is a garnet gneiss, a quarry-faced ashlar in the wall of a cemetery visitors center.

gneiss-garnet

And the other one is a wild boulder near Albany, the New York Albany.

gneiss-newyork

Gneiss is a stone of infinite variety. The pink-and-gray stone in the Wilson Building is probably Morton Gneiss, an extremely old stone quarried in Minnesota. David B. Williams, author of the very fine Stories in Stone, considers it America’s most beautiful building stone. I’ve posted other examples from Oakland here and here.