Oakland building stones: Verd antique

5 September 2016

California’s state rock is serpentine, and in this blog I’ve picked up, photographed and defended it against detractors. I’ve called serpentine beautiful, adorable, inspiring and more, but I’ve never called it a building stone. However, it can be.

Serpentine in its Sunday best is called verd antique. This closeup is from the iconic I. Magnin building at Broadway and 20th Street.

verdantique-closeup

It resembles a particularly psychedelic marble — a translucent deep jade-green shot with intricate white veins. Petrologically, though, it is nothing like marble and a lot like soapstone. It consists of magnesium-based silicate minerals that are harder and more chemically robust than the calcite or dolomite that makes up marble.

Verd antique is all over downtown Oakland, usually as a subsidiary element at the sidewalk level. The building on 17th Street between Telegraph and Broadway has a particularly large expanse of it.

verdantique-17thbway

The Fox Theater building is another notable example. But in this post I want to draw attention to four buildings that do more with verd antique. The I. Magnin building (built 1930) naturally comes first, its entire first story richly faced with this stone.

verdantique-magnin

The Foulkes Building at 419 15th Street (built 1924) has a high facade of verd antique with bronze decorations.

verdantique-15th-st

The Moyles-Kappenman building (built 1928), longtime home of Lobe & Velasco Jewelers, has a lovely front of verd antique on 1617 Broadway and an identical one at its other end on 1618 Telegraph.

verdantique-1617bway

Finally we have the otherwise undistinguished Wells Fargo building at 2040 Franklin Street, where the public-facing elements (entrances and ATM) are framed in verd antique.

verdantique-franklin

The reason that all of these stones look the same is that for nearly a century a single quarry in Rochester, eastern Vermont, has been supplying the trade with Vermont Verde Antique(tm) stone. It is geologically special because it was squeezed two separate times, like twice-cooked fries, between colliding tectonic plates during the Paleozoic Era. As the supplier’s website puts it, it owes its origin to “highly sheared ultramafic rocks that have been rewelded and metasomatized by the process of serpentinization.” The lighter-colored “Cardiff Marble” serpentine from northern Maryland was once popular but is no longer produced. That stone is found in the White House’s Green Room, the National Archives rotunda and other places in Washington DC.

Verd antique gets its name from the Italian verde antico, “ancient green.” It was popular in Byzantine architecture. Oakland has a few pieces of European-style, brecciated verd antique to be seen here and there.

The stone industry considers verd antique a type of marble (it’s metamorphic and takes a polish) and follows ASTM standards for its manufacture, but verd antique is really its own beast.

serp-sierra

You can’t do much with California serpentine except admire it. For an example, see the ecstatic response of an out-of-state geologist to San Francisco’s Marshall Beach exposures.

Oakland building stones: Granite variations

29 August 2016

granite-wilsonbldg

Oakland’s buildings embody history and geology in the different types of stone that compose their facades. So how about granite?

California began producing granite soon after statehood, in a stonecutting district near Sacramento where rail and barge transport was handy.

At the time Oakland’s third City Hall was being designed, around 1910, many large quarries in the Sierra were supplying the market, and the architects chose “Sierra White” stone from a quarry near Madera, probably the McGilvray Quarry. The same district still produces Sierra White granite for the great Coldspring empire.

granite-cityhall

This fine-grained granite is excellent for carving and for resisting the attacks of urban pollution. Its neutral color works well with the building’s superb ceramic tiles and marble. Many of San Francisco’s notable buildings (the St. Francis Hotel, Bank of California and Old Custom House among them) also use Sierra White.

Buildings of later vintage take advantage of stone from more distant sources. The Lake Merritt Tower building at 155 Grand Avenue, built in 1990, features a splendid granite with extremely coarse grains, not typical of California. The granite benches around the building use the same stone in a polished finish that displays it especially well.

granite-bench

The building itself employs the same granite with a flamed or thermal finish. It’s produced by running stone slabs through a set of gas-fired torches. The sudden thermal expansion shatters the minerals and, properly controlled, yields this rugged surface.

granite-lakemerritt-tower

The Caltrans building next door, formally named the Nicholas C. Petris State Building and finished in 1992, is faced with a pink granite from Texas in both polished and sandblasted finishes. Here’s a closeup.

granite-caltrans

What are we looking at here? Time for the geology part. First of all, to stone dealers “granite” means only one thing — a coarse-grained rock (1) hard enough to take a good polish and (2) not marble, serpentine or quartzite — but that thing is a very broad one. Stonecutters came centuries before geologists, and what they say goes.

Geologists don’t call every granite building granite. “Black granite,” for instance, is gabbro. Scrupulous geologists would call all the stones in this post “granitoid,” meaning one of a broad set of coarse-grained rocks made of feldspar and quartz in various proportions. Real petrological granite has a narrower set of proportions. None of these stones has the 20 percent quartz that would qualify it as true granite. I know you’re disappointed. But the feldspar makes a great building stone.

In the picture above, quartz is the clear stuff that looks gray. Feldspar comes in two main types. The white grains are plagioclase feldspar, and the pink ones are alkali feldspar. (A pink color is unusual; it ranges from white to buff to red.) The black grains are typically hornblende, biotite (black mica) or both.

The gorgeous Texas Pink granite of the Lake Merritt Plaza complex, at 20th and Harrison streets, shows these minerals more vividly.

granite-lakemerritt-plaza

I do think this is Oakland’s most beautiful building stone. Texas Pink is a brand (not the kind you burn on steers), also from the Coldspring empire. It’s quarried from Granite Mountain, about an hour out of Austin and near the amazing dome of pink granite preserved in Enchanted Rock State Natural Area.

Texas Pink granite was also used, a bit more adventurously, in the AC Transit headquarters building at 1600 Franklin Street.

granite-actransit

I don’t know in what year the building was constructed, but I have the impression that Oakland architects are finished with Texas Pink granite for the moment. Metal and glass is the default exterior of today’s large urban buildings. In their lavish lobbies and interiors, though, stone is still the right stuff.

The Merritt Sand

22 August 2016

merritt-terrace-madison

Downtown Oakland sits on an unusual bit of geology — a large dune field mapped as the Merritt Sand. San Francisco is famous for its sand dunes, of course, and Points Reyes and Año Nuevo have some too, but the dune fields of Oakland and Alameda are the only ones within the bay. Here they are, labeled “Qds” on USGS Map OF-00-444, which shows the young (Quaternary) deposits of the Bay area.

dunesandmap

They’re just like the dunes in San Francisco. They formed during the ice ages, when the shoreline was out near the Farallon Islands, the Bay was totally dry and the Sierra was full of permanent glaciers (on not quite the same schedule as the great continental ice caps). The rivers carried huge amounts of fresh-ground rock dust from the glaciers to the Bay and beyond, and the Pacific beaches of the time must have been formidable. Think of the brisk summer days we have when the sea wind is being sucked into the Central Valley, and now multiply that. Those winds blew all that sand here.

Brian Atwater of the U.S. Geological Survey mapped ice-age deposits in the Delta, and in 1982 he summarized the overall picture as “a stage on which three related and repetitious plays are presented simultaneously. In one play, wetlands and flood plains appear and expand as tidwater invades from the west, then become sites of erosion after the tidewater retreats. In another play, glacially eroded detritus from the Sierra Nevada builds alluvial fans and, reworked by wind, creates extensive sand dunes. In the third, little-understood play, streams draining the Sierra Nevada foothills and Coast Ranges episodically build alluvial fans. Spanish- and English-speaking persons enter during a major incursion of tidewater and find most of the stage covered with tules.”

Our dunes may sit higher than the buried dunes of the Delta because the conditions that built them were more stable here. There was always a wind gap at the Golden Gate and always lots of sand available on the other side.

In detail, the Merritt Sand (Qms) reaches the edge of Lake Merritt.

merritt-lake-geomap

It differs from the marine terrace deposits (Qmt) that I’ve described before. It consists of very fine sand, with no clay. It’s also higher and less flat. Apparently the original, undisturbed surface featured yardangs — elongated ridges of sand running in the direction of the wind — whereas the dunes of Alameda were the more typical arc shapes known as barchans. All of that is obliterated today.

You can see the edge of the Merritt Sand platform from across the lake where the streets rise abruptly away from the shore. Snow Park is probably the least disturbed exposure.

merritt-terrace-snowpark

Another exposure stands out between Jackson and Madison streets, although it probably has also been excavated.

Merritt terrace at 160-17th St

It’s the back yard of an apartment building at 160 17th Street. The view is nice from there.

merritt-terrace-view

That same agreeable elevation attracted Oakland’s early elite, who put up a row of mansions overlooking the lake. Of those great homes, only the Camron-Stanford House survives.