Archive for the ‘Oakland stone’ Category

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.

Oakland building stones: Lime stones

28 November 2016

There isn’t a good word for the full variety of carbonate building stones — limestone, dolomite, marble, marlstone and travertine. Although I like the word limerock, it apparently doesn’t really exist. In any case, these rocks aren’t very common in Oakland’s buildings, the way they are in, say, Washington DC (all of those memorials, and the Pentagon too).

Part of the reason is that limestone/marble is uncommon in California, and here it’s more valuable as an industrial material than as dimension stone. For limestone, think Indiana and Wisconsin. For marble, think Alabama, Texas and Colorado.

Panels of fossiliferous limestone decorate a building at 9th and Clay.

fossilif-ls-panels

Take a closer look at all the fossils.

fossilif-ls-detail

A builder might call this marble because it takes a polish, but in my language geologists call it marble only if the rock has been metamorphosed, and that would erase all of these fossils.

Its warm color indicates a certain content of clay minerals as well as calcium carbonate minerals (calcite or aragonite). If clay is present in amounts comparable to the carbonates, the stone is referred to as marl or marlstone. The Torrey Pines Bank building, on Webster Street, uses a striking combination of buff and black marlstone. A small building can pull off this look.

marl-bank

The light stone is quarry-faced. The dark stone is fully dressed and then bush-hammer finished to lighten its color.

marl-corner

This stuff has a few fossils in it, too.

marl-fossils

Limestone is a sedimentary rock that forms in warm, shallow seas where sea life, most of it microscopic, takes calcium carbonate out of solution to build its shells. These limy shells pile up and are consolidated into stone at low temperatures and gentle conditions.

You’ll see this environment in the Bahamas, for instance, but the current geological age is not a limestone-building time because the seas are so low. For most of geologic time the low parts of the continents have been warm, shallow seas. Hence the huge limestone beds of Indiana and the rest of the Midwest.

Then there’s travertine. It’s not marble in my language, but it takes a polish so it’s industrial marble. Here it is next door to the Torrey Pines Bank.

travertine-panel

A closeup shows its weird — I mean, attractive character, full of pore spaces and irregular layers.

travertine-detail

It seems like it might be fragile, but it’s very strong. The ancient Romans loved it, and Italy supplies most of the world today. The Getty Center down in L.A. is a veritable Disneyland of travertine.

travertine-getty

Mountain View Cemetery uses a lot of it too.

travertine-mvc

Travertine grows in freshwater marshes, fed by springs of groundwater that’s full of dissolved limestone. It’s actually a renewable resource.

Marble, real marble, metamorphosed and recrystallized limestone, is a common accent and interior stone in Oakland. I like this staircase in City Hall, which pleasingly contrasts detailed tilework against the warmth and translucency of fine marble. The stairs are marble too.

marble-cityhall

“What about dolomite?” you ask. “You mentioned dolomite up there.” The mineral dolomite, magnesium-calcium carbonate, can be present in limestone and marble without affecting the names we give those stones. Pure dolomite rock, or dolostone, is pretty unusual as a dimension stone. But Oakland has a spectacular example of dolomite aggregate used as the facing of the Kaiser Center, which I featured here a few months ago.

Oakland building stones: Slate

10 October 2016

slatedowntown

Slate offers visual texture and a range of colors to the architect, plus superior performance as an exterior stone. It’s not particularly strong among California stone resources, but slate is deeply engrained in European-American culture and geologically interesting to boot. This photo is from a downtown building where slate is used as a wall finish. It presents a naturally textured surface consisting of very thin layers that aren’t boringly flat.

slateordwaybldg

Slate is well displayed at the Ordway Building, where it makes up the pavement around Oakland’s tallest building. Its dignified gray color and organic texture, reminiscent of wood grain, is an excellent foil for the metal and glass around it.

slateordway

Slate is a claystone or shale that has been squeezed enough to start remaking its mineral content. The clay begins to convert to mica, but more importantly the minerals realign their crystals in response to the pressure. This change imposes a strong new fabric upon the stone that allows it to be split into thin sheets. In this photo from the Ordway Building, the dim stripes running almost vertically down the image are remnants of the original bedding in the shale protolith. You can also see, at the upper right, the approximate point where the quarry worker struck the slab to split off this sheet.

In Oakland buildings, slate appears mainly as an accent in the outside facades and sometimes as a floor in interiors. And, of course, as rugged fireproof roofing tiles.

slateorinda

The rest of the photos below are from the eastern U.S. where slate has been produced for hundreds of years from occurrences in the Appalachian mountain chain.

slatewaste

Slate is not rare — California has lots of it in the Sierra foothills — but it’s hard to find deposits with good, flat slaty cleavage. The two biggest slate-producing areas in this country are in Pennsylvania and in Vermont and the adjoining area of New York, where I took this photo of a waste pile. There’s a great deal of waste rock in a slate operation. Stone like this is still good for flagstone.

slatepoultneyjpg

In the slate regions, you’ll sometimes see the stone used in unusual applications like this post office building. Notice the range of colors.

slatesteps

And this staircase in Albany, New York, shows how properly selected slate can perform very well under foot traffic. Another advantage is that it isn’t slippery when wet.

The nearest thing to slate that Oakland has is argillite, which is the same metamorphosed claystone but without the slaty cleavage. I think the stone in the Davie quarry qualifies.