Oakland building stones: Slate


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.


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.


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.


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.


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.


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


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.

One Response to “Oakland building stones: Slate”

  1. Richard Says:

    Another interesting post, thanks. I found a few large mounds of broken slate up in the White Mountains on the roads north of the Bristlecone ancient pine forest. Some really interesting stuff. I took a few small samples for fish tank decoration in my planted discus tanks. A “caretaker” of one of the research stations up there made a beautiful rock garden with numerous slate stones set upright around the front of some of their buildings. Some bigger than 3′ long.

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