The carbonate platform collapsed,” Anita said. “The continental shelf went down and formed a big depression. Sediments pored in.” Much in the way that a sheet of paper bends downward if you move its two ends toward each other in your hands, the limestones and dolomites and the basement rock beneath them had subsided, forming a trough, which rapidly filled with dark mud. The mud became shale, and when the shale was drawn into the heat and pressure of the making of mountains its minerals realigned themselves and it turned into slate. We moved on west a couple of miles and stopped at a roadcut of ebony slate. Anita said, “Twelve thousand feet of this black mud was deposited in twelve million years. That’s a big pile of rock.” The formation was called Martinsburg. It had been folded and cleaved in orogenic violence following its deposition in the sea. As a result, it resembled stacks of black folios, each of a thousand leaves. Just to tap at such rock and remove a piece of it is to create something so beautiful in its curving shape and tiered laminations that it would surely be attractive to a bonsai gardener’s eye. It seems a proper setting for a six-inch tree. I put a few pieces in the car, as I am wont to do when I see some Martinsburg. Across the Delaware, in Pennsylvania, the formation presents itself in large sections that are without joints and veins, the minerals line up finely in dense flat sheets, and the foliation planes are so extensive and straight that slabs of great size can be sawed from the earth. The rock there is described as “blue-gray true unfading slate.” It is strong but “soft,” and will accept a polishing that makes it smoother than glass. From Memphis to St. Joe, from Joplin to River City, there is scarcely a hustler in the history of pool who has not racked up his runs over Martinsburg slate. For anybody alive who still hears corruption in the click of pocket billiards, it is worth a moment of reflection that not only did all those pool tables accumulate on the ocean floor as Ordovician guck but so did the blackboards in the schools of all America.
—John McPhee. “In Suspect Terrain.” 1983