West of Chicago

Gerelateerde afbeelding

West of Chicago and through most of Illinois, you would have been wading on clean sand, the quiet margin of the Canadian craton, which remained above the sea. The limy bottom apparently resumed in Iowa and went on into eastern Nebraska, and then, more or less at Kearney, you would have moved up onto a blistering-hot equatorial beach and into low terrain, subdued hills, rock that had been there a thousand million years. It was barren to a vengeance with a hint of life, possibly a hint of life-rocks stained green, stained red by algae. Wyoming. Past Laramie, you would have come to a west-facing beach and, after it, tidal mudflats all the way to Utah. The waters of the shelf would now begin to deepen. A hundred miles into Nevada was the continental slope and beyond it the blue ocean. If you had turned around and gone back after thirty million years-well into Ordovician time, say four hundred and sixty million years ago-the shelf edge would still have been near Elko, Nevada, and the gradually rising clean-lime seafloor would have reached at least to Salt Lake City. Across Wyoming, there may have been low dry land or possibly flexplek huren rotterdam continuing sea. The evidence has almost wholly worn away, but there is one clue. In southeastern Wyoming, a diamond pipe came up about a hundred million years ago, and, in the tumult that followed the explosion, marine limestone of late Ordovician age fell into the kimberlite and was preserved. In western Nebraska, you would have crossed dry and barren Precambrian terrain and by Lincoln have reached another sea. Iowa, Illinois, Indiana. The water was clear, the bottom uneven-many shallows and deeps upon the craton. In Ohio, the sea would have begun to cloud, increasingly so as you moved on east, silts slowly falling onto the lime. In Pennsylvania, as you approached the site of the future Delaware Water Gap, the bottom would have fallen away below you, and where it had earlier been close to the surface it would now be many tens of fathoms down. “The carbonate platform collapsed,” Anita said. “The continental shelf went flexplek huren almere down and formed a big depression. Sediments poured 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.”

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