North America

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Anita said, choosing North America’s most eminent example, “The Gulf of Mexico is a big geosyncline, if you want. The big birdfoot delta of the Mississippi River is one hell of a sedimentary pile. Drill twenty-two thousand feet down and you’re still in the Eocene. The crust will take about forty thousand feet of sediment-that’s the elastic limit. Then it regurgitates the sediment, which begins to rebound. The sediment is also heated up, melted. Water, gas, and oil come out of the rock. Sedimentary layers move up with thermal drives as well as with isostasy. Sedimentary layers also move laterally, and are thus thrust sheets. In Cambra-Ordovician time, fifty million years or so before the Taconic mountains came up, the continent was to the west of co-working space nijmegen us here, the coastline was in central Ohio, and to the east of us, where the Atlantic shelf is now, stood an island arc like Japan. There are volcaniclastic sediments of that age from Newfoundland to Georgia-just about the length of Japan. The present coast of Asia is the Ohio coastline in that story. Picture the sediment that is pouring off the Japanese islands into the Sea of Japan. The Martinsburg slates were shed not from the continent-not from Ohio-but mainly from the east, from the island arc offshore. You pile up forty thousand feet of sediment and it pops. The Martinsburg popped. The Taconic mountains came up. Once the process starts, it keeps itself going. You push up a mountain range, erode it into the west. The material depresses the crust. It is low-density material and it is brought down into the regime of high-density material. When enough has been piled on, the low-density material comes back up. That is how orogenic waves co-working space arnhem propagate themselves, each mountain mass being cannibalized to produce a new mountain mass to the west. But I still don’t know what started the process.” Historical geologists, in the olden days, pieced together that narrative. Economic geologists, in their pragmatic way, cared less. In describing the minable Martinsburg-the blue-gray true unfading slate-C. H. Behre, Jr., wrote in i933, “Sedimentary rocks are often compressed from the sides through what may be loosely described as shrinking of the crust of the earth; how this shrinking is brought about is, for the present purpose, beside the point. It has the wellrecognized effect, however, that layers or bedding planes are wrinkled or thrown into ‘folds.’ “

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