Innumerable jigsaw puzzles

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They are like odd pieces from innumerable jigsaw puzzles. The rock column-a vertical representation of the crust at some point on the earth-holds a great deal of inferable history, too. But rock columns are generalized; they are atremble with hiatuses; and they depend in large part on well borings, which are shallow, and on three-dimensional seismic studies, which are new, and far between. To this day, in other words, there remains in geology plenty of room for the creative imagination. All tlrn more amazing is the extent to which the early geologists, who flexplek huren amsterdam travelled the Appalachians in the eighteen-twenties and thirties, not only catalogued the evident rock but also worked out stratigraphic relationships among various formations and began to see composite structure. Starting close up, with this rock type, tl1at mountain, this formation, that valley-with what they could see and know-they gradually began to form tentative regional pictures. Piece by piece over the next century and a half, they and their successors would put together logically sequential narratives. presenting the comprehensive histo1y of the mountain belt. As new evidence and insight came along, old logic sometimes fell into discard. When plate tectonics arrived, its revelations were embraced or accommodated but by no means universally accepted. The Appalachians, meanwhile, continued slowly to waste away. The debate about their origins did not. Observing the flexplek huren amsterdam zuidas valley scene, the gapped and distant ridgeline, Anita said that mountains in this region had come up and been worn down not once but a number of times: the Appalachians were the result of a series of pulses of mountain building, the last three of which had been spaced across two hundred and fifty million years -the Taconic Orogeny, the Acadian Orogeny, and the Alleghenian Orogeny. The first stirrings of the Taconic Orogeny began nearly five hundred million years ago.

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