. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography "* ' - * ^**^y.â â â .^r â -!-â : \ ' i â '\- 'â¢-â ^H â s ,N » ⢠s V » - > I * * . t - -. v ⢠- / v> * . ', ,-, - V âo:«-V -t,;':; si â¢'. >,x, -W'.v * - V - - -%V-1 * *.$SIt<^' sf. i: k rr*.^: APPALACHIAN PLATEAU. CRUMPLING OF EARLIER GEOSYNCLINAL COUPLET, apparently laid down in late Precambrian and early Paleozoic time more than 4S0 million years ago, produced the Appa- lachian foldbelt. The four-part se
. Collected reprints / Atlantic Oceanographic and Meteorological Laboratories [and] Pacific Oceanographic Laboratories. Oceanography "* ' - * ^**^y.â â â .^r â -!-â : \ ' i â '\- 'â¢-â ^H â s ,N » ⢠s V » - > I * * . t - -. v ⢠- / v> * . ', ,-, - V âo:«-V -t,;':; si â¢'. >,x, -W'.v * - V - - -%V-1 * *.$SIt<^' sf. i: k rr*.^: APPALACHIAN PLATEAU. CRUMPLING OF EARLIER GEOSYNCLINAL COUPLET, apparently laid down in late Precambrian and early Paleozoic time more than 4S0 million years ago, produced the Appa- lachian foldbelt. The four-part sequence shows how the miogeocline, or western part of the geosynclinal couplet, was folded into the series of ridges between the Blue Ridge line and the Allegheny front. The eugeocline, altered by heat, pressure and volcanism, formed a lofty range of mountains, now almost completely eroded, east of the Blue Ridge line. 36 ticularly evident in miogeoclines, which can attain a total thickness at their sea- ward edge of five kilometers even though they are entirely composed of beds de- posited in shallow water. This phenome- non is nicely accounted for by plate tec- tonics: the margins of rift oceans inher- ently have, as one geologist has expressed it, a "certain sinking ; Let us take as an example the Atlantic Ocean between the and the bulge of Africa. This new ocean basin was cre- ated about 180 million years ago by the insertion of a spreading rift that split North America away from Africa [see illustration on apposite page]. Attendant swelling of the mantle arched the con- tinents upward along the rift line by about two kilometers. Erosion then bev- eled the raised edges, thinning the mar- gins of the two continental plates. A modem example of crustal arching associated with incipient rifting of the crust can be observed in the high dorsal of Africa from Ethiopia southward. The Red Sea provides a more advanced stage of a newly open
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