During the Devonian Period, 400-350 million years ago (MYA), the shallow Kaskaskia Sea covered much of eastern North America. When the Avalon subcontinent collided with the northeast edge of North America, about 380 MYA, the Acadian Mountains crumpled skyward, stretching from the mid Atlantic region to the Canadian Maritimes. In concert, downwarping of the crust to the west of this range created a deep bay (the Catskill Basin)in the Kaskaskia Sea; known as the Catskill Sea, this deeper water covered much of New York State and northeastern Pennsylvania.
Throughout the remainder of the Devonian and into the Mississippian Period, the Acadian Mountains eroded from lofty peaks to modest hills; much of the erosional debris was carried into the Catskill Sea, gradually filling its basin and pushing its shoreline further to the west (geologists refer to this process as the Catskill Delta formation). Elsewhere, the Kaskaskia Sea retreated to its component basins, which gradually filled with marine sediments and carboniferous deposits (the latter from vast swamplands and coal forests). Then, from 300-250 MYA, Earth's continents merged into Pangea; the collision of North America and Africa lifted the Southern Appalachians and the adjacent Appalachian Plateau. The latter, comprised of sediments that collected in the Catskill and Allegheny Basins, stretches from New York to Alabama (and was once continuous with the Ozarks of Missouri-Arkansas).
The Catskills of New York and Poconos of Pennsylvania mark the eastern end of the Appalachian Plateau and are composed primarily of debris from the Acadian Mountains, now hardened into sandstones and conglomerate rock. Pleistocene glaciers scoured northern sections of the Plateau and its entire length has been carved into a maze of ridges and valleys by a vast network of streams. Composed of more resistant rock, the Catskills harbor the highest elevations of the Appalachian Plateau; Slide Mountain climbs to nearly 4200 feet and almost 100 peaks rise above 3000 feet.
