During the middle Ordovician Period, about 440-480 million years ago, a change in plate motions set the stage for the first Paleozoic mountain building event Taconic orogeny in North America. The once quiet, Appalachian passive margin changed to a very active plate boundary when a neighboring oceanic plate, the Iapetus, collided with and began sinking beneath the North American Craton. With the birth of this new subduction zone, the early Appalachians were born.
Along the continental margin, volcanoes grew, coincident with the initiation of subduction. Thrust faulting uplifted and warped older sedimentary rock laid down on the passive margin. As mountains rose, erosion began to wear them down. Streams carried rock debris downslope to be deposited in nearby lowlands.NASA image of the Appalachian Valley and Ridge province. These rock layers were folded during the series of continental collisions that formed the Appalachians during the Paleozoic Era.
Much more recent (Cenozoic) uplift and erosion produced the landscape we see today. The ridges are made up of erosion-resistant sandstone (green on image), while the valleys are made up of limestone and other less-resistant rock layers The major river is the Susquehanna. To appreciate the scale of this image, note the white arrow indicating the town of Sunbury. Two bridges and a dam across the river.
This was just the first of a series of mountain building plate collisions that contributed to the formation of the Appalachians. Mountain building continued periodically throughout the next 250 million years Caledonian, Acadian, Ouachita, Hercynian, and Allegheny orogenies. Continent after continent was thrust and sutured onto the North American craton as the Pangean supercontinent began to take shape. Microplates, smaller bits of crust, too small to be called continents, were swept in, one by one, to be welded to the growing mass.
By about 300 million years ago, Pennsylvanian Period Africa was approaching North American Craton. The collisional belt spread into the Ozark-Ouachita region and through the Marathon Mountain area of Texas. Continent vs. continent collision raised the Appalachian-Ouachita chain to lofty, Himalayan-scale ranges. The massive bulk of Pangea was completed near the end of the Paleozoic Era Permian Period when Africa Gondwana plowed into the continental agglomeration, with the Appalachian-Ouachita mountains near the core.
By the end of the Mesozoic Era, the Appalachian Mountains had been eroded to an almost flat plain. It was not until the region was uplifted during the Cenozoic Era that the distinctive topography of the present form. Uplift rejuvenated the streams, which rapidly responded by cutting downward into the ancient bedrock. Some streams flowed along weak layers that define the folds and faults created many millions of years earlier. Other streams down cut so rapidly that they cut right across the resistant folded rocks of the mountain core, carving canyons across rock layers and geologic structures.