The Southern Appalachians, which stretch from New York to Alabama, formed as the North American and African Plates collided, about 250 million years ago. Folding, faulting and uplift along this swath created a broad chain of highlands, characterized today (west to east) by the Appalachian Plateau, the Ridge and Valley Province and the Smoky-Blue Ridge Mountains.
The above collision occurred during the assembly of Pangea, the Permian mega-continent that formed as the Continental Plates merged. During this process, pressure was also exerted along the southern edge of the North American Plate, extending the Appalachian Orogeny westward from what is now northern Alabama. At that time, much of Mississippi, eastern Arkansas and southeastern Missouri did not exist; the Ozark Plateau was a westward extension of the Appalachian Plateau and the Ouachita Mountains of Arkansas-Oklahoma were continuous with the southern end of the folded Appalachian chain. Then, as Pangea broke apart, the Gulf of Mexico opened and the Mississippi River Embayment split the south-central edge of the North American Plate, producing a broad, northward extension of the Gulf (as far north as southern Illinois).
Today, the Southern Appalachian and Ouachita Mountains, once resembling the Rockies, have eroded into less-imposing, tree-covered ranges and are separated by the broad Gulf Plain of the lower Mississippi Valley; the latter formed as the sea retreated and as erosional debris, from the regional mountains and vast interior, was deposited by the Mississippi River and its numerous tributaries. In reality, the Ozarks and the Ouachitas are the Western Appalachians!
