Earth's landscape forms and reforms itself over and over again from the shifting of the tectonic plates, which is why the continents are the way they are now but at one time was one single land mass.

First of all, you need to understand that the outer most layer of the Earth (the lithosphere) is broken into many pieces (plates) that move about and interact with each other. The history of the Appalachian Mountains starts 100’s of millions of years ago when the plate carrying what would eventually be Africa and the plate containing what would become North America began to collide with each other. This collision created a chain of volcanic mountains on Africa as the oceanic lithosphere was shoved under it. Approximately 390 million years ago, the oceanic lithosphere began to run out and the continents themselves began to collide. This collision created a mountain range much like the present day Himalayas. This collision also brought together two of the major pieces of continent in the creation of the super continent of Pangaea. Then, about 195 million years ago, North America began tearing away from Africa and these continental masses began to move toward their current locations. Since the end of the collision that generated the Appalachians, erosion has been slowly tearing them down. What we see today was once very deep beneath the surface.

tectonic plates pushed aginst each other and formed the appalachain