The Appalachian Mountains The Appalachian mountain system dominates the Eastern United States and seperates the Eastern seaboard from the interior with a belt of uplands that extends nearly 1,500 miles from northeastern Alabama to the Canadian border.; They are old, complex mountains, eroded from much older and greater ranges. The present topography is a result of erosion that has carved the weak rocks away, leaving a skeleton of resistant rocks behind as highlands. Because of this weathering, geologic differences are refelected in topography. In the Appalachians these differences are sharply demarcated and neatly arranged, so that all the major subdivisions except for New England lie in strip