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