What physical geographical feature in the old northwest served as an internal borderland?

The physical geographical feature in the old northwest which served as an internal borderland was the Appalachian mountains. This system of mountains. that was formed due to continent-continent convergence 480 million years. is still be a barrier to east-west travel. Because of its location and geographical position, all of its valleys are oriented in opposition to the roads that lead east or west, therefore it is can be considered as internal borderland.