The mountain range that formed the western boundary of the original states is the Appalachian Mountains. The original thirteen colonies were established along the Atlantic coast of North America, and the Appalachian Mountains formed a natural barrier to westward expansion. The mountains were a major obstacle for early settlers, who had to find passes or build roads through them in order to reach the Ohio River Valley and beyond. The Appalachians also played a significant role in the American Revolution, as they provided a natural defensive barrier for the colonists against the British.