What natrual barrier prevented early colonial expansion westward

Most people are going to answer with geographical features, the Appalachians and other features ... But the real reason was the the natives. Until the population pressure was strong enough to overweight the pressure from the natives the colonists stayed near the coasts. The Appalachians are pretty and scenic but they aren't all that much of a barrier. They were much less of a hurdle than the Great Plains.

The Appalachian mountains were essentially the barrier of westward expansion from the east coast of New England.