As the european settlers arrived on the coast of the united states and began to push into the country they were blocked by the presence of

the Appalachian Mountain

Explanation:

The European settlers in what is now the United States quickly started to push the natives of their lands and create their colonies. As they were doing so, they started to establish towns and villages, create their own plantations, implement their rules and laws. But this was all happening along the eastern coastline and in close proximity to it. The reason for this was that they had a very big natural barrier in front of them that didn't allowed them to spread out toward the west. This natural barrier were the Appalachian Mountains. The Appalachians are a mountain range that separates almost all of the eastern coastline area from the interior of the country. These mountains are relatively high, had very dense impenetrable forests, and were very dangerous for crossing, so the Europeans avoided them until they developed better means of travel and transportation.