The maps of the period of the early explorations and settlements by the British tell us that they were not well aware of the size of the continent, as well as its interior, so they were mainly depicting the eastern coast and the close surrounding of it. Also, we can easily see that the settlements and the activities by the early settlers were along the eastern coast of what is now the United States, and that was the starting place from were they slowly started to expend further west in the years to come.
