The correct answer to this open question is the following.
Isolated bodies of settlement” refers to new towns in the region.
We are referring to an excerpt of "The Significance of the Frontier in American History" written by Frederick Jackson Turner, chapter 1.
The Westward expansion to settle those inhabited territories in the mid and late 1880s was an important part of American history. The colonization of new territories made a lot of people move from the Eastern portion if the country to the West, looking for new opportunities or a new life.
