How does the movie "The Searchers" represent the frontier?

Ford's film can represent the concept of the frontier in a
couple of ways.  The first is that the men embark on a search without a fixed definition
or end.  Their search is for their families,  but they have little idea as to where they
are actually going.  They progress deep into the West, further and further into the
frontier.  In many respects, they cross several lines of demarcation.  The frontier is
the line that marks how the men have to leave their homes in order to facilitate their
pursuit of justice.  It is also a line that separates law from lawlessness, different
conceptions of culture, as well as what is known and "the other."  The journey that the
men undertake is also an example of the frontier in that it reflects how there is
fluidity in the concept.  It is always changing.  At first, the frontier marked the line
between the men seeking the return of their cattle.  Then, it marked the line between
seeing their families killed and seeking justice for the wrongs that had been done. 
Another example of the Frontier might lie with Ethan, who walks away in the distance at
the end, establishing and simultaneously crossing into a new
frontier.