By 1900, most historians agree the Western Frontier of America was closed. Can you explain three reasons why they have concluded this?
In 1890 the Census Bureau declared that the frontier was closed. It meant that no ne lands could be discovered.
Frederick Jackson Turner is the key historian who coined the idea of frontier which has an almost mythical dimension.
Find details: http://www.pbs.org/weta/thewest/people/s_z/turner.htm
His famous speech:http://nationalhumanitiescenter.org/pds/gilded/empire/text1/turner.pdf
The development of the railroads had been the main factor contributing to the conquest of the West.
Tremendous amounts of resources could be extracted thanks to those discoveries.