1890 - As part of its series of analytical publications reporting results from the 1890 census, the Census Bureau issued a report about the western frontier. Since the first census in 1790, each census report had contained a map showing where the population density was high and where it was low. Areas with a population density of less than two persons per square mile were deemed wilderness. A "frontier line" was drawn on the map as the line dividing the wilderness from the populated areas (that had more than two persons per square mile). The first frontier line ran from the middle of the Maine coast down through the middle of New York, then through Virginia and down to Georgia. There were less than four million Americans and they were clustered against the Atlantic Ocean. As far as they were concerned, the continent was largely uncharted and unorganized.
It was routine for Americans, and new immigrants to America, to move out to the frontier. Land there was either cheap or actually free. After Lewis and Clark got back from their historic expedition in the early 1800s, Thomas Jefferson thought it would take one hundred generations - over 2,000 years - for the frontier to be filled up.
It took only eighty years: in 1890, the census bureau announced that so many people had filled in pockets throughout the West that it was no longer meaningful to talk about a "frontier line." The frontier was closed.