The timing of the American frontier's closing can be determined through an analysis of the patterns and rates of settlement growth in the United States—based on historical county boundaries, population data from U.S. censuses, and a minimum of two people per square mile to classify a region as settled. Trends in the settlement of the contiguous areas of the country indicate three periods of population settlement—1790 to 1840, 1840 to 1910, and 1910 to 1990. The first period of rapid frontier growth ended in 1840, and a second, more moderate, one ended in 1910, marking the final closure of the frontier.

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