U.S. Census Bureau, Following the Frontier Line, 1790-1890 (2012)

In 1890, the Superintendent of the U.S. Census, Robert P. Porter, claimed that the nearly unbroken line of settlement that had been moving across the country over the last 100 years, the frontier line, had disappeared.  As described in the Census report, "the unsettled area [of the country] has been so broken into by isolated bodies of settlement that there can hardly be said to be a frontier line" (p. xlviii). This interactive map from the U.S. Census Bureau shows this gradual westward movement of the U.S. population between 1790 and 1890, culminating in the supposed end of the American frontier. Using the time slider at the bottom of the map, viewers can select a specific year to observe the population density, or press the play button to watch the gradual progression of population density over time. There is also an option to show recognizable cities on the map by checking the "Selected Cities" box next to the map.