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Following the Frontier Line: Population Density in the Contiguous U.S. 1790 to 2010

In 1890, the Superintendent of the Census described an unbroken line of settlement stretching across the country. The advance of the frontier line that characterized shifts in population distribution for the previous 100 years was complete. This series of maps highlights population expansion into new territory, subsequent gains in population density, and increasing urbanization.

Data from the Census Bureau; maps are based on county-level data at the time of each census.

Current state boundaries are shown for reference. Much of the territory shown as having densities of under 6 people per square mile was not part of the U.S. in early decades. Data do not include “Indians not taxed” prior to 1900.

In 1790, high population densities are found closest to the Atlantic coast, with the areas around Boston, New York City, and Philadelphia showing the highest numbers of people per square mile.

By 1800, the generally settled area (6 people or more per square mile) covers a large swath of the Northeast, the Mid-Atlantic, and stretches south into Georgia. Pockets of settlement grow west of the Appalachian Mountains.

Population growth in the Ohio River Valley and upstate New York is more prominent in 1810.

In 1820, the sparsely settled frontier territory gains population density west of the Appalachians and in northern New York along the Erie Canal. New Orleans and the Lower Mississippi Delta achieve densities above 6 people per square mile.

The edge of the frontier reaches the Mississippi River below its confluence with the Ohio River by 1830 and previously settled areas around cities such as Cincinnati and Pittsburgh show increased density.

Excluding pockets of mountainous terrain, there is an area of contiguous settlement reaching inland from the Atlantic Ocean to the Mississippi River in 1840.

The frontier line crosses the Mississippi River in 1850 and settlement jumps across the Midwest and Interior West into California. Eastern seaboard settlement around Boston, New York City, Philadelphia, and Baltimore as well as the “old” frontier gateway cities of Pittsburgh and Cincinnati show very high densities in this decade.

While the population continues to surge westward, gains in density are apparent in Albany, along the route of the Erie Canal, and across the Great Lakes in cities such as Milwaukee, Chicago, and Detroit in 1860. This is the first census decade for which the contiguous U.S. covers its full extent.

From a lightly settled area to a dense city, Chicago begins to rival other major cities in density in 1870. The line of the frontier is approaching the 100th meridian while the western outposts of San Francisco, Portland, and Salt Lake City grow.

In 1880, areas with sparse or moderate settlement are seen expanding rapidly in the Midwest, with the line of contiguous settlement reaching well into Nebraska and Kansas.

By 1890, large areas of moderate density expand. Of note are the Upper Midwest, the South, and eastern Texas. Settlement in the West remains very dispersed except for major, emerging urban centers. However, towns and cities occur regularly enough to disrupt the appearance of a clear, unbroken line separating the generally settled area from the frontier.