Bill Rankin at Radical Cartography created a set of maps that show the growth of slavery in the US between 1790 and 1870. The population of the US is indicated by the size of dots on each 250-square-mile cell, and the percentage of those people who were enslaved is indicated by color. At the site, you can mouseover the map titles to advance to the next map. As you can see by the map of 1830 here, the population surrounding Charleston, South Carolina, was over 95% enslaved. The other highest concentration is around the lower Mississippi River. As you advance the maps, you can see the spread and concentrations of slavery move as the country grew, and the growing dependence of southern agriculture on slave labor. Another map at the site charts the year of peak slavery for each area. -via Digg