One of the great innovations in visual information display was demographic maps created by Alexander Dallas Bache of the U.S. Coast Survey in 1860. These showed visually the concentration of slavery in narrow regions even within the South, and were among the most powerful tools of abolitionists. Blache was the first to use the now-familiar method of shading regions to show their demographic traits.
I was struck looking at this map how closely it resembles the current mapping of non-Hispanic blacks in the U.S. (Year 2000 data; from Social Explorer). Look closely at some part of both maps; you'll find that even at a county-by-county level, the concentration of blacks in 2000 is closely related to the slave population in 1860.