This map shows a strong correlation between 2008 presidential election results (red counties = McCain, blue = Obama) and cotton production in 1860 (each dot = 2,000 bales).
Long story short:
- Marine deposits from Upper Cretaceous period form Selma Chalk, which underlies the area in the crescent through Mississippi and Alabama.
- That area, and other soils suitable for cotton production, attract plantation owners who concentrate slave labor on the most productive soils growing the most lucrative cash crop, King Cotton.
- Despite migrations after emancipation, the areas where cotton was king continue to have a large proportion of African-Americans.
- African-Americans voted overwhelmingly for Obama, producing the blue streak through the otherwise red south.