How soil type affected the 2008 election map

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.

This correlation was first noted on by Allen Gathman. a genetics professor at Southeast Missouri State University, with further elaboration at The Vigorous North and Strange Maps.