This is a picture of the American South, showing the 2008 presidential election results by county; red for Republican, blue for Democrat.
You'll notice that although it's mostly red, there's an inverted arc of blue that sweeps down from Virginia and the Carolinas in the east, to Arkansas and Mississippi in the west.
Now look at this:
This is a geological picture of the same area, showing Cretaceous rock formations - following the same path as the blue in the first picture - in green.
During the Cretaceous period, 139-65 million years ago, much of what is now the southern U.S was covered by shallow seas. That green path was where the coastline was. As a result of being bathed in fertile tropical waters for millions of years, massive chalk formations arose along that ancient coast.
When the seas withdrew, the chalk gave rise to well-drained and fecund soil along an arc which later became known as the Black Belt, originally because of the soil's dark rich colour, and later because it had such a high African-American population. One led to the other: the fertility of the soil made it exceptionally productive for cotton farmers, who required plenty of slaves to work the land for them.
African-Americans still make up over 50% of the population along the Black Belt. It has a distinct cultural and social identity; as the author of the wonderful post from which I'm borrowing points out, if you take two counties from either end of the belt you'll find that they have an extraordinary amount in common, from average household incomes, to place-names and pastimes.
Politics, too. Despite running through heavily Republican states, the counties along this band regularly elect Democrats. The Black Belt has become the Blue Belt.
I urge you to read the whole thing, over at Deep Sea News.