That’s a portion of the 2012 US Presidential election map of the southern states broken down by county: blue ones went Barack Obama’s way and counties in red voted for Mitt Romney.
But let’s go back to the Cretaceous Period, which lasted from 145 million years ago to 65 million years ago. Back then, the coastline of what is now North America looked like this:
Along that ancient coastline of a shallow sea, plankton with carbonate skeletons lived and died in massive numbers, accumulating into large chalk formations on the bottom of the sea. When the sea level dropped and the sea drained through the porous chalk, rich bands of soil were left right along the former coastline. When that area was settled and farmed in the 19th century, that rich soil was perfect for growing cotton. And cotton production was particularly profitable, so slaves were heavily used in those areas.
McClain, quoting from Booker T. Washington’s autobiography, Up From Slavery, points out: “The part of the country possessing this thick, dark and naturally rich soil was, of course, the part of the South where the slaves were most profitable, and consequently they were taken there in the largest numbers.” After the Civil War, a lot of former slaves stayed on this land, and while many migrated North, their families are still there.
The counties in which slave populations were highest before the Civil War are still home to large African American populations, which tend to vote for Democratic presidential candidates, even as the whiter counties around them vote for Republicans. The voting pattern of those counties on the map follows the Cretaceous coastline of 100 million years ago — the plankton fell, the cotton grew, the slaves bled into that rich soil, and their descendants later helped a black man reach the White House.