Mesozoic coastlines and modern politics

Virgil once wrote, "Felix qui potuit rerum cognóscere causas"-- "Happy is the one who knows the causes of things," and I am certainly happy to know the cause of a band of Democratic-voting counties in the otherwise mostly Republican Deep South, namely that this geographical region used to be the coastline of an ocean that split North America during Cretaceous times. Intermediate links in the causal chain are plankton, chalk, cotton, and slavery:

During the Cretaceous, 139-65 million years ago, shallow seas covered much of the southern United States.   These tropical waters were productive–giving rise to tiny marine plankton with carbonate skeletons which overtime accumulated into massive chalk formations.  The chalk, both alkaline and porous, lead to fertile and well-drained soils in a band, mirroring that ancient coastline and stretching across the now much drier South....
Over time this rich soil produced an amazingly productive agricultural region, especially for cotton.  In 1859 alone a harvest of over 4,000 cotton bales was not uncommon within the belt. And yet, just tens of miles north or south this harvest was rare.  Of course this level of cotton production required extensive labor...
As Washington notes further in his autobiography, “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."