Black Belt is a physical geography term referring to a roughly crescent-shaped geological formation of dark fertile soil in the Southern United States.
[1][2] During the Cretaceous period, about 145 to 66 million years ago, most of what are now the central plains and the Southeastern United States were covered by shallow seas.
Tiny marine plankton grew in those seas, and their carbonate skeletons accumulated into massive chalk formations.
The Black Belt arc was the shoreline of one of those seas, where large amounts of chalk had collected in the shallow waters.
Later, and especially since the war, the term seems to be used wholly in a political sense—that is, to designate the counties where the black people outnumber the white.Since the 1920s the term Black Belt fell out of favor as a term outside of the specialized field of physical geology, but various authors have written about the fact that the Black Belt geographical formation contained a large number of slaves before the American Civil War, many of whom worked the cotton plantations.