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You are describing part of what is referred to as the Black Belt in the American South. It can be seen here in a map of majority-black US counties.
Your suspicions for the reason for this are quite correct. This corresponds to the interior of the old cotton belt. This area was historically the richest river-fed southern soil, with the largest plantations, ...
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T.E.D.'s answer (+1) is complete with regard to the human history of the Americas, but the rich soil in the "cotton belt" or "black belt" has a geological explanation as well. The article "How presidential elections are impacted by a 100 million year old coastline" by Craig McClain, "Executive Director of the Lousiana University Marine Consortium", describes ...
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