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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, ...
44
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 ...
5
For what this website defines as history (roughly, "the story of humanity"), there's rather a lot of "pre-Columbian history" in South America, and you can't really depict it all in a single map. Population density maps for 6500BCE, 3500BCE, and 1491 AD would all look quite different from each other. For the purposes of the rest of the answer, I'm going to ...
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