On examination, this is apparently the Rosebud Indian Reservation, which has been shattered into a million different pieces. There's got to be some history to that. (I suspect, probably some trickery/abuse on the part of the US related to supression of Native Americans, but I may be wrong) https://www.openstreetmap.org/way/9869788#map=14/43.4802/-100.5684#gis#mapping#SouthDakota
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