Complex sentence: The Ocmulgee immigrants were either already practicing agriculture or adopted it shortly after arriving, and settled in upland areas suitable for agriculture, which were largely unexploited by the wetlands-oriented Cades Pond people.
Split into simpler sentences: The Ocmulgee immigrants were either already practicing agriculture or adopted it shortly after arriving, and settled in upland areas suitable for agriculture. These had been largely unexploited by the wetlands-oriented Cades Pond people.