Complex sentence: Chronologically this area became Mississippianized later than the Middle Mississippian area (about 1000 CE as compared to 800 CE) to its northwest and is believed that the peoples of this area adopted Mississippian traits from their northwestern neighbors.
Split into simpler sentences: Chronologically this area became influenced by Mississippian culture later than the Middle Mississippian area (about 1000 CE as compared to 800 CE) to its northwest. It is believed that the peoples of this area adopted Mississippian traits from their northwestern neighbors.