Complex sentence: Cahokia Mounds, a prehistoric Mississippian culture urban complex and World Heritage Site near Collinsville, Illinois was also named for this historic clan, although it was built by an earlier, potentially unrelated people and reached its peak in the 13th century.
Split into simpler sentences: Cahokia Mounds, a prehistoric Mississippian culture urban complex and World Heritage Site near Collinsville, Illinois was also named for this historic clan. It was built by an earlier, potentially unrelated people and reached its peak in the 13th century.