Early European settlers named the nearby ( and long - abandoned ) Cahokia Mounds in Madison County after the Illini clan .
The UNESCO World Heritage Site and State Historic Park actually reflects an extensive prehistoric Mississippian culture urban complex active from 900AD to 1500AD .
Early European settlers named the nearby ( and long - abandoned ) Cahokia Mounds in Madison County after the Illini clan , although the UNESCO World Heritage Site and State Historic Park actually reflects an extensive prehistoric Mississippian culture urban complex active from 900AD to 1500AD .