Cahokia is a village in St. Clair County, Illinois, United States. It is located east of the Mississippi River in the Greater St. Louis metropolitan area. As of the 2010 census, 15,241 people lived in the village, a decline from 16,391 in 2000. The name refers to one of the clans of the historic Illini confederacy, who met early French explorers to the region. Early European settlers named the nearby (and long-abandoned) Cahokia Mounds in present-day Madison County after the Illini clan. But the UNESCO World Heritage Site and State Historic Park was developed by the prehistoric Mississippian culture, active here from AD 900 to AD 1500. They created an extensive urban complex, the largest of the farflung Mississippian culture territory through the Mississippi and Ohio river valleys.
French Canadian colonists founded Cahokia village in 1696 as a Catholic mission. The historic Church of the Holy Family is the oldest continually active Catholic parish in the United States, as well as the oldest church west of the Allegheny Mountains. Other significant colonial and Federal-period buildings listed on the National Register of Historic Places include the Cahokia Courthouse (c. 1740, in the French Colonial style); and the Jarrot Mansion (c. 1810). Archeologists ascribe the earthwork mounds Cahokia complex to the Mississippian culture, an earlier indigenous people who are not believed to have been ancestral to the Illini. The city site reached its peak in the 13th century and was abandoned centuries before European contact.
information provided by wikipedia.org