Cahokia Mounds, approximately 85 surviving Native American earthworks, most in Cahokia Mounds State Historic Site, SW Ill., near East St. Louis; largest group of mounds N of Mexico. Monks' Mound, a rectangular, flat-topped earthwork, 100 ft (30.5 m) high with a more than 14-acre (5.7-hectare) base, is named for Trappist monks who settled there in the early 19th cent. It is the largest surviving earthen mound in the Americas. Evidence of an extensive stockade and other wood structures also has been found. The earliest known settlement in the area dates to c. A.D. 700. The Mississippiean people who constructed the mounds were village dwellers who lived in a fertile river-bottom area; their culture arose about 100 years later. They flourished from 11th to 14th cent.; the population of the area at its height ranged from 10,000 to 20,000. The mounds are a national historic landmark.