Cahokia

The largest pre-Columbian city north of Mexico, abandoned by 1350

Across the Mississippi from St. Louis, on the flat floodplain called the American Bottom, a grassy hill rises in four terraces to 30 metres and covers some six hectares. Monks Mound is the largest prehistoric earthen structure in the Americas, raised in fourteen stages, and it was the heart of Cahokia, the greatest pre-Columbian city north of Mexico. Its original name is lost; the mounds were later named for the Cahokia, an Illiniwek people living nearby when French explorers arrived in the 17th century, centuries after the city was empty.

People had lived here since at least 600 AD, in villages of 50 to 100, when something remarkable happened around 1050. In what archaeologists call the Big Bang, the scattered settlements were swept into a planned city laid out along a grid oriented to the north, with a Grand Plaza, a causeway and dozens of mounds, and newcomers poured in from as far away as the lower Ohio, the Lower Mississippi and the southern plains. Within fifty years the dense core had grown from perhaps 1,400 to 2,800 people to between 10,000 and 15,000, and at its peak around 1100 the whole city spread across about six square miles with some 120 earthworks and perhaps 15,000 to 20,000 inhabitants, comparable to London or Paris at the time. Every mound was built by hand, with earth carried in woven baskets, some 55 million cubic feet of it. A circle of tall wooden posts, Woodhenge, tracked the sun.

Cahokia sat near the meeting of the Mississippi, Missouri and Illinois rivers and traded with the Great Lakes and the Gulf Coast in copper, Mill Creek chert, shark teeth and lightning whelk shells. It was the first and greatest centre of the Mississippian culture, whose towns spread across the Midwest and South, and a powerful hierarchy organised its labour, farming and trade. A wooden palisade almost two miles round enclosed the ceremonial centre, and a section of it has been rebuilt.

Then it emptied. After its peak around 1100 the population shrank, and by about 1350 the city was abandoned. Exhausted land, political and social unrest, climate change and long droughts have all been blamed, and no one knows for sure. Strikingly, the anthropologist Thomas Emerson has noted, Cahokia barely appears in Native American oral histories, as if what happened there left a bad taste.

Today the Cahokia Mounds State Historic Site in Collinsville, Illinois, protects 2,200 acres and about 80 mounds. It has been a UNESCO World Heritage Site since 1982.