The Largest North American City Before 1780 Was Across the River from St. Louis

Cahokia was a Mississippian city built on the floodplain east of present-day St. Louis between roughly 1050 and 1350 CE. At its peak around 1100 it held somewhere between 10,000 and 40,000 people — large enough that, on the high estimates, no city in what would become the United States surpassed it until Philadelphia crossed 40,000 in the 1780s. Twelfth-century Cahokia was probably more populous than contemporary London or Paris.

The central monument, Monks Mound, is a flat-topped earthen pyramid that covers 13.8 acres at its base and rises about 100 feet. It was built in roughly ten distinct stages over the course of two centuries from somewhere on the order of 814,000 cubic yards of basket-loaded earth. Eight hundred meters west of the mound stands Woodhenge — a series of timber circles, the largest more than 400 feet in diameter, whose posts mark solstice and equinox sunrises against the silhouette of the central pyramid. The radiocarbon dates put successive Woodhenges between about 900 and 1100 CE, each larger than the last.

Why Cahokia emptied is still being argued. Tree-ring records show drought from about 1100 to 1250, paleoflood evidence captures massive Mississippi floods around 1100–1260 and 1340–1460, and excavations show signs of palisade-building and intercommunal violence late in the city's life. By 1350 the site was empty. The DeSoto expedition came through the lower Mississippi about 200 years later and met Mississippian successors, but the people who built Cahokia were gone, and the city's name was lost.