Cahokia: The City Larger Than London

Across the Mississippi from Saint Louis, on the Illinois bottomlands, sit the remains of a city that most Americans have never heard of, even though at its peak it was larger than London.

Cahokia flourished between roughly six hundred and thirteen fifty of the Common Era, reaching its height around the year eleven hundred. At that moment, this Mississippian metropolis stretched across four thousand acres, encompassed about one hundred twenty earthen mounds, and housed an estimated ten thousand to twenty thousand people. Medieval London held perhaps fifteen thousand. The people who built Cahokia had no writing system we can read, which is partly why their story remains unfamiliar. The name itself was borrowed from a nearby tribe encountered by French explorers centuries later. What the original inhabitants called their city is lost.

The centerpiece is Monks Mound, the largest prehistoric earthen structure in the Western Hemisphere. Built in fourteen stages over centuries, it covers six hectares and rises in four terraces to a height of thirty meters. A temple or palace stood on its summit. Surrounding it, the city's layout was deliberate and complex.

Just west of Monks Mound, archaeologists found the postholes of what they call Woodhenge: a great circle of wooden posts whose alignment tracks the summer and winter solstices and the spring and fall equinoxes precisely. The calendar was rebuilt four times, each time with twelve additional poles.

Cahokia began declining after twelve hundred. Deforestation, soil erosion, flooding, political fracture, the exact combination is still debated. By the mid-thirteen hundreds the city was empty. In nineteen eighty-two, UNESCO designated it a World Heritage Site. It is the largest archaeological site in North America, lying just across the river from the Gateway Arch.