"Cahokia was one big piece in the mosaic of chiefdoms that covered the lower half of the Mississippi and the Southeast at the end of the first millennium A.D. Known collectively as "Mississippian" cultures, these societies arose several centuries after the decline of the Hopewell culture, and probably were its distant descendants. At any one time a few larger polities dominated the dozens or scores of small chiefdoms. Cahokia, biggest of all, was preeminent from about 950 to about 1250 A.D. It was an anomaly: the greatest city north of the Rio Grande, it was also the only city north of the Rio Grande. Five times or more bigger than any other Mississippian polity, Cahokia's population of at least fifteen thousand made it comparable in size to London, but on a landmass without Paris, Cordoba, or Rome."