In last week's issue of the journal Science, Andrew Lawler gives a lengthy report on the forgotten city of Cahokia. For a while now archaeologists have known about this Native American settlement beneath modern East St. Louis, but many believed it was what Lawler calls a "seasonal encampment." A new round of archaeological digs, done in preparation for a bridge being constructed across the Mississippi River between Missouri and Illinois, has unearthed evidence of "a sophisticated, sprawling metropolis stretching across 13 kilometers on both sides of the Mississippi" that existed about a thousand years ago, Lawler writes:
[A] millennium ago, this strategic spot along the Mississippi River was an affluent neighborhood of Native Americans, set amid the largest concentration of people and monumental architecture north of what is now Mexico.
Back then, hundreds of well-thatched rectangular houses, carefully aligned along the cardinal directions, stood here, overshadowed by dozens of enormous earthen mounds flanked by large ceremonial plazas. … Cahokia proper was the only pre-Columbian city north of the Rio Grande, and it was large even by European and Mesoamerican standards of the day, drawing immigrants from hundreds of kilometers around to live, work, and participate in mass ceremonies.
Archaeologists believe people began to gather at Cahokia around the year 1000 A.D. Inspired perhaps by the sighting of Halley's Comet in the year 989, settlers erected ceremonial mounds at the site, some of which line up with the sun's position during the winter solstice. Around the year 1100 they began to build Monks Mound — the largest mound, reaching some 100 feet off the ground, created from millions of baskets of dirt. A vast palisade that enclosed Monks Mound and other parts of the settlement was constructed around the year 1200. For reasons still debated, the whole city failed around the start of the following century.
The latest excavations have uncovered evidence of more than five hundred thatched houses and signs of workshops where residents created various goods. The homes surrounded the ceremonial sites, and at its peak the settlement may have expanded out into a primitive metropolitan area that served as residence to tens of thousands of Native Americans. But as a city Cahokia lacked the density of Mayan or European settlements; instead it appears to have organized itself more along the lines of "modern American urban sprawl," Lawler writes.
While settlement at Cahokia was short-lived, its cultural impact appears to have been widespread. Researchers working as far away as Wisconsin have found evidence of Cahokia-style pottery and housing. Why exactly the city disappeared it still a matter of conjecture. The leading assumptions point to political problems, a changing climate, or a combination of both. In a paper published in a 2009 issue of the journal American Antiquity, a research team led by Larry Benson of the U.S. Geological Survey presented climate-related evidence that "a series of persistent droughts occurred in the Cahokian area" which may have contributed to the city's abandonment:
By A.D. 1150, in the latter part of a severe 15-year drought, the Richland farming complex was mostly abandoned, eliminating an integral part of Cahokia's agricultural base. At about the same time, a 20,000-log palisade was erected around Monks Mound and the Grand Plaza, indicating increased social unrest. During this time, people began exiting Cahokia and, by the end of the Stirling phase (A.D. 1200), Cahokia's population had decreased by about 50 percent and by A.D. 1350, Cahokia and much of the central Mississippi valley had been abandoned.
It's fitting that the construction of a new bridge has led to additional revelations about the lost city. As Glenn Hodges reported in National Geographic in early 2011, it was the implementation of the Interstate Highway System that led to a surge of new interest in the settlement, by providing funds for excavation near highway sites. Interstate 55/70 in East St. Louis now bisects what was Cahokia's north plaza, Hodges writes.