Excavations in the Midwest have turned up evidence of a massive ancient fire that likely marked “the beginning of the end” for what was once America’s largest city, archaeologists say.

The digs took place in southern Illinois, just meters away from the interstate highways that carve their way through and around modern-day St. Louis. But 900 years ago, this was the heart of Greater Cahokia, a civilization whose trade routes and religious influence stretched from the Great Lakes to the Deep South, and whose culture shaped the lifeways of the Plains and Southern Indians.

Here, researchers with the Illinois State Archaeological Survey have discovered a widespread layer of charcoal and burned artifacts among the foundations of ancient structures — evidence of a great and sudden conflagration that consumed perhaps as many as 100 buildings.

While there’s only “circumstantial evidence” as to what caused the fire, the researchers say, what’s even more striking is that the event seems to mark an ominous turning point in Cahokian culture.

The structures destroyed by the fire were never rebuilt, the excavations showed. Meanwhile, other large, important buildings, like distinctive ceremonial “lodges” or houses for local elites, stopped appearing altogether throughout the region. And soon after the fire, a great palisade wall went up around the nearby city center — known to archaeologists as Downtown Cahokia — most likely for protection.

“My colleagues and I believe that we have pinpointed a major turning point in ancient Cahokia’s history,” writes Dr. Tim Pauketat, archaeologist at the University of Illinois, in a statement.

“We have found, we think, the beginning of the end of this American Indian city.”

Pauketat, author of Cahokia: Ancient America’s Great City on the Mississippi , is also lead author of a paper describing the find in the Journal of Field Archaeology.

The end, in this case, began at a site known today as the East St. Louis precinct, a large walled compound some 10 kilometers from Downtown Cahokia that was likely the site of important civic and religious ceremonies.

During the culture’s heyday, from about 1050 to the mid-12th century, East St. Louis was the second-largest ceremonial center in all of eastern North America — after Downtown Cahokia itself, which at its peak was home to as many as 10,000 people.

The compound included dozens of pole-and-thatch structures, along with a large leveled plaza, and at least two pyramids made of packed earth.

Inside some of the buildings, instead of the usual wares of daily life, the scientists found pigment stones, crystals, and “unusual” half-spheres made of fired red clay — items thought to be key to Cahokian rituals.

Many sites were also littered with uneaten corn, yet not enough to suggest that it was being stored there. Instead, Pauketat conjectures, it and other goods may have been put there in “token amounts,” as if made in offering.

Radiocarbon dates of the charred remains place the fire at around 1170 CE, near the midpoint of Cahokia’s century-long prime.

Around this time, the culture’s political and religious tendrils ran for hundreds of kilometers in every direction, as did its trade routes, through which ideas were spread and ritual goods were obtained. [Read about Cahokia’s ceremonial beverage: “Ancient Americans Pounded Vomit-Causing ‘Black Drink’ 6 Times Stronger Than Coffee“]

But decades’ worth of excavations all around Greater Cahokia have shown evidence of economic hard times and political strife in the 1100s that could have led to instability — even rebellion. [Learn about human sacrifice at Cahokia: “Infamous Mass Grave of Young Women in Ancient City of Cahokia Also Holds Men: Study“]

For its part, however, the research team suspects that the fire may have been set intentionally, by Cahokians themselves, for ritual purposes. It could have been done to commemorate the burial of elites possibly interred in a nearby mound, for instance, though this can never be confirmed since the mound was demolished by settlers in the 1870s.

What’s more, soil layers above the burned ruins show that the sites were carefully cleaned and maintained after the fire — scorched earth and charcoal having been neatly swept in to fill the foundations.

After the fire, the team found, a handful of earthen pyramids were built in the East St. Louis precinct, but the construction of wooden structures stopped.

Meanwhile, they note, other excavations have found that building patterns in the farming communities surrounding East St. Louis also changed around this time, hinting at a major cultural shift.

“Before 1170, the East St. Louis site was heavily populated, and the Cahokians living there and across the Metro-East region were known for their special … religious buildings or elite houses,” Pauketat writes.

“But after 1170, Cahokians stopped constructing these special buildings. At the same time, East St. Louis site was burned and emptied of its people. Only temple mounds were constructed at the site in later years, as if the place had become a ghost town, remembered and celebrated, but not lived in.”

And while the great wall erected in Downtown Cahokia has not been definitively dated, he adds, “[p]resumably, that palisade wall was built either in anticipation of or in response to the events of 1170.”

From there, the team notes, other changes in material culture quickly began to manifest themselves all over Greater Cahokia — like new methods of making clothing and pottery, and even new visual symbols showing up on ceramic decorations — all part of a gradual but undeniable social reorganization.

In the end, Pauketat says, what caused the epic fire may be less important than the pivot point in history that it seems to mark.

In that way, the team’s excavations are more of a benchmark from which future research may resume, rather than a final answer to the riddle of Cahokia’s demise.

“While we don’t know yet why this all-important piece of Cahokia was burned,” he says, “we are beginning to piece together the puzzle of Cahokia by linking old and new excavations together.”

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Timothy Pauketat, Andrew Fortier, Susan Alt, & Thomas Emerson (2013). A Mississippian conflagration at East St. Louis and its political-historical implications Journal of Field Archaeology DOI: 10.1179/0093469013Z.00000000054