Archaeology mystery of the Cahokia exodus: Why was this settlement abandoned?

Why did the once popular settlement of Cahokia Mounds in now modern-day St Louis, suddenly have a mass exodus in 1400?

Cahokia Mounds State Historic Site in Illinois
Cahokia Mounds State Historic Site in Illinois. Photo: Joe Angeles / Washington University

Around 900 years ago, the Cahokia Mounds settlement located across the Mississippi River in modern-day St Louis was bursting with 50,000 people. This settlement was one of the largest communities in the world, yet, by the year 1400 the once-popular site had seen a mass exodus.

One theory surrounding this desertion is that the Cahokia residents left the settlement after a prolonged drought which caused crops to fail. A new study in the journal The Holocene by Natalie Mueller, an assistant Professor of Archaeology in Arts and Sciences at Washington University St Louis, and Cailtin Ranking, a PhD ’20, suggests that there may have been another reason that the residents of Cahokia left.

Caitlin Rankin took soil samples at the historic settlement to collect Carbon Isotopes, a type of atom that is left behind by the plants. All plants use one of two types of Carbon, carbon 12 and Carbon 13 for photosynthesis, but not every plant photosynthesise in the same fashion. Plants that are adapted to live in a drier climate such as maze or prairie grasses, incorporate carbon into their bodies at a rate that will leave a signature when they die.

Most of the other plants that the Cahokians would have grown and harvested as food, such as squash, sumpweed, and goosefeet, would leave behind a different signature which they share with other plants from wetlands and native forests.

Carbon isotope evidence

Caitlin Rankin’s samples displayed that the ratios of Carbin 12 and 13 stayed consistent, suggesting that there was no shift in the types of crops they were growing at the settlement. “We saw no evidence that prairie grasses were taking over, which we would expect in a scenario where widespread crop failure was occurring,” said Mueller.

The Cahokians were known for their ingenuity and Rankin thinks that they may have had the irrigation skills and the engineering to be able to keep crops alive under difficult circumstances such as drought. “It’s possible that they weren’t feeling the impacts of the drought,” said Rankin.

Mueller has also suggested that the settlement may have also included a storage system for grains and other foods, and residents would have eaten varied diets including fish, birds, bears, deer, and forest fruits and nuts. So even if some food sources disappeared they had plenty of others to choose from.

Mueller hopes to build a database of palaeobotanical evidence from the Midwest to gain a better understanding of the diets and agricultural practices of Indigenous people. “Gathering that information would help us see if people switched to different crops in response to climate change,” she said. She also hopes to be able to grow some crops within a controlled environment to study how the plants would have responded to droughts and other challenges.

So, why did the Cahokian people decide to leave? Mueller thinks it may have been a gradual process. “I don’t envision a scene where thousands of people were suddenly streaming out of town,” she said. “People probably just spread out to be near kin or to find different opportunities.”

“They put a lot of effort into building these mounds, but there were probably external pressures that caused them to leave,” Rankin said. “The picture is likely complicated.”

Source of the news:

Rankin, C.G. and Mueller, N.G. (2024). Correlating Late-Holocene climate change and population dynamics at Cahokia Mounds (American Bottom, USA). Holocene.