Strategic Communications and Marketing News Bureau

North ‘plaza’ in Cahokia was likely inundated year-round, study finds

CHAMPAIGN, Ill. — The ancient North American city of Cahokia had as its focal point a feature now known as Monks Mound, a giant earthwork surrounded on its north, south, east and west by large rectangular open areas. These flat zones, called plazas by archaeologists since the early 1960s, were thought to serve as communal areas that served the many mounds and structures of the city.

New paleoenvironmental analyses of the north plaza suggest it was almost always underwater, calling into question earlier interpretations of the north plaza’s role in Cahokian society. The study is reported in the journal World Archaeology.

Researcher in a trench.

Caitlin Rankin stands in a trench dug in Mound 16, one of four mounds that delineate the north plaza.

Photo by Ann Merkle

Delete

Edit embedded media in the Files Tab and re-insert as needed.

Cahokia was built in the vicinity of present-day St. Louis, beginning in about A.D. 1050. It grew, thrived for more than 300 years and was abandoned by 1400. Many mysteries surround the culture, layout and architecture of the city, in particular its relationship to water. Cahokia was built in a flood plain below the confluence of the Mississippi and Illinois rivers and would have been regularly infiltrated with flowing water, said Caitlin Rankin, a geoarchaeologist at the Illinois State Archaeological Survey who conducted the new research.

Photo of wetland with stumps and logs

Researchers took samples from nearby wetlands to determine the environmental history of the north plaza in Cahokia.

“Cahokia is the largest archaeological site in North America, but only about 1% of it has been excavated, so there’s so much about the site that we don’t know,” Rankin said.

Early in her encounters with the city’s layout, Rankin was baffled by the location and height of the north plaza.

“It’s a really strange area because it’s at a very low elevation, like the lowest elevation of the site,” she said. “And it’s in an old meander scar of the Mississippi River.”

Two creeks ran through the area, and it likely flooded whenever the Mississippi swelled after heavy rains.

Grace Ward collecting sample in wetland.

Grace Ward, a paleobotany graduate student at Washington University in St. Louis, collects baseline samples from a nearby wetland.

To investigate the site, Rankin conducted test excavations and extracted sediment cores around the four mounds that define the north plaza. She also took soil samples in the same meander scar less than 5 kilometers from the plaza and analyzed stable carbon isotopes in these modern soils to determine isotope differences between wetlands, seasonal wetlands and prairie environments. Comparing these with carbon isotopes from ancient soils chronologically associated with the mounds gave insight into what types of plants had grown there in the past.

“What I learned is that this area remained wet throughout the year,” Rankin said. “There may have been some seasonal dryness, but overall, it was a wetland.”

Researcher in a prairie

Rankin collects baseline data from a nearby prairie.

Her findings challenge previous notions about this site being a plaza, which is generally thought of as a dry open area across which people walk and congregate. “Generally, those places aren’t underwater,” Rankin said.

Team with sediment coring machine

Rankin takes sediment cores at various locales to determine the paleoenvironmental history of the north plaza in Cahokia.

How the north plaza was used remains a mystery, she said, but the study adds to the evidence that water was a central element of the city.

Rankin standing with a shovel in a very deep trench.

Sediments from excavations at Mound 5 reveal that the north plaza was a wetland prior to, and after, mound construction.

Photo courtesy Caitlin Rankin

Delete

Edit embedded media in the Files Tab and re-insert as needed.

“Water was important to the people of Cahokia for a number of reasons,” she said. “They had a whole agricultural suite of wetland plants that they domesticated and relied on as food.” Water also was essential to their trade with people up and down the Mississippi River. And the cosmological beliefs of many Indigenous groups include creation stories that involve complex interactions with sky, water and earth.

“At Cahokia, you have these mounds emerging from this watery sphere,” she said. “And so that was a significant feature that probably resonated with their creation stories and their myths and their worldview.”

The Illinois State Archaeological Survey is a division of the Prairie Research Institute at the University of Illinois Urbana-Champaign.

The National Geographic Society and National Science Foundation supported this work.

Caitlin Rankin with measuring tape in wetland.

The author collected baseline information from nearby wetlands.

Editor’s notes

To reach Caitlin Rankin, email rankinc@illinois.edu

The paper “The exceptional environmental setting of the north plaza, Cahokia Mounds, Illinois, USA” is available online and from the U. of I. News Bureau.

DOI: 10.1080/00438243.2022.2077824

Read Next

Life sciences Photo of Michael Ward standing in tall grass on a riverbank.

How are migrating wild birds affected by H5N1 infection in the U.S.?

Each spring, roughly 3.5 billion wild birds migrate from their warm winter havens to their breeding grounds across North America, eating insects, distributing plant seeds and providing a variety of other ecosystem services to stopping sites along the way. Some also carry diseases like avian influenza, a worry for agricultural, environmental and public health authorities. […]

Announcements Marcelo Garcia, professor of civil and environmental engineering at The Grainger College of Engineering.

Illinois faculty member elected to National Academy of Engineering

Champaign, Ill. — Marcelo Garcia, a professor of civil and environmental engineering in The Grainger College of Engineering, has been elected to the National Academy of Engineering.

Social sciences Male and female student embracing on the quad with flowering redbud tree and the ACES library in the background. Photo by Michelle Hassel

Dating is not broken, but the trajectories of relationships have changed

CHAMPAIGN, Ill. — According to some popular culture writers and online posts by discouraged singles lamenting their inability to find romantic partners, dating is “broken,” fractured by the social isolation created by technology, pandemic lockdowns and potential partners’ unrealistic expectations. Yet two studies of college students conducted a decade apart found that their ideas about […]

Strategic Communications and Marketing News Bureau

507 E. Green St
MC-426
Champaign, IL 61820

Email: stratcom@illinois.edu

Phone (217) 333-5010