Clare Eigenbrode’20
February 03, 2022

Central question

Why did residents of what’s now Cahokia, Illinois, abandon their thriving settlement a millennium ago?

Geoarchaeologist Caitlin Rankin’14 and her colleagues are untangling this mystery with the latest tools, an interdisciplinary mindset, and a willingness to challenge long-held assumptions.

By 7 a.m. on certain summer days, foreheads have already started dripping sweat onto researchers’ clipboards at the Cahokia Mounds State Historic Site in southwestern Illinois. Seven-day work weeks are common, and scientists armed with trowels wouldn’t seem too paranoid for triple-checking their ankles at the end of a long day for Lyme-carrying ticks.

But in return, sunburned researchers like geoarchaeologist Caitlin Rankin’14, who conducted most of her research at Cahokia as a Ph.D. student at Washington University in St. Louis, might be rewarded with a discovery a thousand years in the making.

•••

About a millennium ago, a settlement larger than contemporaneous London thrived near what’s now Cahokia, Illinois. At its height, the city’s metropolitan area was home to as many as 40,000 residents, with four large public plazas acknowledging the four compass directions and dozens of earthen mounds, including a 30-meter platform mound that remained the tallest manmade structure in the United States until 1867.

Such a structure might sound familiar to Beloiters, but unlike the mounds on campus, which were constructed by ancestors of the Ho-Chunk people and other tribes, the Cahokia mounds were works of monumental architecture and living spaces as opposed to burial or memorial sites. Both are permanent reminders that all of us are living on Indigenous land.

These days, Cahokia is regarded by archaeologists as one of North America’s first urban centers, but just 400 years after its population peaked, the site had been all but abandoned. Exactly what happened may be the most obvious question to ask, but it’s also the one that’s kept archaeologists hooked for decades.

A mural depicts the golden age of Cahokia, when the population was greater than London. A mural depicts the golden age of Cahokia, when the population was greater than London.
Credit: Cahokia Mounds State Historic Site

In 1993, researchers at Southern Illinois University-Edwardsville studied archaeological evidence of increased local wood consumption toward the end of Cahokia’s occupation. They formed a hypothesis that former residents had over-harvested timber from forests nearby, which could have caused erosion and, eventually, destructive flooding. That could mean that Cahokia’s residents were driven out of their homes by careless consumption of their own environment. That hypothesis of self-inflicted ecologic disaster was generally accepted by archaeologists for the better part of 30 years, but the research of Caitlin Rankin’14 has found that the evidence it was built upon was about as stable as an eroded hillside.

To reconsider what happened, Rankin and her team employed a branch of soil science called micromorphology, which investigates the components of soil at a microscopic level. Collaborating with micromorphologists in a German laboratory, they used a technique that treats soil and other unconsolidated sediments like a rock by impregnating them with epoxy so that they’re glued together. They then examined the soil’s microstratigraphy as a geologist might. Using this approach, Rankin was able to spot evidence of Cahokia residents trampling certain paths and even sweeping their earthen floors.

She was also able to check for indications of ancient flooding and erosion. Her team discovered that while flooding did occur in Cahokia, it happened relatively early in the city’s construction and didn’t appear to have deterred human settlement. Erosion and unstable soil do appear in the region’s history, too, but Rankin’s research didn’t find evidence of them until the mid-19th century—when European colonizers had already begun mining for coal in the area and removing vegetation from nearby bluffs.

These discoveries make way for new hypotheses about what really happened to the Cahokia settlement, including the possibility that it simply ran its course. They also carry implications for less-studied areas of the same site, and they expose a colonial tendency to “project our own problems, and how we think the world should be” onto societies for which they were not applicable, Rankin said recently.

That impulse to interpret human activity as inherently bad for the environment has created some convenient cautionary tales, but it doesn’t paint an accurate picture of pre-contact people’s understanding of the natural world, or what we can learn from ancient practices on this continent.

“You could easily write a dissertation that confirms a theory that’s already out there, but the kind of work that really makes an impact is the kind that challenges assumptions,” Bill Green says of Rankin’s research. Green is professor of anthropology and director emeritus of the Logan Museum of Anthropology and one of Rankin’s mentors while she was an anthropology student at Beloit.

“Beloit was where I learned how to do research,” Rankin says. “That’s where I was taught how to teach myself … Bill gave me the tools and taught me the steps, then gave me the space to figure it out.”

There was never a question that Rankin would seek an indoors-optional career, forehead sweat and all. When she arrived at Beloit in 2010, she was already sure she wanted to study archaeology, and had her eye on classics. But she’d grown up on a farm in Pennsylvania, and quickly found that classics was a little too indoorsy. After a couple of geology courses with Professors Sue Swanson, Jim Rougvie, and Carl Mendelson, “the field trips reeled me in,” she says. She graduated in 2014 with a double major in environmental geology and anthropology.

Those two interests neatly accommodate the interdisciplinary field of geoarchaeology, which examines the natural physical processes that impact archaeological sites. It takes an individual who’s accustomed to a liberal arts brand of holistic thinking to enter a field like this one, Green says. “Not every geologist understands the human part of what they’re studying, and not every archaeologist understands the dirt,” he says.

While working toward her degree at Beloit, Rankin lived with classmates in the now-defunct Geology House and took breaks from geology field trips to work in the basement of the Godfrey Anthropology Building as Green’s research assistant. Following a summer at the Angel Mounds State Historic Site near Evansville, Ind., through the National Science Foundation’s Research Experiences for Undergraduates program, she decided to head straight to graduate school for more of the same.

It was difficult to leave Beloit behind entirely, however. During a visit to Washington University in St. Louis, her tour guide was John Kelly’69. Green, a longtime friend of Kelly’s, describes him as a “walking encyclopedia of Cahokia” and one of the site’s most influential scholars. Not long after Rankin applied, she got a call from Kelly. “So do you want to come down?” he asked.

The two Beloiters were able to work closely together at Cahokia in the years that followed. “Caitlin was just phenomenal,” Kelly said recently. “I wasn’t holding her hand or anything. She wasn’t as excited about artifacts as I was, but she was excited about dirt.”

Kelly and his wife, the archaeologist Lucretia “Cricket” Schryver Kelly’69, met during a 15-week field school in Northern Wisconsin, made possible during the Beloit Plan, the year-round curricular program from their undergraduate years. Comparing and reminiscing about the two different Beloits they’d known and loved quickly became a favorite step in their research process.

And that era might not be over for the two of them. Kelly sees an important role for Rankin’s research in an installment of the Oxbow Books series “American Landscapes” that he’s writing about Cahokia with another colleague. It’s slated for publication next fall. Like much of his recent work, the book will focus on “trying to make sure that American Indians obtain full control over their history, including archaeology,” Kelly says.

Caitlin Rankin and her colleagues' discoveries challenge assumptions about why residents abandone... Caitlin Rankin and her colleagues' discoveries challenge assumptions about why residents abandoned Cahokia.
Credit: Dilip Vishwanat
Rankin, who now works as a research geoarchaeologist for the Illinois State Archaeological Survey through the University of Illinois-Urbana/Champaign, is also seeking to remedy what she calls a “lack of collaboration with Indigenous people” in her most recent research. She is working with a variety of tribes and the Missouri Department of Conservation to help grow native plants using Indigenous agricultural strategies.

Meanwhile, her paradigm-shifting findings, which make up one part of her Ph.D. thesis, are gaining national attention. After earning her doctorate in May 2020, she published her work with two other researchers in the journal Geoarchaeology in February 2021, and it was picked up by outlets like National Geographic, National Public Radio, Smithsonian Magazine and the New York Times.

There’s excitement about her discoveries because of the technology her team utilized, which didn’t exist when the Southern Illinois University-Edwardsville researchers published their own hypothesis, but also because many in the scientific community are eager for opportunities to re-examine their own fields’ chronically colonial thinking. Rankin says it’s a step in the right direction that we are “recognizing these assumptions in the first place.”

Rankin repurposes a beloved Beloit phrase when she speaks about the importance of interdisciplinary education in her life and career. To her, visualizing a sea filled with floating turtles, each bearing invaluable knowledge but unable to communicate with one another, illustrates when disciplines like anthropology and geology don’t collaborate. Then she pictures the turtles stacked on top of each other, passing knowledge up and down and enriching one another’s understanding of the world.

The future of scientific research, Rankin says, is “turtles all the way down.”


Clare Eigenbrode’20 is a writer currently working for an AmeriCorps program called City Year Milwaukee.


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