Built from an estimated 22 million cubic feet of earth and standing about 100 feet high, Monks Mound is the tallest structure at the Cahokia Mounds State Historic Site.

This American Indian metropolis was mysteriously abandoned. Archaeologists want to know why.

Cahokia was a robust economic and cultural center for the Mississippian people. They built hundreds of massive mounds but suddenly left the city in the 1300s.

Built from an estimated 22 million cubic feet of earth and standing about 100 feet high, Monks Mound is the tallest structure at the Cahokia Mounds State Historic Site.
Ira Block/National Geographic Image Collection

East of the Mississippi River in southern Illinois lie the remains of an American Indian metropolis: the Cahokia Mounds. This urban center occupied nearly 4,000 acres during its peak between A.D. 1000 and the 1200s. Cahokia’s inhabitants built scores of burial mounds, public buildings, and even an astronomical observatory, “Woodhenge,” whose wood posts aligned with the sun throughout the year. Dominating the landscape then, as it does now, was Monks Mound—the largest earthwork north of Mexico. Its four terraces soared about 100 feet high.

Cahokia Mounds stands at the center of what was once the greatest civilization between the deserts of Mexico and the North American Arctic. The site of one of North America’s first cities and arguably one of American Indians’ finest achievements, it is the most visible example left today of the Mississippian culture, an agricultural civilization that spread across the U.S. Midwest and Southeast starting around A.D. 800 and peaking around the 13th century.

Overlooked

Today the site is an Illinois state historic site, a national historic landmark, and a UNESCO World Heritage site. Nevertheless, until recently it was largely familiar mostly to people around St. Louis, Missouri. That ignorance has deep roots. The first white person to have written a detailed account of Cahokia’s mounds was Henry M. Brackenridge, a lawyer and amateur historian who came upon the site and its massive central mound while exploring the surrounding prairie in 1811.“I was struck with a degree of astonishment, not unlike that which is experienced in contemplating the Egyptian pyramids,” he wrote about the 100-plus mounds he counted. “What a stupendous pile of earth! To heap up such a mass must have required years, and the labors of thousands.”

(This little-known Native American society was once as powerful as the Aztecs and Incas.)

Newspaper accounts of his discovery were widely ignored. Brackenridge complained about this in a letter to a friend, former President Thomas Jefferson; with friends in such high places, word of Cahokia did eventually get around.

Unfortunately, it was not news that most Americans, including subsequent presidents, were particularly interested in hearing. Andrew Jackson’s Indian Removal Act of 1830, which ordered the relocation of eastern Native Americans to land west of the Mississippi, was premised on the white supremacist notion that American Indians were “savages,” incapable of making “good use” of land. Evidence of a prehistoric city—one even larger than Washington, D.C., at the time—would have threatened that narrative. Nineteenth-century historians theorized that the mounds were built by an almost comic array of alternate peoples—Phoenicians, Vikings, or even perhaps a lost tribe of Israel—rather than acknowledge the skills and efforts of Native Americans.

(Sequoyah, the U.S. state that almost existed, disappeared when Indigenous lands were folded into Oklahoma.)

Even American universities took scant notice of Cahokia and other local sites before the second half of the 20th century. They preferred sending archaeologists to Greece, Mexico, and Egypt, where the stories of ancient civilizations were comfortably distant and romantic. Not until the 1880s did years of fieldwork by the Smithsonian Institution’s Cyrus Thomas, originally a skeptic himself, definitively prove the mounds were of Native American origin. Still, relatively few people championed Cahokia and its neighboring mound centers in East St. Louis and St. Louis (once nicknamed “Mound City”), which fought a mostly losing battle against development, neglect, and treasure hunters for the better part of a century.

Though Monks Mound, named for French monks who once lived in its shadow, became a tiny state park in 1925, it was used for sledding and campgrounds. The rest of Cahokia was largely ignored—built on and only sporadically studied—until the 1960s. Ironically, the biggest construction project to tear into Cahokia would also put it on the map. President Dwight Eisenhower’s interstate highway program contained provisions for the study of archaeological sites in its path, which meant more money for excavations than had ever been available, as well as a clear agenda for where to dig, when, and how fast. With two highways slated to skewer the ancient city (I-55/70 now bisects Cahokia’s north plaza, creating a road sandwich with Collinsville Road, a quarter mile to the south), archaeologists began to study the site systematically. What they found was nothing less than revelatory.

Birth of a city

It became apparent that Cahokia was more than just a stupendous pile of earth or an occasional meeting spot for scattered tribes. Nearly everywhere they dug, archaeologists found homes—indicating that thousands of people had once formed a community there— and many of their homes were built in a very brief span of time.

Research revealed that the whole city seemed to have sprung to life almost overnight around 1050. People streamed in from surrounding areas, building houses and the infrastructure of a new city—including several mounds with buildings on top and a grand plaza the size of 45 football fields, used for everything from sporting events and communal feasts to religious celebrations.

From the flat top of the colossal Monks Mound—with a footprint of 14 acres, the mound is larger at its base than the Great Pyramid of Khufu—the view encompasses the vast floodplain known as the American Bottom. After directing the construction of what would have been the highest geographic feature in 175 square miles, a chief or high priest would have had a bird’s-eye view of the land under his sway.

That scenario presumes that Cahokia had such a single leader, which is not universally agreed upon. It is not even known what the site was called—the name Cahokia is borrowed from a tribe that lived nearby in the 1600s—or what the people who lived there called themselves. With no written language, they left behind a smattering of meager clues ripe for dispute.

Despite the many points of contention among Cahokia scholars, there are still points of general consensus. Experts largely agree that the city quickly developed a couple centuries after corn became an important part of the local diet; that it drew together people from the American Bottom; and that it dwarfed other Mississippian communities in size and scope. Scholars tend to be divided on the size of its population, the nature of its government and economy, and the extent of the city’s influence.

Podcast: The Descendants of Cahokia
Listen Now

(In this episode of our podcast Overheard, we chat with an anthropologist working to protect the remaining burial mounds and sacred shrines of Cahokia so that the descendants of the ancient city's founders can keep its legacy alive. Listen now on Apple Podcasts.)

Mississippian culture

The Cahokia Mounds and the people who lived among them belonged to what is known today as the Mississippian culture. While the Cahokia site is the largest settlement found to date, archaeologists have also found the remains of Mississippian settlements in the Southeast, into the Midwest, and north into the Great Lakes region. Like Cahokia, many of them feature giant earthworks, wooden palisades and fortresses, and artifacts crafted from copper, shell, and stone.

It is difficult to pinpoint the exact origin of the Mississippian culture, but many experts place it around A.D. 800. Around that time villages began to form along the central Mississippi River Valley, and farmers began growing maize (which became the dominant staple), beans, and squash. Similar settlements appeared in other river valleys across the Southeast and Midwest. In these fertile lands of North America, Mississippian peoples, for the most part, enjoyed a mild climate, plentiful water, and abundant natural resources, including timber, nuts, fish, and wild game.

The kinds of artifacts found at Cahokia and other Mississippian sites demonstrate the existence of large trade networks among the different villages across North America. Mound 34 at the site has been identified as the only Native American copper works in North America. Copper would be brought to Cahokia from the Great Lakes region. The nuggets would be crafted into objects—some sacred objects, others diplomatic gifts—that have been found at other Mississippian sites across the continent.

Touching the sky

As many as 120 mounds used to rise above the Cahokia landscape. Today 80 of them have been preserved and are part of the Cahokia Mounds State Historic Site. The city itself was organized on a grid that aligned with astronomical movements of the sun and moon. Plazas, houses, public buildings, the mounds, and connecting infrastructure were all oriented according to this heavenly plan.

Study of the mounds has revealed that they served different purposes in Cahokia’s culture. Based on the mounds’ construction, archaeologists have grouped them into three categories: flat top, round top, and ridge top. Flat-top mounds generally had some kind of building on top, while round-top mounds were for burials. The ridge-top mounds, some of which also contain burials, were classified as “directional” because they seemed to mark different areas of the city.

Named for French monks who once lived nearby, Monks Mound is the largest of all the structures. It covers about 14 acres and stands about 100 feet tall. Monks Mound’s highest terrace was once topped with a structure that may have been the home of a chief or priest or perhaps served as a ritual space. Archaeologists estimate that it took 22 million cubic feet of earth to build Monks Mound. Analysis of the soils found that the earth came from local pits. It was excavated with stone tools and then transported in baskets.

Mysterious decline

As many as 15,000 people lived in Cahokia at its peak, some estimate, but by 1400 the city was deserted. Cahokia’s demise is perhaps an even greater mystery than its emergence. By the 1400s it was abandoned. The American Bottom and substantial parts of the Mississippi and Ohio River Valleys had become so depopulated they are referred to as the Vacant Quarter. Historians note that the city grew to prominence during an especially favorable climate phase and began shrinking around the time the climate became cooler, drier, and less predictable. For an agricultural community dependent on regular crop yields, the changing conditions could have been anything from stressful to catastrophic.

(Why was the ancient city of Cahokia abandoned? New clues rule out one theory.)

The fact that between 1175 and 1275 Cahokia’s inhabitants built (and rebuilt, several times) a stockade encircling the city suggests that conflict or the threat of conflict was a regular part of life—perhaps because competition grew fierce. Furthermore, dense populations create environmental problems—pollution, disease, diminished resources—that can be difficult to counter and that have brought down many societies. 

One of the most popular explanations is the so-called deforestation hypothesis. In 1993 researchers from Southern Illinois University Edwardsville suggested that Cahokia’s decline might have been caused by cutting down thousands of trees to build stockades and other improvements. Fewer trees meant more erosion, more flooding, and degraded harvests. Their hypothesis found wide acceptance among Cahokia scholars.

In spring 2021 geoarchaeologist Caitlin Rankin turned that idea on its head. She published her findings in the journal Geoarchaeology, showing that it was impossible for deforestation and flooding to cause the city’s demise. Rankin’s excavations at the site found no evidence of flooding during Cahokia’s occupation. As scholars look to other explanations, some are examining if increased conflict among different groups caused the city to decline. Others are looking to see if a major drought in the region could have led the Cahokia people to seek more fertile lands and leave their city behind.

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