Cahokia, It’s Disintegration, And Our Need For Explanation

"Anyone" Charles Mann wrote in 1491: New Revelations of the Americas Before Columbus, "who traveled up the Mississippi in 1100 A.D. would have seen it looming in the distance: a four-level earthen mound bigger than the Great Pyramid of Giza. Around it like echoes were as many as 120 smaller mounds, some topped by tall wooden palisades, which were in turn ringed by a network of irrigation and transportation canals; carefully located fields of maize; and hundreds of red-and-white-plastered wood homes with high-peaked, deeply thatched roofs like those on traditional Japanese farms... Covering five square miles and housing at least fifteen thousand people, Cahokia was the biggest concentration of people north of the Rio Grande until the eighteenth century."

Cahokia is the name given to the site of the largest, and at the time most populated, of American Indian societies throughout the eastern half of North America, ranging from Louisiana up to Canada, characterized by large earthen mounds. It is the name of the site but not of the people who built it, nor the name of the language they spoke. This information isn't known, and the name Cahokia is that of a foreign group who moved in to the area much later. Despite this, there is a fair deal known about Cahokian society. Investigation has determined, for example, that the mounds supported dwellings or temples for the society's elite, which included their priests.

Humans constructed these mounds, though the reasons for them vary. The oldest of these Mound Builder sites is in northeastern Louisiana and dates back nearly five and a half thousand years, preceding the advent of agriculture in North America. This is surprising because the organization of labor necessary for such construction implies a hierarchical social stratification and a food surplus great enough to feed the multitudes of people who were not spending their time obtaining or cultivating all of that food themselves. A surplus of this kind is generally believed to require sedentary agriculture, yet these people were hunter-gatherers, and so it has changed the assumptions about the architectural capabilities of pre-agricultural humans. More puzzling is their utility. These mounds have consistently evaded any clear explanation of apparent purpose. It is known however that mounds constructed in the following millennia in other places often served different purposes in different areas, most notably as burial sites or locations of religious temples.

Herb Roe's map of various Mound Builder sites, taken from Wikipedia. 

The site called Cahokia is situated in the Mississippi River Valley. It is almost directly across Mississippi River from modern day St. Louis, where another Mound Builder site used to sit but has long since been destroyed. Unlike its oldest predecessors in Louisiana, it was constructed around the end of the first millennium A.D. and was "preeminent from about 950 to about 1250 A.D." and the only city north of the Rio Grande. This point bears elaboration though. While James Daschuk misleadingly refers to Cahokia as a "metropolis" in his book Clearing The Plains (2014), it was no city in the modern sense. Mann explains that a "city provides goods and services for its surrounding area, exchanging food from the countryside for the products of its sophisticated craftspeople. By definition, its inhabitants are urban- they aren't farmers. Cahokia, however, was a huge collection of farmers packed cheek by jowl." Despite its peak population numbering around fifteen thousand people, almost all were farmers, despite it being a center for regional trade. Mann builds on this apparent dissimilarity with the observation that "having never seen a city, its citizens had to invent every aspect of urban life for themselves." This point is interesting when comparing Cahokia to the cities of Tenochtitlan in the Triple Alliance (Aztec Empire) or Qosqo (Cusco) in the Inka Empire. These cities in modern-day Latin America were true metropolises, and interrogating the reasons for the stark differences between them and Cahokia might yield insights into the project of civilization building.

Monks Mound today. Source: https://newarkearthworkscenter.blogspot.com/2015/10/americas-largest-earthwork-cahokias.html?m=0

The largest of the mounds at Cahokia (or anywhere) is named Monk's Mound after Trappist monks who once lived nearby. Archaeologist and geographer William Woods has headed its excavations and discovered that its comprised of multiple layers. At its core is a giant clay slab measuring around 900 feet in length, 650 feet in width, and 20 feet in height. The innovative engineering abilities of Homo sapiens screams out from the mouthpiece of this architectural feat. Cahokia is located in what's called the American Bottom, a floodplain of the Mississippi river about 175 square miles in area. The soil is mostly clay, which is difficult to till and especially prone to flooding, as was the general area. Clay quickly absorbs water through capillary action, causing it to expand, and then can also dry out to impressive degrees of desiccation, progressively reducing in volume as it does. This repeated expansion and constriction makes it an exceptionally poor building material, especially for large structures. William Woods' excavations have demonstrated the ingenuity of Cahokia's solution to this problem. The mound had to remain constantly moist, which was more easily accomplished at the bottom levels of the mounds as the clay sucked up ground water but became an issue as that water evaporated on the upper levels. Mann explains:

"[T]he Cahokians encapsulated the slab, sealing it off from the air by wrapping it in thin, alternating layers of sand and clay. The sand acts as a shield for the slab. Water rises through the clay to meet it, but cannot proceed further because the sand is too loose for further capillary action. Nor can the water evaporate; the clay layers atop the sand press down and prevent air from coming in. In addition, the sand lets rainfall drain away from the mound, preventing it from swelling too much. The final result covered almost fifteen acres and was the largest earthen structure in the Western Hemisphere; though built out of unsuitable material in a floodplain, it has stood for a thousand years."

                                             Herb Roe's rendering of mound structure, taken from Wikipedia.

Atop the mounds were the dwellings of the Cahokia elites and the temples where they performed ceremonial religious rites in efforts to secure safety and prosperity. The ceremonies were held up there in secret from the peasants, who were also symbolically separated from the elites by a palisade that encircled Monks Mound, but which they could likely pass through freely at any time. At one point the elites of Cahokia added a low platform on one side for priests to perform their rites in public. According to Woods, his acoustic tests demonstrated that everyone below should have heard the priest or other leader with clarity. 

The real mystery of Cahokia is its demise. It should come as no surprise that the remains of a pre-literate society that ceased to exist over half a millennium ago would offer no clear explanation for its disappearance, but that hasn't stopped researchers from trying.

The staple crop of the Cahokia and much of the region was maize. This grew plentifully in the clay floodplains and spurred a population growth which may be tied to Cahokia's disintegration and abandonment. There is no clear answer to why Cahokia was fully abandoned by the early fifteenth century. Ecological overreach does not seem to be the whole story but it almost certainly played a role. As the population grew, Cahokians felled more upland trees for construction and cooking fires. With demand increasing, they had to clear more and more forest and also obtain more water. Mann cites Woods and his research, explaining that they likely diverted the Cahokia Creek to meet the Canteen Creek, which flowed near Cahokia and fed its population, bolstering its flow. (Ecological engineering did not begin with modern society, it became more pronounced.) This allowed access to a greater volume of water that made for a convenient transport network for felled trees, increasing the processes' efficiency. This also increased the creek's likelihood of flooding. The probability of flooding was further increased by the consistent upland deforestation, which Mann explains meant "rainfall sluiced faster and heavier into the creeks, increasing the chance of floods and mudslides." The hypothesis for deforestation is evidenced by a decrease in pollen from the relevant tree species in that area in mud sediments dating to that era. Much of Wood's hypothesis for the collapse of Cahokian society is premised on this and the assertion that floods destroyed maize crops and houses to a severe enough degree as to cause social disintegration. However, Mann contradicts, there "is little indication that the Cahokia floods killed anyone or even led to widespread hunger." 

Reproduction of Cahokia at its peak. Source: http://gotravelaz.com/cahokia/#photo_1

Floods didn't cause Cahoka's abandonment, and there's no evidence it was the result of warfare either. Needing an answer, Mann takes the estimates of the seismologist Otto Nuttli and asserts positively that an earthquake in the early thirteenth century rocked Cahokia and toppled the west side of Monks Mound, causing fires from the splintering houses and falling torches and cooking embers, burning much of Cahokia to the ground. The quake would also have, he says, made the rivers "[slosh] onto the land in a mini-tsunami." This would have led to social breakdown and, as Woods says, a civil war. In my attempts to find Nuttli's seismological estimates, which Mann does not directly cite, I could find none asserting an earthquake likely occurred around the end of the fifteenth century. Of course, it is possible I missed something, but one would expect such positive declarations to be well cited by their claimants. 

Mann is not the only one eager to offer an explanation with all airs of apparent authority. In Clearing The Plains James Daschuk claims with equal certainty that the direct stimulus for decline and disintegration was catastrophic climate change. According to him, that climate change was caused by a massive volcanic eruption in 1259 A.D. which induced a "Little Ice Age" that came on suddenly in the last quarter of the 13th century.

With contradicting claims, all made without qualification, I was befuddled while pondering which was accurate, if any were. My own research brought me no closer to substantiating any one with anything that would even be analogous to a  confidence interval of 95% (the standard in scientific research for a finding to be considered statistically significant). Remembering an old Smithsonian article I'd read had said that the reason for the society's disappearance was unknown, I emailed American History professor Steven Gimber of West Chester Pennsylvania. His response was really a lesson in just how honed the edge of Occam's razor is:

"Frequently ppl/ scholars want to proclaim that they have determined THE ONE thing that led to something major - like the end of a civilization thousands of years ago - but in reality, it's never really just one thing.  Combinations of factors lead to human actions/ decisions and it's difficult to say with certainty why ppl did (or do) something.  We can't ask them so we have to piece together answers from what they left us and what remains of their time that is accessible to us (evidence of conflicts, droughts, floods, invasions, or migrations)." 

There is no clear answer. The further removed one is from the period and conditions of the subjects one is studying, the less evidence there will be for study. The desire to be the researcher who has determined the certain cause of this or that historical phenomenon is at times like these more an answer to one's confusion than those the researcher provides. There is useful information for speculation, but nothing more. Any voice proffering definitive explanations ought to be suspected. Against our deepest desires, sometimes we lack any good answer to serious questions.