What Caused the Fire that Destroyed America’s most prosperous Ancient City of Cahokia?

The ancient city of Cahokia, which was built around 800 AD, was once home to 20,000 inhabitants, stretches of farmland, wealthy communities and surrounded by 120 earthen mounds. However, in 1170 AD, it was ravaged by a massive blaze which left the city in ruins, leading to dramatic changes in their society, culture and architecture. A study has presented strange new evidence leading to exciting theories about the cause of the fire.

An artist depicts the Monks Mound as found within the interpretive center at Cahokia Mounds State Park.

The secrets of the ancient city of Cahokia lie buried below where St. Louis, Missouri stands today in an area which is considered the largest and most complex archaeological site north of the great Pre-Columbian cities in Mexico. Cahokia covered a vast area of about 6 square miles. 

The 'Cahokia Woodhenge' site was discovered as part of salvage archaeology in the early 1960s interstate highway construction boom. The Cahokia Woodhenge was a series of large timber circles located roughly 2,790 feet to the west of Monks Mound at the Mississippian culture Cahokia archaeological site in Collinsville, Illinois. They are thought to have been constructed between 900 and 1100 AD; with each one being larger and having more posts than its predecessor. One of the circles was reconstructed in the 1980s. The circle has been used to investigate archaeoastronomy at Cahokia. Annual equinox and solstice sunrise observation events were held at the site.

The mounds were believed to have been built as a place of worship and seem to have had religious significance to the dwellers, with tombs below and places of ceremony on top.  Like the Mayans of Mexico, the civilization was also known to make human sacrifices, including dismembering and burying people alive.

However, one of the greatest mysteries surrounding this World Heritage (1982) listed site, is the devastating fire which ripped through the main ceremonial plaza in the center of the city, destroying many of the buildings which were wooden with thatched roofs. Surrounding this peculiar event is the fact that after the fire the city was a changed place: new architectural designs sprung up, along with new defensive walls. In the original city, the rich and powerful lived in large homes whereas following the blaze all the structures became more regulated and smaller. There was also a sudden influx of clay plates featuring sun symbolism. Was this a sign of a new spiritual or political regime in the area? What is clear is that the fire marked a major turning point in Cahokia's civilization and perhaps the beginning of an end; but why?

University of Illinois at Urbana-Champaign archaeologist Timothy R. Pauketat and his colleagues pored over new dig sites in East St. Louis, and examined the evidence for the fire, and what they found was extremely strange.

The 100 or so thatched buildings that had been destroyed were all packed with luxury items such as clay pots, pipes, and animal bones used in ceremonies, items which were not typically found in regular homes. No garbage pits or normal household items could be found. The houses also appeared to have been hastily constructed, indicating that they were more like temporary structures, and they were placed much more closely together than elsewhere in the city. 

Another strange finding was that the homes which were burned were not rebuilt. Previous digs in Cahokia showed that if houses burnt down, the dwellers would rebuild on top however in this instance, the ashes were swept into piles and left untouched.

Monks Mound with reconstructed stairs in a 2007 photo; repairs done to the mound at Cahokia in 2005 shored up the mound and kept it from further collapse.

The researchers believe that all these clues point to the fact that instead of the fire being an accident or being set by an enemy, it was in fact a mass sacrifice. It wasn't uncommon for the mound builders to burn the structures they built at the top of mounds in ceremonial events. But if this fire was sacrificial, it was on a scale that was unprecedented.

One theory is that the fire marked a decline in the city's power, and the sacrifices were part of an on-going effort to restore the city's former status. If this was the case, it was not successful because by 1400AD, Cahokia and its vicinity had been almost entirely abandoned. It lost power and never regained its reputation again.

Compiled by Neil Gale, Ph.D.

Monks Mound at Cahokia was built in decades, not 250 years as previously thought.

By studying plant seeds and spores in the soil used to construct Monks Mound, the largest prehistoric earthen structure in North America, archaeologists have determined that it was not built over the course of 250 years, as previously thought, but in decades, a small fraction of that time.

An artist’s depiction of the Monks Mound is found within the interpretive center at Cahokia Mounds State Park in Collinsville, Illinois.

Monks Mound is in the ruins of the ancient city of Cahokia in Illinois. At its height, about 1,000 years ago, Cahokia was home to as many as 20,000 people. The mound was a series of rectangular terraces that reached 10 stories or 100 feet in height, and its footprint covered 14 acres or 610,000 square feet. The structure had a large public building at its apex, perhaps a temple. There are many other mounds at the site, but Monks Mound towers over them. It was named after Trappist Monks who lived for a very short time on a nearby mound.

Researchers say their new study of the soil in the mound, which began collapsing in 2005, shows that the presence of annual plant seeds and spores, as opposed to perennials, shows the mound was probably built within a few decades. The workers got the soil and sediments from a nearby borrow pit (an area where material like soil, gravel or sand has been dug up for use at another location). Archaeologists surmise that workers got soil from a nearby borrow pit and used it to build the mound. They did so without wheels or beasts of burden, carrying the soil by hand.

The team, led by Dr. Neal Lopinot of Missouri State University, took advantage of the collapse in 2005 and took samples from 22 exposed areas of the mound to study sediments from the floodplain used in constructing it. Apart from remains of perennial plants used for food, they found seeds and spores from wild annual plants that grow once and then die. They concluded from this that the borrow pits where the soil was taken were disturbed frequently.

Monks Mound with reconstructed stairs in a 2007 photo; repairs done to the mound at Cahokia in 2005 shored up the mound and kept it from further collapse.

That leads them to conclude that Monks Mound was built much quicker than surveys in the 1960s seemed to show. Researchers had theorized in the 1960s, based on nine cores taken, that the mound was built in 14 stages over 250 years. The theory seemed credible given Monks Mound’s size and that it was built by hand. Another thing the researchers found was that the seeds were not burned or carbonized, which makes them believe the seeds were covered quickly and not exposed to campfires or cookfires. Plus, they found that soil was cut in sod-like blocks and laid upside-side down in the mound. So some of the mound was built with sod instead of baskets full of soil.

In 2005 experts did emergency, high-tech repairs to the mound to shore it up. The completed repairs have saved Monks Mound from further collapse. It took intelligence to build it to last over 1,000 years without modern technology.

Compiled by Dr. Neil Gale, Ph.D.

Ceremonial Road Discovered in the Ancient City of Cahokia.

A new study published in the Midcontinental Journal of Archaeology revealed a major ceremonial road running through the heart of Cahokia. Such a road has been the subject of debate and conjecture since the 1920s, and confirming its existence changes our understanding of the prehistoric city. 

A reconstruction of Cahokia with Monk's Mound in the distance.

The ancient indigenous people city of Cahokia near Collinsville, Illinois, is known to have been one of the most sophisticated pre-Columbian settlements north of Mexico. At its peak, it was home to 20,000 people and sprawled over 4,000 acres.

Cahokia was once composed of a collection of agricultural communities across the Midwest and Southeast starting around 800 AD and flourishing between the 11th and 12th centuries. It is a striking example of a complex chiefdom society, with many satellite mound centers and numerous outlying hamlets and villages. It was also a place where the indigenous people made pilgrimages for special spiritual rituals linked to the origin of the cosmos. At its peak, Cahokia boasted some 120 mounds, the largest of which is a ten-story earthen colossus known as Monks Mound. The giant mound is the largest prehistoric earthwork in the Americas, covering over 12 acres and is 100 feet high. An estimated 78,500 square feet of earth was used to build the mound between 800 and 1,350 AD, but not long after this time, Cahokia was mysteriously abandoned. 

The newly discovered ancient road, dubbed the "Rattlesnake Causeway," is an elevated embankment about 60 feet wide that stretches from Cahokia's Grand Plaza south through the center of the city, where it dead-ends in the middle of the burial feature known as Rattlesnake Mound.

Monks Mound with reconstructed stairs in a 2007 photo; repairs done to the mound at Cahokia, in Collinsville, Illinois, in 2005 shored up the mound and kept it from further collapse.

Dr. Sarah Baires of the University of Illinois has suggested that the causeway may have been a literal and symbolic centerpiece of the city, as it is aligned 5° east of north, forming a central "axis" around which the community seems to have been built. Previous research had indicated that the city's major mounds, plazas, and households were oriented along this 5° alignment. Now it appears the causeway marked the axis itself.

Previous research has suggested that Cahokia's buildings align with a celestial event known as the "major lunar standstill," when the moon rises at its southernmost point in the sky. The event occurs once every 18.6 years, and, as seen from Cahokia's Grand Plaza, it is visible over the bluffs south of Rattlesnake Mound, where the causeway ends.

Dr. Baires has suggested that the road's relationship to some of the city's most important mortuary mounds is a key to understanding its purpose. For example, Rattlesnake Mound is a major burial mound with at least 140 individuals buried there, and midway down the road's length is Mound 72, the site of hundreds of burials, including mass graves of sacrificial victims. Baires said that these spatial relationships suggest that the Rattlesnake Causeway served as a conduit between the realms of the living and the dead.

Compiled by Dr. Neil Gale, Ph.D.