Ceremonial Road Discovered in the Ancient City of Cahokia.

A new study published in the Midcontinental Journal of Archaeology has revealed the presence of a major ceremonial road running through the heart of Cahokia. The existence of such a road had 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 some 20,000 people and sprawled over nearly 4,000 acres.

Cahokia was once composed of a collection of agricultural communities that reached across the Midwest and Southeast starting around 800 AD and flourishing between the 11th and 12th century. 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 stood 100 feet high. An estimated 78,500 square feet of earth was used to build the mound between the years of 800 and 1,350 AD, but it was not long after this time that 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 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 centrepiece 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. These spatial relationships suggest that the Rattlesnake Causeway served as a sort of conduit between the realms of the living and the dead, Baires said.

Compiled by Neil Gale, Ph.D.