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.