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.