Monks Mound at Cahokia built in decades, not 250 years as previously thought
By studying plant seeds and spores in the soil used to construct Monk’s 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 a fraction of that time.
Monk’s Mound is in the ruins of the ancient Native American city of Cahokia in the U.S. state of Illinois. At its height, about 1,000 years ago, Cahokia was home to as many as 15,000 people. The mound was a series of rectangular terraces that reached 10 stories or 30 meters (100 feet) in height, and the area of its base was larger than the Empire State Building in New York City. The structure had a large public building at is apex, perhaps a temple.
There are many other mounds at the site, but Monk’s Mount towers over them. It was named after Trappist monks who lived for a time on a nearby mound.
Monk’s 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. (Photo by Skubasteve834/Wikimedia Commons)
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 in a few decades. The workers got the soil and sediments from a nearby borrow pit.
An artist’s depiction of Monks Mound as found within the interpretive center at Cahokia Mounds State Park
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.
That leads them to conclude Monk’s 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 Monk’s Mound’s size and that it was built by hand, Western Digs says.
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 cook fires.
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—repairs that have saved it from further collapse. Lopinot said it took intelligence to build it to last 1,000 years without high technology.
Featured image: Monk’s Mound, Cahokia (Oakton Community College)
By Mark Miller