Monks Mound

The largest Pre-Columbian earthwork in the Americas

Monks Mound is the largest Pre-Columbian[1] earthwork in the Americas and the largest pyramid north of Mesoamerica[2]. Located at the Cahokia Mounds UNESCO World Heritage Site near Collinsville, Illinois, the mound size was calculated in 1988 as about 100 feet high and 955 feet long, including the access ramp at the southern end 775 feet wide.

The builders constructed 120 earthen mounds, with the largest being “Monks Mound” (named after a community of Trappist monks[3] who settled on the mound), a 950-foot-long platform consisting of raised terraces, surrounded on its north, south, east, and west by large rectangular open areas.

These areas were thought to serve for communal gatherings, however, a new paleoenvironmental analysis[4] of the north plaza suggests that it was almost always underwater. Cahokia was built in a flood plain below the confluence of the Mississippi and Illinois rivers.

Monks Mound is roughly the same size at its base as the Great Pyramid of Giza (13.1 acres). The perimeter of its base is larger than the Pyramid of the Sun at Teotihuacan. As a platform mound, the earthwork supported a wooden structure on the summit. Unlike Egyptian pyramids which were built of stone, the platform mound was constructed almost entirely of layers of basket-transported soil and clay.

Because of this construction and its flattened top, over the years, it has retained rainwater within the structure. This building technique causes slumping[5] which is happening now and most likely for the mound’s entire existence.

Archaeological evidence suggests that the city was founded around AD 950, reaching an apex of 20,000 inhabitants until it was abandoned during the AD 1400s. Hand coring with an augur, conducted by archaeologists from Southern Illinois University Edwardsville, was undertaken in late July and early August. At 25 feet–15 feet above the stone cobbles–the augur hit a layer of coarse material, at which point groundwater prevented further coring. A.D. 950.

There are several layers of coarse material, a sandy silt loam, alternating with ones of clay, above a 20-foot-high clay core inside the mound. These layers, believed to have functioned as groundwater drains, were built into the mound during its construction beginning ca. There is a book titled “The Great Knob: Interpretations of Monks Mound” written in 1988 by Mikels Skele that is a 2013 reprint of the original hard to find book. The drawings I used above were from this book which is available on Amazon.

  1. In the history of the Americas, the pre-Columbian era spans from the original settlement of North and South America in the Upper Paleolithic period through European colonization, which began with Christopher Columbus’s voyage of 1492. Usually, the era covers the history of Indigenous cultures until significant influence by Europeans. This may have occurred decades or even centuries after Columbus for certain cultures. [Back]
  2. Mesoamerica is a historical region and cultural area in southern North America and most of Central America. It extends from approximately central Mexico through Belize, Guatemala, El Salvador, Honduras, Nicaragua, and northern Costa Rica. Within this region, pre-Columbian societies flourished for more than 3,000 years before the Spanish colonization of the Americas. Mesoamerica was the site of two of the most profound historical transformations in world history: primary urban generation, and the formation of New World cultures out of the long encounters among indigenous, European, African, and Asian cultures. [Back]
  3. The Trappists, officially known as the Order of Cistercians of the Strict Observance and originally named the Order of Reformed Cistercians of Our Lady of La Trappe, are a Catholic religious order of cloistered monastics that branched off from the Cistercians. They follow the Rule of Saint Benedict and have communities of both monks and nuns that are known as Trappists and Trappistines, respectively. They are named after La Trappe Abbey, the monastery from which the movement and religious order originated. The movement first began with the reforms that Abbot Armand Jean le Bouthillier de Rancé introduced in 1664, later leading to the creation of Trappist congregations, and eventually the formal constitution as a separate religious order in 1892. [Back]
  4. The aim of paleoenvironmental analysis, or paleoecology, is to reconstruct the biological, chemical, and physical nature of the environment at the collection site at the time of deposition, based on the rock’s paleontological record. Information can be reconstructed for depositional environments, paleobathymetry, positions of ancient shorelines, paleoclimate, degree of oxygenation of the bottom water and sediment, and salinity of the waters. [Back]
  5. A slump is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials or a rock layer moves a short distance down a slope. Movement is characterized by sliding along a concave-upward or planar surface. Causes of slumping include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of a slope. Translational slumps occur when a detached landmass moves along a planar surface. Common planar surfaces of failure include joints or bedding planes, especially where a permeable layer overrides an impermeable surface. Block slumps are a type of translational slump in which one or more related block units move downslope as a relatively coherent mass. [Back]

Further Reading


Cahokia Mounds
Heritage Daily
Hankering For History
Western Digs
Archaeology Archive

Author: Doyle

I was born in Atlanta, moved to Alpharetta at 4, lived there for 53 years and moved to Decatur in 2016. I've worked at such places as Richway, North Fulton Medical Center, Management Science America (Computer Tech/Project Manager) and Stacy's Compounding Pharmacy (Pharmacy Tech).

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