Monks Mound, Cahokia

900 CE - 1200 CE

c. 900-1200 CE

Platform mound

Summit Plateau and Ceremonial Buildings

The summit of Monks Mound supported a massive wooden structure, the largest in the entire Cahokia Mounds urban zone. Located at the northern end of the summit plateau, this building exceeded 100 feet in length. Archaeological investigations using backhoes in the 1970s revealed the outline of this presumed temple or palace, along with a significant post-hole approximately three feet in diameter. Evidence suggests that several large ceremonial buildings on the southwest corner were destroyed by fire around 1150 CE, prior to the eventual decline of the urban society.

Origin of the Name Monks Mound

The mound's modern name derives from a group of French Trappist monks who settled on a smaller nearby mound in 1809. They utilized the terraces of the large mound to grow produce, taking advantage of the elevation to protect crops from flooding. They cultivated wheat on the upper levels and garden produce on the south terrace until they were forced to leave in 1813. During this period, Henry Brackenridge visited the site and published the first detailed description of the earthwork, officially naming it 'Monks Mound' in his writings.

Post-Mississippian History and Abandonment

Following the decline of Cahokia around 1300 CE, the mound saw various uses before its eventual abandonment by Native American populations around 1400 CE. In 1735, French missionaries established a chapel on the west end of the south terrace to serve the Illiniwek community. During the American Revolutionary War in 1776, a trading post known as the Cantine was established adjacent to the mound, which was then referred to as the 'Great Nobb.' By the time European settlers claimed the land in the early 19th century, the mound had become heavily overgrown with trees.

Manual Earthwork Construction

Unlike the stone pyramids of Egypt, Monks Mound was constructed almost entirely of layers of soil and clay transported manually in baskets. Deep excavations and test borings confirmed that builders utilized several types of earth and clay sourced from different locations. To improve structural stability and permit steeper slopes than earth alone would allow, the Mississippian builders incorporated specialized bulwarks made of clay and sods harvested from the Mississippi flood-plain. This massive undertaking was completed in stages between approximately 900 and 1200 CE.

Historical and Modern Structural Slumping

The mound's composition of clay and soil, combined with its flattened top, has historically caused it to retain rainwater, leading to 'slumping' or avalanche-like sliding of its sides. Excavations indicate that slumping was a problem even during the original construction phase. A massive failure beginning around 1200 CE caused the western half of the summit plateau to become significantly lower than the eastern side, resulting in the collapse of the large summit building. Modern slumping was exacerbated in the 1950s when falling groundwater levels caused the internal clay layers to dry and crack.

Engineering Interventions and Discoveries

To combat severe slumping in 2007, engineers employed a radical repair method by digging out massive sections of the mound to reach internal slippage zones. They created anti-slip 'steps' across the exposed faces before replacing the original earth. This work was performed during high summer to prevent water from entering the mound's interior. During these repairs, workers discovered a previously unknown mass of stone deep within the earthwork. The excavation required to address the eastern sliding zone was unexpectedly large, reaching 50 feet wide and 65 feet above the mound's base.

19th-Century Exploration and Ownership

In 1831, T. Amos Hill purchased the mound, built a house on the upper terrace, and sank a well that revealed human bones and other archaeological remains. Later, in 1864, Thomas I. Ramey acquired the site and initiated a period of more responsible stewardship. Ramey attempted to explore the mound's interior by excavating a tunnel nearly 98 feet into the north face, though it yielded no significant historical findings. Modern understanding of the site as a whole began in the 1880s with a survey conducted for Dr. John R. Patrick.

Revised Construction Timeline

While earlier theories suggested a long-term construction process, botanical remains found within the mound suggest it may have been built much more rapidly than previously believed. Current evidence indicates the primary structure might have been completed over several consecutive decades rather than centuries. The construction began between 900 and 950 CE on a site previously occupied by other buildings. The final major addition, the lower south terrace, was added before 1200 CE, possibly as a structural measure to minimize the slumping that was already occurring at the northern end.