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Monks Mound, located in Collinsville, Illinois, United States, is the largest Pre-Columbian earthwork in the Americas and the largest pyramid north of Mesoamerica. The construction of this monumental structure began between 900 and 955 CE. It’s situated at the Cahokia Mounds, a UNESCO World Heritage Site.

The mound’s dimensions, calculated in 1988, are approximately 100 feet high, 955 feet long (including the access ramp at the southern end), and 775 feet wide. This makes Monks Mound roughly the same size at its base as the Great Pyramid of Giza. As a platform was an issue even during the mound’s construction.

Unlike the stone-built Egyptian pyramids, Monks Mound was constructed almost entirely of layers of soil and clay transported in baskets. Over time, rainwater retention within the structure has caused slumping, or the avalanche-like sliding of large sections of the sides at the highest part of the mound. Recent excavations have revealed that slumping culture began the construction of Monks Mound around 900–950 CE, on a site that had already been occupied by buildings. The original concept appears to have been a much smaller mound, now buried deep within the northern end of the current structure. Upon completion around 1100 CE, a building over 100 ft long, the largest in the entire Cahokia Mounds urban zone, was placed on an area slightly raised at the northern end of the summit plateau.

Botanical remains suggest that Monks Mound was built much more quickly than previously thought, perhaps over several consecutive decades. Deep excavations in 2007 confirmed that several types of earth and clay from different sources were used successively. The mound’s stability was improved by incorporating bulwarks, some made of clay, others of sods from the Mississippi flood-plain, allowing for steeper slopes than the use of earth alone. The structure rises in four terraces to a height of 100 feet, with a rectangular base covering nearly 15 acres and containing 22 million cubic feet of adobe, carried to the site basket by basket.

The latest addition to the mound, made before 1200 CE, is the southern lower terrace. This was added once the northern end had reached its maximum height, possibly to help reduce the already occurring slumping. Today, the western half of the summit plateau is significantly lower than the eastern half due to substantial slumping that began around 1200 CE. This caused the collapse of the west end of the large building and may have led to the mound’s high status being abandoned. Subsequently, various wooden structures were built on the south terrace, and waste was discarded at the base of the mound. By 1300, the urban society at Cahokia Mounds was in severe decline, and when the eastern side of the mound began to slump severely, no repairs were made.

Settlers

European settlers There is no evidence of significant Native American settlement in the Cahokia Mounds urban area for centuries after about 1400 CE. In 1735, French missionaries constructed a chapel at the western end of the mound’s south terrace. The River L’Abbe Mission served a small Illiniwek community until they were forced to leave the area by rival tribes around 1752. During the American Revolutionary War in 1776, a trading post known as the Cantine was established next to the mound, then known as the Great Nobb, but it only lasted until 1784.

In the early 19th century, people of French descent claimed the land, and Nicholas Jarrot held a deed for most of it. He donated some to a small group of French Trappist monks who settled on one of the smaller mounds in 1809. They used the terraces of the large mound to grow crops, which were elevated above the flood risk: wheat on the upper levels and garden produce on the south terrace. During their brief stay in the area, which lasted until 1813, Henry Brackenridge visited the site and published the first detailed description of the largest mound, which he named Monks Mound.

In 1831, T. Amos Hill purchased the plot that included the Mound. He built a house on the upper terrace and dug a well. This work uncovered various archaeological remains, including human bones.

Archaeology

Surveying the Cahokia Mounds site from the summit of Monks Mound, Thomas I. Ramey, who became the owner of the site in 1864, ushered in a period of conscientious stewardship and promoted archaeological exploration. Numerous artifacts were discovered on or near the surface. Ramey commissioned a tunnel nearly 30 m (98 ft) into the mound’s north face, but it yielded no historically significant findings. Around this time, the mound began to be viewed more in its broader context. A survey conducted for local dentist Dr. John R. Patrick in the 1880s signaled the onset of a modern comprehension of the Cahokia site in its entirety, and its connection to other regional sites.

Since then, the mound has been the subject of numerous archaeological studies. One of the most extensive commenced in the 1960s when Nelson Reed, a local entrepreneur and historian of indigenous cultures, was granted permission to carry out excavations. He aimed to find the high-status building (temple or palace) believed to have been situated at the top of Monks Mound. His team drilled cores at various locations on the mound, revealing its construction phases from the 10th to 12th centuries CE. They found remnants of a relatively recent house (presumably Hill’s), but no temple.

In 1970, Reed resumed work at the mound and implemented a novel approach: he used a backhoe to remove the topsoil from several 5 m2 (54 sq ft) patches to a depth of approximately 60 cm (24 in). This swiftly uncovered various features, including what seemed to be the footprint of the temple. Additional backhoe work in 1971 validated the shape of the supposed temple, measuring over 30 m (98 ft) long, making it the largest building discovered at Cahokia. This method was criticized by professional archaeologists as it obliterated centuries of stratification across most of the mound’s peak, which was crucial for artifact placement and evaluation. Reed’s backhoe digs unearthed other notable features, such as a hole that appeared to be the socket for a post about three feet (one meter) in diameter. These significant discoveries prompted the Governor of Illinois to allocate funds for the expansion of the Cahokia Mounds State Park.

Preservation

Following the collapse of the original urban society, the grand mound was overrun with trees, the roots of which helped to stabilize its steep slopes. In the 20th century, the trees were removed during mound and park preparation work. The lowering of groundwater levels in the Mississippi floodplain in the 1950s led to the mound drying out, damaging the internal clay layers. Heavy rainfall triggered new slumping, beginning around 1956. The problem has been worsened by the increasingly severe weather of recent decades. In 1984-5, several slumps occurred, and the state government imported surplus soil to repair the significant scar on the eastern side. A decade later, further slumping on the western side was so irregular that repair was unfeasible. Drains were installed to mitigate the impact of heavy rain. During this process, workers found a large stone mass deep within the mound.

Reed

The repair efforts of the 1980s and 1990s were only partially successful. More severe slumping episodes took place in 2004-5. These showed that the addition of new earth to fix the major slump on the east side had been a mistake. Experts decided on a new strategy. In 2007, backhoes were used to excavate the entire mass of earth from this slump and another at the northwest corner, down to a level beyond the internal slippage zone. Engineers constructed a series of anti-slip “steps” across the exposed face before the original earth (minus the imported repair material) was reinstated at its original level. To prevent water from penetrating deep into the mound’s interior, the work was carried out during the height of summer, and as swiftly as possible. Concurrently with the repair work, archaeological teams examined the evidence that was being uncovered.