is the largest Pre-Columbian earthwork
in America north of Mesoamerica
. Located at the Cahokia Mounds UNESCO World Heritage Site
near Collinsville, Illinois
, its size was calculated in 1988 as about 100 feet (30 meters) high, 955 feet (291 meters) long including the access ramp at the southern end, and 775 feet (236 meters) wide . This makes Monk's Mound roughly the same size at its base as the Great Pyramid of Giza
(13.1 acres / 5.3 hectares), but unlike Egyptian pyramids which were built of stone, it was constructed almost entirely of layers of basket-transported soil and clay. Because of this construction, and because it is not a true pyramid but has a flattened top, rainwater retained within the structure has tended to cause "slumping", the avalanche-like sliding of large sections of the sides at the highest part of the mound. Its designed dimensions would have been significantly smaller than its present extent, but recent excavations have revealed that slumping was a problem even while the mound was being made.
Construction and abandonment
Construction of Monk's Mound by the Mississippian culture
began about 900-950 CE, on a site which had already been occupied by buildings. The original concept seems to have been a much smaller mound, now buried deep within the northern end of the present structure. At the northern end of the summit plateau as finally completed, around 1100 CE, is an area raised slightly higher still, on which was placed a building over 100 feet (30 metres) long, the largest in the whole Cahokia Mounds urban zone. Deep excavations in 2007 confirmed findings from earlier test borings, that several types of earth and clay from different sources had been used successively. The most recent section of the mound, added some time before 1200 CE, is the lower terrace at the south end, which was added after the northern end had reached its full height, and may partly have been intended to help minimise the slumping which by then was already under way. Today, the western half of the summit plateau is significantly lower than the eastern; this is the result of massive slumping, beginning about 1200 CE which also caused the west end of the big building to collapse. and apparently led to the abandonment of the mound's high status, following which various wooden buildings were erected on the south terrace, and garbage was dumped at the foot of the mound. By about 1300, the urban society at Cahokia Mounds was in serious decline, and when the eastern side of the mound started to suffer serious slumping, it was not repaired.
There is no evidence of significant settlement in the Cahokia Mounds urban area for hundreds of years after about 1400 CE. However, in 1735 French missionaries built a chapel at the west end of the south terrace of the mound, which served a small Illiniwek
community (the true Cahokia), until they were forced to abandon the area by rival tribes about 1752. In 1776, during the American Revolutionary War, a trading post called the Cantine was established next to the mound (by then known as the Great Nobb), but it only lasted until 1784. Next Europeans to settle in the vicinity were Trappist monks, who lived on one of the smaller mounds from 1809, and took advantage of the big mound's terraces to grow food out of danger from flooding: wheat on the upper levels, garden produce on the south terrace. During their short stay in the area, which lasted until 1813, the first detailed description of the mound was published by Henry Brackenridge, and thus the site gained its modern name. In 1831 the plot including the Mound was bought by one T. Amos Hill, who built himself a house on the upper terrace, and sunk a well, revealing various archaeological remains, including human bones.
Thomas I. Ramey, who bought the site in 1864, began an era of more responsible ownership, and encouraged archaeological investigation. Many artifacts were found at or near the surface, but Ramey's own tunnel driven nearly 30 metres into the north face of the mound revealed nothing of historic interest. By this time, the mound was beginning to be considered more within its context, and the survey made for local dentist Dr. John R. Patrick in the 1880s marked the beginning of modern understanding of the Cahokia site as a whole, and its relationship to other sites in the area. There have been many archaeological investigations of the mound since then. One of the biggest began in the 1960s, when Nelson Reed, a local businessman and historian of native cultures, obtained permission to conduct excavations to attempt to locate the high-status building (temple or palace) which was presumed to have stood at the peak of Monk's Mound. His team also drilled cores at various points on the mound, revealing the various stages of its construction from the 10th to 12th centuries CE. Remains of a fairly recent house (presumably Hill's) were found, but no temple. In 1970 Reed returned to the mound, and adopted a new strategy, scraping away the topsoil from several 5-metre-square patches with a backhoe
, to a depth of around 60cm. This quickly revealed various features, including what appeared to be the outline of the temple. Further backhoe work in 1971, while causing considerable distress to traditional archaeologists due to its destruction of several hundred years of stratification
over most of the mound's summit, confirmed the shape of the presumed temple (at over 30 metres long, the largest building found at Cahokia) and found other significant features such as a hole which seemed to have been the socket for a post about a metre in diameter. As a bonus, the dramatic finds also encouraged the Governor of Illinois to budget for an expansion of the Cahokia Mounds State Park.
From the time the original urban society collapsed, the great mound became overgrown with trees, the roots of which conveniently stabilised its steep slopes. In the 20th century the trees were removed. Also, reduction of groundwater levels in the Mississippi floodplain during the 1950s caused the mound to dry out, damaging the clay layers within. Heavy rainfall thus began to cause new slumping, starting about 1956, and the increasingly violent weather of recent decades has exacerbated the problem. In 1984-5 there were several slumps, and the state government brought in some surplus soil to make repairs to the major scar on the eastern side. A decade later, there was further slumping on the western side, so irregular that repair was impractical, but drains were installed in hopes of reducing the effects of heavy rain. It was during this work that a mass of stone was discovered deep within the mound (at the cost of losing a drill bit).
The repairs of the 1980s and 1990s were only partly successful, and in 2004-5 there were more serious slumping episodes. These demonstrated in particular that the importation of earth to repair the major slump on the east side had been a mistake, and it was decided that a new approach was needed. In 2007, backhoes were used to dig out the entire mass of earth from this slump and another at the northwest corner, to a level beyond the internal slippage zone, then a series of anti-slip "steps" were created across the exposed face before the original earth (minus the imported repair material) was replaced at its original level. In order to avoid introducing water deep into the interior of the mound, this was carried out in high summer, and as quickly as possible, but in parallel with the repair work, teams of archaeologists studied the evidence that was being revealed. Unfortunately, the eastern sliding zone was found to be deeper within the mound than originally estimated, and the excavation consequently had to be very large- 50 feet (15 metres) wide, to a height of 65 feet (20 metres) above the mound base- which heightened TriNotch/Cahokia2007 about a conflict between conservation and archaeology.
- Price, Douglas T. and Gary M. Feinman. Images of the Past, 5th ed. New York: McGraw-Hill (2008) ISBN-13:9780073405209. pp. 282-285.
Further reading on the 2007 excavations
Monks Mound (Mound 38)
, Cahokia Mounds State Historic Site website