Monks Mound

Monks Mound is the largest Pre-Columbian earthwork in the Americas and the largest pyramid north of Mesoamerica. The beginning of its construction dates from 900 to 955 CE. Located at the Cahokia Mounds UNESCO World Heritage Site near Collinsville, Illinois, the mound size was calculated in 1988 as about 100 feet (30 m) high, 955 feet (291 m) long including the access ramp at the southern end, and 775 feet (236 m) wide. This makes Monks Mound roughly the same size at its base as the Great Pyramid of Giza (13.1 acres / 5.3 hectares). 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 has caused 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 of Monks Mound by the Mississippian culture began about 900–950 CE, on a site that 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 ft (30 m) long, the largest in the entire Cahokia Mounds urban zone. Excavations on the southwest corner found that several large ceremonial buildings had burned around 1150 CE.

Botanical remains from Monks Mound suggest it was built much more quickly than previously thought, perhaps on the order of several consecutive decades, providing an alternative view of its construction history. Deep excavations in 2007 confirmed findings from earlier test borings, that several types of earth and clay from different sources had been used successively. Study of various sites suggests that the stability of the mound was improved by the incorporation of bulwarks, some made of clay, others of sods from the Mississippi flood-plain, which permitted steeper slopes than the use of earth alone. The structure rises in four terraces to a height of 100 feet (30 m) with a rectangular base covering nearly 15 acres (6.1 ha) and containing 22 million cubic feet of adobe, carried basket by basket to the site.

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. It may partly have been intended to help minimize 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. This also caused the west end of the big building to collapse. It may have 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. When the eastern side of the mound started to suffer serious slumping, it was not repaired.

There is no evidence of significant Native American settlement in the Cahokia Mounds urban area for hundreds of years after about 1400 CE. In 1735, French missionaries built a chapel at the west end of the south terrace of the mound. The River L'Abbe Mission served a small Illiniwek community, 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). It lasted only until 1784.

In the early 19th century, the land was claimed by people of French descent, and Nicholas Jarrot had 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 from 1809. They took advantage of the big mound's terraces to grow produce, which was elevated above the danger of flooding: wheat on the upper levels, garden produce on the south terrace. During their short stay in the area, which lasted until 1813, Henry Brackenridge visited the site and published the first detailed description of the largest mound. He named it Monks Mound.

In 1831 T. Amos Hill bought the plot including the Mound. He built a house on the upper terrace, and sank a well. This work revealed various archaeological remains, including human bones.