Description: Monk's Mound 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). Its base circumference is larger than the Pyramid of the Sun at Teotihuacan.
Unlike Egyptian pyramids which were built of stone, Monk's 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.