Cahokia Mounds, in south-western Illinois directly across the Mississippi River from St. Louis, is the site of the largest pre-Columbian city north of Mexico: a Mississippian-culture settlement that flourished from about 1050 to 1350 CE, covered roughly 16 km² (6 square miles) at its height around 1100, and held somewhere between 10,000 and 20,000 people, with some estimates higher. Its centrepiece, Monks Mound, is about 30 metres (100 ft) high with a base of roughly 5.6 hectares (13.8 acres), the largest pre-Columbian earthwork in the Americas. About 120 mounds once stood here; around 80 survive inside an 890-hectare (2,200-acre) state historic site between East St. Louis and Collinsville, which UNESCO listed as a World Heritage Site in 1982.
1050 CE: The Year a Village Became a City
People had lived in the American Bottom — the wide floodplain of the Mississippi below its meetings with the Missouri and Illinois rivers — for a very long time before the city existed. Settlement at Cahokia proper dates to about 600 CE, and through the Late Woodland period people lived in hamlets of 50 to 100 people that lasted only five or ten years. Maize became an important crop around 900 CE, and by the late 900s a nucleated village of 35 to 70 hectares had formed at Cahokia.
Then, around 1050, something changed fast enough that archaeologists call it the “Big Bang”. Three linked urban precincts — Cahokia itself, East St. Louis and St. Louis on the opposite bank — were laid out at once. A planned grid, oriented to the cardinal directions, was imposed over the older village, with the Grand Plaza, the Rattlesnake Causeway and dozens of mounds as its framework. Pottery and house styles across the region became uniform. Non-local ceramics show that newcomers arrived from the lower Ohio valley, the lower Mississippi, the upper Midwest and the Caddo country of the southern plains, and many settled in farming villages in the eastern uplands known as the Richland Complex.
The city’s reach extended across much of the continent. Hoes made of Mill Creek chert from southern Illinois spread through the Mississippian world; Cahokian pottery turns up at the Silvernale site near Red Wing, Minnesota; and copper from Lake Superior, shells from the Gulf of Mexico and goods from as far as Pennsylvania have been excavated at Cahokia.
6,000 to 40,000: How Many People Lived at Cahokia?
No census survives, so every population figure is an estimate drawn from house counts, excavated neighbourhoods and assumptions about household size. A 2007 study in Quaternary Science Reviews calculated that between 1050 and 1100 the population of the dense central zone, just 1.8 km² in area, rose from 1,400–2,800 to 10,200–15,300. Later excavations found more residential areas to the west, pushing whole-city estimates up. The commonly quoted figure for the city at its peak around 1100 is 15,000 to 20,000; one estimate puts it at no fewer than 22,400 around 1124; and the full range archaeologists have proposed runs from 6,000 to 40,000.
Even the middle of that range is striking. If the highest estimates are right, no city in what became the United States was larger until Philadelphia passed 40,000 people in the 1780s. In the 12th century Cahokia was comparable in size to London or Paris. The city also had problems that came with density: researchers have pointed to polluted waterways and food shortages, possibly made worse by droughts between 1100 and 1250, and suggest that a steady flow of immigrants was needed to keep the population up.
100 Feet High on 14 Acres: Monks Mound
At the heart of the grid stands Monks Mound, a platform mound with four terraces. Measured in 1988, it is about 30 metres (100 ft) high, 291 metres (955 ft) long including its southern ramp and 236 metres (775 ft) wide. Its base is roughly the same size as that of the Great Pyramid of Giza, and its perimeter is greater than that of the Pyramid of the Sun at Teotihuacan. It holds about 622,000 cubic metres of earth, every load dug and carried in woven baskets.
Construction began between 900 and 955 CE, with a much smaller mound now buried inside the northern end. It grew through as many as ten building episodes until about 1100, when a building more than 30 metres (100 ft) long — the largest in the city, probably a temple or the residence of the paramount chief — stood on the summit. The lowest terrace at the south end was added before 1200. Because it is made of layered soil and clay with a flat top, the mound holds rainwater, and large sections of its sides slump; the problem was already under way while it was being built, and modern slumps began again around 1956 after groundwater levels in the floodplain fell. Slumps in 2004–05 led to a major repair in 2007, when backhoes dug out the failed earth on the east and north-west sides and engineers rebuilt them in anti-slip steps.
The name is recent. French Trappist monks lived on a nearby mound from 1809 to 1813 and grew wheat and vegetables on the big mound’s terraces; the traveller Henry Brackenridge, who visited during their stay, published the first detailed description and called it Monks Mound.
120 Mounds, 48 Posts and 270 Burials
Monks Mound was only the largest of about 120 earthworks. To date 109 have been located, and about 80 survive. They come in three forms: flat-topped platform mounds that carried buildings, conical mounds and ridge-top mounds used for burials. In total, Cahokia’s builders moved some 1.6 million cubic metres (55 million cubic feet) of earth. South of Monks Mound, the Grand Plaza covered about 20 hectares (50 acres); soil studies show that its flat surface was deliberately levelled from an originally undulating landscape.
About 850 metres west of Monks Mound stood a series of timber circles now called Woodhenge. Archaeologist Warren Wittry found their post holes in the early 1960s during salvage digs ahead of interstate highway construction. Five circles, built between about 900 and 1100, have since been identified, each larger than the last and with 12 more posts. A reconstruction of Woodhenge III, with 48 posts in a ring and a 49th at the centre, was built in 1985 on the original post positions. The Illinois Historic Preservation Division holds public sunrise viewings there at the equinoxes and solstices.
Mound 72, a small ridge-top mound south of the central precinct, held the site’s most startling burials. Excavations recovered the remains of about 270 people. One man was laid on a bed of about 10,000 marine-shell disc beads arranged in the shape of a falcon, a burial known as the Birdman, with a cache of finely made arrowheads in styles from four different regions. Around 62 percent of the other burials appear to have been sacrificial victims, including a mass grave of more than 50 young women around 21 years old, four young men buried without hands or heads, and about 40 men and women who were killed violently, some perhaps buried alive. Wood from the mound has been radiocarbon-dated to between 950 and 1000 CE, early in the city’s history.
A Two-Mile Palisade and Abandonment by 1350
Around 1160–1170 a large walled residential compound in the East St. Louis precinct was burned, possibly in unrest. At about the same time, around 1175, people built the first version of a palisade around Cahokia’s core: a wooden wall about 3.2 km (2 miles) long with protective bastions. It cut through existing neighbourhoods, and as the population fell it was rebuilt several times to enclose ever smaller areas. Whether it was mainly for defence or for separating the elite from everyone else is debated, though bastions usually indicate warfare.
By the mid-13th century the population had fallen by half or more, and by 1350 the city was abandoned. Explanations include drought and flooding, deforestation and pollution (although recent research finds no evidence of human-caused erosion or flooding), disease, the strain of holding together immigrants with different languages and customs, and conflict, since burned villages from the 13th century have been found in the Illinois valley to the north. Many people seem to have moved south, into the Cairo Lowlands of southern Illinois and the central Mississippi valley, and later to the Cumberland Basin of central Tennessee; Cahokian-style copper plates and engraved shell turn up at powerful centres such as Moundville and Etowah only after 1250. Between about 1400 and 1600 the region was so empty that archaeologists call it the Vacant Quarter.
The city’s original name is unknown. The mounds take their name from the Cahokia, an Illinois Confederation people who lived in the area when French explorers arrived in the 17th century — centuries after the city was abandoned. The Osage Nation now works with archaeologists on the management of the site, and Ponca oral tradition remembers the city as P’ahé’žíde, “red hill”.
1982 to Today: UNESCO, Highways and Suburbs
Illinois first protected the site in 1923 by authorising a state park. It became a National Historic Landmark on 19 July 1964 and was listed on the National Register of Historic Places in 1966. In 1982 UNESCO inscribed it as a World Heritage Site. Surviving the 20th century was not straightforward. The federal highway programme of the 1950s cut into the site and reduced its integrity — though the salvage digs it funded found Woodhenge and much else. Across the river, most of the mounds that once gave St. Louis the nickname “Mound City” were levelled for fill in the mid-19th century. Today the park lies within the St. Louis metropolitan area, between Collinsville and East St. Louis, with the visitor centre, Monks Mound and the Woodhenge reconstruction all inside its 2,200 acres.
The rivers that made the American Bottom fertile are covered in our guide to the Mississippi, America’s great waterway, and the floodplains and bluffs of the state around Cahokia are described in the geography of Illinois.