What was the Cahokia Woodhenge?

Cahokia Woodhenge was a series of large timber circles located roughly 850 metres (2,790 ft) to the west of Monks Moundat in the Mississippian culture of the old Cahokiaarchaeological site near Collinsville, Illinois. They are thought to have been constructed between 900 and 1100 CE; with each one being larger and having more posts than its predecessor. It was discovered as part of salvage archaeology in the early 1960s interstate highway construction boom and one of the circles was reconstructed in the 1980's by Dr. Warren Wittry. The circle has been used to investigate archaeoastronomy at Cahokia. Annual equinox and solstice sunrise observation events are held at the site.

The majority of the site contained village house features, a number of unusually shaped large post holes were also discovered. The post holes were 7 feet (2.1 m) in length and 2 feet (0.61 m) in width and formed sloping ramps to accommodate the insertion and raising of the estimated 20 feet (6.1 m) tall posts to a 4 feet (1.2 m) depth into the ground. When the holes were plotted out it was realized that they formed several arcs of equally spaced holes. Detailed analytical work supported the hypothesis that the placement of these posts was by design.

Wittry hypothesized that the arcs could be whole circles and that the site was possibly a calendar for tracking. He began referring to the circles as "woodhenges"; comparing the structures to England's well-known circles at Woodhenge and Stonehenge.