structural:scholar_edition, quality:correction, information:highlighting
Cahokia Woodhenge

The Cahokia Woodhenge was a series of large timber circles located roughly 850 m (2,790 ft) to the west of Monks Mound. They are thought to have been constructed between 900 and 1100 CE, with each one being larger and having 12 more posts than its predecessor. The site was discovered during salvage archaeology undertaken by Dr. Warren Wittry in the early 1960s interstate highway construction boom. Although the majority of the site contained village house features, a number of unusually shaped, large post holes were also discovered. When the holes were plotted out, they formed several arcs of equally spaced holes. Detailed analytical work supported the hypothesis that the placement of these posts was by design, and Wittry hypothesized that the arcs could be whole circles. He began referring to the circles as "woodhenges", comparing the structures to England's well-known circles at Woodhenge and Stonehenge.

The post holes found by Wittry are an example of how soil retains the memory of organic materials that decayed in the soil from thousands of years ago. Many soils have a naturally acidic component that breaks down most organic material pretty quickly, but leaves behind dark discoloration in the soil. The original posts of Cahokia Woodhenge left behind this same discoloration, allowing researchers to easily identify the soil as having once contained wooden posts.

Additional excavations in the 1960s–1980s used predictions based on verified posthole locations and spacing to locate other postholes and confirm the existence of five separate timber circles in the general vicinity. The circles are now designated Woodhenges I through V in Roman numerals. In 1985, a reconstruction of Woodhenge III was built with the posts being placed into the original excavated post positions. The circle, which has 48 posts in the circle and a 49th central post, has been used to investigate archaeoastronomy at Cahokia. The Illinois Historic Preservation Division that oversees the Cahokia site hosts public sunrise observations at the vernal and autumnal equinoxes and the winter and summer solstices. Out of respect for Native American beliefs, these events do not feature ceremonies or rituals of any kind.