The U.S. state home to Cahokia and its solstice-aligned Woodhenge circles is Illinois.
Cahokia was a major Mississippian city near present-day Collinsville, directly across the Mississippi River from St. Louis. At its height around 1100 CE, it covered several square miles and included enormous earthworks, plazas, residences, and platform mounds such as Monks Mound.
The Cahokia Woodhenges were large timber circles built in stages between about 900 and 1100 CE. Archaeologists identified their locations from the surviving dark soil marks left by decayed posts. The circles appear to have helped track solar events, including equinoxes and solstices; Woodhenge III has been reconstructed with 48 perimeter posts and a central observation post.
The site is sometimes confused with Cahokia, Illinois, the modern city, but the archaeological complex lies mainly in the Collinsville area. Cahokia Mounds became a UNESCO World Heritage Site in 1982 and is distinct from the later historic Cahokia people who lived in the region.