Cahokia Mounds State Historic Site will host a nature walk through its fields, marshes and woods. Guides will lead the free hike Saturday starting at 9:30 a.m. at the Interpretive Center. It's expected to conclude around 1 p.m. and will be canceled if there's rain at the start time. Cahokia Mounds is the largest Native American earthworks north of Mexico and was part of a city built by the Mississippian culture about 1,000 years ago. The site is near St. Louis in Collinsville, Illinois. It is administered by the Illinois Historic Preservation Agency.