The Cowan Creek Circular Enclosure is a prehistoric earthworks site in southwestern Ohio within Cowan Lake State Park and the main visible survivor of a larger ceremonial and burial landscape along Cowan Creek. Surveyed in 1949, it measured about 90 feet across and had so little height left that its shape was easiest to see from the air. Evidence from the nearby Cowan Creek Mound, excavated that year under Raymond Baby, points to construction around AD 500 near the end of the Adena culture’s presence in the region. Many nearby mounds were later lost beneath the reservoir, and the enclosure was listed on the National Register of Historic Places in 1974.