Cedar Bog State Nature Preserve is a fen left behind by the retreating glaciers of the Wisconsin glaciation about 12,000-18,000 years ago. A protected area of about 450 acres (180 ha) of fen remains from the original area of approximately 7,000 acres (28 km2). Cedar Bog was purchased in 1942 by the Ohio Historical Society and was the first nature preserve purchased with state monies. In 1967, Cedar Bog was designated as a National Natural Landmark by the National Park Service.