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 remain 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. It was designated a National Natural Landmark in 1967.