As the Pleistocene Glaciers pushed into the American Midwest, the vegetation zones shifted southward in advance of the ice. The periglacial zone was characterized by trees and plants that favor a cool, moist climate; today, such vegetation is found across the vast northern forest of Canada.
When the last Glacier, the Wisconsin, began retreating northward, some 15,000 years ago, this boreal forest moved along with it and was soon replaced by the deciduous woodlands that characterize the Midwest today. But pockets of the periglacial vegetation remained behind, finding a cool refuge in deep gorges or shaded stream valleys. Hemlock, Canada yew, mountain maple and northern white cedar are among the plants that typify these areas. Known as glacial relics, they are usually found on north-facing slopes where the soil is moist and the summer sun is less intense.
Ohio is especially rich in glacial relic vegetation. Clifton Gorge, near Yellow Springs, Hocking State Park, southwest of Logan, and the Cuyahoga Valley National Park, south of Cleveland, harbor some of the better remnants of periglacial forest.
