Figure 17.33

North American recessional moraines. There are many recessional moraines in the Great Lakes region. The moraines mapped in the dark blue area were formed by the Laurentide ice sheet some 15,000 years ago. Moraines in southern Illinois, Indiana, and Ohio (mapped in gray) are from the previous glacial period, called the Illinoian glaciation, that occurred some 150,000 years ago. Southwestern Wisconsin does not have drift (or glaciofluvial deposits) and is called the “driftless area.” Many scientists think the glaciofluvial deposits in the driftless area were washed away long ago in a large flood event caused by the sudden draining of one of the ancient lakes in the area.