Corrugated ground moraine in Story County, Iowa consists of low relief (1-2 m) ridges which most commonly consist of late Wisconsinan till. The average spacing between ridges is about 105 m, and they generally parallel the configuration of the Bemis Moraine, the terminal moraine of the Des Moines glacial lobe. Till fabric (pebble orientations and anisotropy of magnetic susceptibility), facies variations and landform fabric analyzed at three sites in typical corrugated ground moraine landscape near Ames in Story County, Iowa indicate that at sites 1 and 2 the ridges consist of till probably deposited by lodgement. At site 3, two ridges consist of till and massive to cross-bedded sand. Till at site 3 is also probably lodgement till. The nature of the till and related glaciofluvial sediments at each site suggests that the corrugation ridges formed in basal cracks or crevasses in the Des Moines Lobe. The cracks are postulated to have formed during extending flow of the Des Moines lobe as it advanced toward its terminal position. Retreat of the Des Moines lobe was rapid enough to preclude significant accumulations of supraglacial sediment upon corrugated ground moraine.