Stratigraphic continuity of late Wisconsinan (Woodfordian) tills in the Decatur sublobe area with those in the Peoria sublobe area of the Lake Michigan lobe and garnet-to-epidote ratios indicative of a Superior and Southern Province source rather than a Granville Province source are evidence that the Decatur sublobe was part of the Lake Michigan lobe rather than the Saginaw Bay, Huron, or Erie lobes. Garnet-to-epidote ratios indicate that till of the Iroquois Moraine was deposited by an eastern source lobe, probably a coalesced Huron-Erie lobe. The Decatur sublobe formed in a broad pre-Woodfordian lowland between drift-covered bedrock uplands to the northwest in McLean and Livingston Counties and to the southeast in Vermilion and Edgar Counties, Illinois. A coalesced Huron-Erie ice lobe at times deflected flow of the Lake Michigan lobe ice westward. Slightly lobate end moraines in the Decatur sublobe area probably are the result of strong advances or readvances of a broad ice margin; more lobate end moraines probably formed during recession when local topography exerted greater control on the shape of the ice margin. Some of the moraines may have formed as a result of surge-like readvances of a portion of the ice margin.