Four late Wisconsinan stratigraphie units crop out in coastal bluffs of Lake Michigan, northern Illinois. The lowermost unit is a muddy, matrix-supported, stratified diamicton exhibiting abundant sedimentary structures. Stratification is manifested by silt laminations, clay beds, layers of fractional gravel, and coarse-tail grading. Some beds are deformed by loading or folding. Intraformational units have concave erosional contacts with variable dip directions and slopes. Pebble fabrics are random or weakly developed, with wide dispersion of pebbles about the mean axes. These characteristics of the lower diamicton are consistent with resedimentation by subaqueous sediment gravity flows.

The lower diamicton is overlain by a coarsening-upward sequence of lacustrine silt and clay and proglacial deltaic and fluvial sediment. This unit was deformed by an ice advance that deposited lodgement till characterized by massive structure, uniform grain size distribution, and well-developed pebble fabrics with most elongate pebbles aligned parallel to ice flow. Subglacial meltwater eroded channels into underlying deformed lacustrine sediment and deposited stratified sand. The entire sequence is overlain by lacustrine sediment deposited in glacial Lake Chicago following retreat of the ice margin northward into the Lake Michigan basin.