The Green Bay and Des Moines lobes of the Laurentide ice sheet: Evidence for stable and unstable glacier dynamics 18,000 to 12,000 BP

Landform-sediment assemblages in the Central Lowlands reflect different styles of glacier dynamics during deglaciation. Assemblages of the Green Bay Lobe suggest a stable, progressively retreating margin prior to 14,000 BP, whereas after 14,000 BP the margin became unstable. During a still-stand of the Green Bay Lobe at its maximum position about 18,000 BP, hummocky moraines, tunnel channels, and large drumlins formed. During deglaciation, subglacial and ice marginal assemblages were superimposed as the terminus retreated about 50 m/yr. Flutes, small drumlins, and small moraines formed when ice margins were thin and retreating.; Assemblages of the Des Moines Lobe reflect an unstable margin. Des Moines Lobe advanced out of phase with the rest of the southern Laurentide ice margin at 14,000 BP. Between 14,000 BP and 12,000 BP the lobe flowed rapidly, and then stagnated 6 times. Supraglacial assemblage dominates the deglaciated surface and reflects deposition in zones of dead-ice. Evidence for stable flow, such as streamlined landforms and end moraines are not present. Aligned hummocks, ice-walled-lake plains, meltout till, and supraglacial sediment provide evidence of longitudinal compression and regional stagnation.; Both lobes were thin ({dollar}<{dollar}1000 m thick) with gently sloping profiles. Basal shear stress within 100 km of the terminus varied from 5 to 25 kPa for the Green Bay Lobe and was less than 5 kPa for the Des Moines Lobe. During drumlin forming phases average basal shear stress was generally between 10 and 25 kPa for the Green Bay Lobe. During periods of rapid ablation when flutes, eskers, and small push moraines formed average basal shear stress was less than 10 kPa. Mass-balance calculations suggest that the equilibrium line altitude must have been less than 1200 m if the Des Moines and Green Bay Lobes were in steady-state.; Conditions may have changed during deglaciation from those at the glacial maximum. The decrease in supraglacial sediment near the terminus of the Green Bay Lobe as deglaciation progressed, suggests the disappearance of a zone of freezing-on. Because the Des Moines Lobe surface is covered by deglaciation features little is known about conditions prior to deglaciation.