We apply a numerical model of the late Wisconsin (circa 20,000 years B.P.) Lake Michigan Lobe (LML), Laurentide Ice Sheet, to investigate how fine-grained subglacial sediment might influence lobe behavior, particularly rapid millennial-scale marginal oscillations observed in the geologic record. Over the Canadian Shield, we assume a rigid bed (''hard bedded'') basal boundary condition. In areas overlain by fine-grained sediment (''soft bedded''), the base of the ice is coupled to a deformable sediment layer, using a rate-dependent stress-strain law. Geotechnical tests of clay-rich till deposited by the LML provide control for sediment rheologic parameters. Simulated cross-sectional profiles are consistent with reconstructions from geologic evidence. Time-dependent simulations suggest that a soft-bedded lobe could have reached steady state in about 20,000 years or less, in contrast to 50,000 to 60,000 years for an otherwise identical hard-bedded lobe. A soft-bedded lobe with sediment viscosity at the experimentally determined value is about twice as responsive to millennial-scale shifts in accumulation or ablation as a nonsliding hard-bedded lobe, but in both cases the response is slower than that indicated by the geologic record. Results suggest that while strong millennial-scale changes in accumulation and ablation can produce responses in hard-bedded or soft-bedded ice that are consistent with the geologic record, changes in subglacial sediment viscosity, even relatively modest changes (whether independent or in conjunction with climate change), might more readily account for millennial-and submillennial-scale fluctuations of the lobe margin. These observations do not exclude a role for sliding, but they do provide some perspective from which to evaluate relative contributions of the various processes that influence lobe behavior. ¿ American Geophysical Union 1996