SOUTHERN LAURENTIDE ICE SHEET DEAD-ICE TOPOGRAPHY: THE ‘WISCONSIN VIEW' AND NEW OPPORTUNITIES

Much of the Late Glacial Maximum margin of the Laurentide Ice Sheet in Wisconsin is marked by extensive tracts of dead-ice topography. These areas are characterized by numerous irregular hummocks and swales occurring together in a band parallel to the former ice-margin position and extending several kilometers up ice. Ice-walled-lake plains are a common feature in these landscapes. We have analyzed the geomorphology and internal composition of these hummocks while making county maps throughout the state. This dead-ice topography is produced by gradual melting of stagnant ice that had thick supraglacial debris. The supraglacial debris was derived from the ice bed by freezing-on, thrusting, and stacking near the margin. There is abundant evidence that the ice lobes that left behind hummocks topography advanced into permafrosted terrain and, in many cases, likely at fast-flow rates (surging or streaming). These two factors, along with up-gradient bed slopes in places, accentuated the marginal thrusting processes and increased the amount of supraglacial debris. In places, preferred orientation of hummocks likely reflects structures in the parent ice. The presence of tunnel channels cutting through hummocky terrains is also an expression of ice with a frozen toe and rapid ice flow. Following stagnation, the presence of permafrost controlled the rate and timing of dead-ice melting.

The chaotic pattern and meso-scale of ‘classical hummocky topography’ has precluded sophisticated morphometric analysis, but the increasing availability of LIDAR elevation data will likely yield greater insight into the geomorphology and genesis of dead-ice topography.