DELAYED LANDSCAPE STABILIZATION FOLLOWING RECESSION OF ACTIVE ICE FROM THE WESTERN GREAT LAKES AREA: EVIDENCE, TIMING, AND IMPLICATIONS

Geomorphic features that formed as buried ice melted and overlying sediment collapsed are common in formerly glaciated landscapes. In the western Great Lakes area these include widespread hummocky glacial topography, numerous kettles and extensive collapsed outwash surfaces, and features such as ice-walled lake plains, the surface trace of buried tunnel channels, and remnants of meltwater drainage ways.

Along the western margin of the Green Bay Lobe in central Wisconsin, and along recessional margins of the Superior Lobe in western Wisconsin, the surface trace of tunnel channels extends through deposits marking multiple ice-margin positions. In these areas geomorphic relationships indicate that ice occupying abandoned tunnel channels persisted through multiple glacial advances. The final melting of buried ice and the collapse of overlying sediment is clearly shown by active seismic data. In these areas the length of time that buried ice persisted is hard to evaluate because closely limiting dates are lacking and the time between ice advances may have been quite short.

In the central part of Wisconsin’s Northern Highlands geomorphic and stratigraphic relationships indicate that buried glacial ice persisted through multiple episodes of stagnation and readvance along the margins of the Wisconsin Valley Lobe and the western Langlade Lobe. Permafrost and debris cover combined to preserve buried ice and delay the final formation and stabilization of the late-glacial landscape. A comparison of existing chronologies of permafrost and ice-margin positions indicates this delay may have lasted several thousand years.

In evaluating ice-margin chronology and the evolution of late-glacial landscapes it is important to understand how long buried glacial ice persisted and how long that ice delayed the final stabilization of the landscape. Only parts of the landscape that did not contain buried ice stabilized soon after the recession of active ice. In northern Wisconsin high-level outwash surfaces and the crests of drumlins stabilized early and accumulated and preserved loess while other areas did not. In landscapes that contained abundant buried ice, these areas that stabilized early provide the best sites for determining absolute ages that place limits on ice-margin chronology.