ICE WASTAGE AND LANDSCAPE EVOLUTION ALONG THE SOUTHERN MARGIN OF THE LAURENTIDE ICE-SHEET, NORTH-CENTRAL WISCONSIN

The Chippewa and Wisconsin Valley Lobes of the Laurentide Ice Sheet re ached their maximum extent in north-central Wisconsin about 20 000 yea rs ago. Their terminal positions are marked by a broad area of hummock y topography, containing many ice-walled-lake plains, which is bounded on the up-ice and down-ice sides by ice-contact ridges and outwash fa ns. The distribution of these ice-disintegration landforms shows that a wide zone of stagnant, debris-covered, debris-rich ice separated fro m the active margins of both lobes as they wasted northward during deg laciation. Accumulation of thick, uncollapsed sediment in ice-walled l akes high in the ice-cored landscape indicates a period of stability. In contrast, hummocky disintegration topography indicates unstable con ditions. Thus, we interpret two phases of late-glacial landscape evolu tion. During the first phase, ice buried beneath thick supraglacial se diment was stable. Supraglacial lakes formed on the ice surface and so me melted their way to solid ground and formed ice-walled lakes. Durin g the second phase, buried ice began to melt rapidly, hummocky topogra phy formed by topographic inversion, and supraglacial and ice-walled l akes drained. We suggest that ice wastage was controlled primarily by climatic conditions and supraglacial-debris thickness. Late-glacial pe rmafrost in northern Wisconsin likely delayed wastage of buried ice un til after about 13 000 years ago, when climate warmed and permafrost t hawed.