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 reached their maximum extent in north-central Wisconsin about 20 000 years ago. Their terminal positions are marked by a broad area of hummocky topography, containing many ice-walled-lake plains, which is bounded on the up-ice and down-ice sides by ice-contact ridges and outwash fans. The distribution of these ice-disintegration landforms shows that a wide zone of stagnant, debris-covered, debris-rich ice separated from the active margins of both lobes as they wasted northward during deglaciation. Accumulation of thick, uncollapsed sediment in ice-walled lakes high in the ice-cored landscape indicates a period of stability. In contrast, hummocky disintegration topography indicates unstable conditions. Thus, we interpret two phases of late-glacial landscape evolution. During the first phase, ice buried beneath thick supraglacial sediment was stable. Supraglacial lakes formed on the ice surface and some melted their way to solid ground and formed ice-walled lakes. During the second phase, buried ice began to melt rapidly, hummocky topography formed by topographic inversion, and supraglacial and ice-walled lakes drained. We suggest that ice wastage was controlled primarily by climatic conditions and supraglacial-debris thickness. Late-glacial permafrost in northern Wisconsin likely delayed wastage of buried ice until after about 13 000 years ago, when climate warmed and permafrost thawed.