ICE-WALLED-LAKE PLAINS IN THE MID-CONTINENT—WHAT THEY TELL US ABOUT LATE GLACIAL ICE-MARGINAL PROCESSES AND ENVIRONMENTS

Thousands of ice-walled-lake plains formed in the stagnant marginal zone of the southern Laurentide Ice Sheet. They are conspicuous, but little-known, landforms within the broad bands of hummocky glacial topography extending from the Great Lakes to Alberta. They are flat-topped hills up to 50 m high and up to a few kilometers wide. Laminated offshore sediment up to tens of meters thick underlies the middle part of many of these features, and coarser shore sediment is typically present around their edges. These lake plains provide evidence of the final disintegration of the ice and the distribution of debris from the melting ice.

Some of the ice-walled lakes persisted in debris-covered ice through hundreds or thousands of years during which climate was cold enough for permafrost to exist and for the margin of the ice sheet to retreat. Some may have persisted into the earliest Holocene, when they were inhabited by plants and animals in a climate only slightly cooler and wetter than today, indicating surrounding ice was well insulated by rock debris. Many smaller ice-walled-lake plains, especially those in Alberta and Minnesota, seem to have been walled by ice but contain more till-like material than lake sediment, indicating a continuum from ice-walled-lake plains with abundant lake sediment to similar features with abundant till-like sediment to collapse hummocks. In some areas hummocks surrounding ice-walled-lake plains are composed mostly of collapsed lake sediment.

Ice-walled-lake plains are important in evaluating the possible subglacial or supraglacial origin of glacial hummocks. Ice-walled-lake plains and hummocks formed where supraglacial debris slumped down ice slopes. Some debris was reworked by supraglacial streams, or in ice-walled lakes. We know of no evidence that any substantial amount of subglacial material was squeezed up into the hummocks or the ice-walled lakes. Compressive flow near the ice margin, enhanced by permafrost or readvances of ice into areas of stagnant ice, likely contributed to the accumulation of thick masses of debris-rich ice that yielded thick supraglacial debris as they melted. The hummocky moraine surrounding the ice-walled-lake plains formed when the supraglacial debris was let down as the last ice melted.