SUBTLE ICE-WALLED LAKE TERRACES IDENTIFIED AND MAPPED WITH SHADED RELIEF MAPS OF 2-FT DEMS FROM AERIAL PHOTOGRAPHY OR LIDAR

High-quality shaded relief maps of 2-ft DEM data from aerial photography in NE Illinois have been instrumental in creating a new paradigm for mapping surficial deposits of the last glaciation at a scale of 1:24,000. In particular, lake terraces are now observed in places where they had not been identified previously. Many mapping areas have two levels of fossiliferous, laminated sediment: high-level terraces associated with ice-walled lakes, and lower level terraces associated with slackwater lakes. The high level lake sediment was deposited when ice remained on the landscape, whereas the low level sediment was deposited after the ice melted, resulting in a subtle inverted landscape. This experience indicates that ice-walled lakes are likely more extensive than they have been mapped, and that their identification will be facilitated as additional LIDAR-type data become available.

Two features of the sediment successions that form the high lake terraces in NE Illinois indicate that they formed in ice-walled lakes. First, fossiliferous lake sediment is continuous to the edge of most deposits; the topographic relief in some cases indicates that the confining ice wall was at least 10 m high. Second, the sediment contains a deep-water ostracode assemblage (Limnocythere friabilis, Cytherissa lacustris, Fabaeformiscandona rawsoni). The only known modern analog assemblage lives in Lake Michigan in water deeper than 15 m. The collective evidence thus suggests water depths of at least 25 m. Another interesting feature of these ice-walled lake deposits is that the thickest lake sediment is observed in the outer portion of the mound; the thinnest lake sediment occurs in the mound's interior. This suggests, perhaps, that a diapir of diamicton squeezed into the center of the mound as the opening in the ice filled with sediment. Another idea is that the diamicton high was formed by material freezing on to super-chilled water as it discharged into the lakes.

The ice-walled lake sediment successions are implicated in having an important role in modern groundwater storage and recharge. Generally about 10 to 15 m thick, the successions are comprised of basal sand and gravel, laminated silt, weathered sand and gravel, and loess. This succession is inset into subtle, hummocky morainic topography formed of much less permeable uniform loam to silty clay loam diamicton.