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

Shaded relief maps of 2-ft DEM data from aerial photography and LiDAR in Illinois have been instrumental in advancing mapping concepts of continental glacial deposits. In particular, subtle lake terraces are now observed in places where they had not been identified previously. More than 2,200 mounds of the last deglaciation have been mapped in northeastern and central Illinois. Many mapping areas have high-level terraces interpreted as ice-walled lake plains (also known as DeKalb mounds). Sediment architecture implies that the fossiliferous, rhythmically bedded very fine sand and silt loam that form the mounds were deposited in depressions in dead ice permafrost. The mounds are flat-topped, and circular, lenticular or multilobate in map view. They are from 0.1 to more than 10 km across. Mound density is as great as about 2 per km². Rim ridges occur on many mounds. Generally about 8 to 12 m thick, the mounds are formed of basal sand and gravel, laminated silt, weathered sand and gravel, and loess. These successions are inset into depressions formed in stiff, hard diamicton (till). South of about 41° N, the mound successions are 2-4 m thick and formed partly of diamicton (deformation till).