DeKalb Mounds: A Possible Pleistocene (Woodfordian) Pingo Field in North-Central Illinois
Ronald C. Flemal, Kenneth C. Hinkley, James L. Hesler, 1973. "DeKalb Mounds: A Possible Pleistocene (Woodfordian) Pingo Field in North-Central Illinois", The Wisconsinan Stage, Robert F. Black, Richard P. Goldthwait, H. B. Willman
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More than 500 circular to elliptical mounds occur in the late Pleistocene (Woodfordian) deposits of north-central Illinois. The mounds rise from 1 to 5 m above the general ground level and are either flat-topped or have slightly depressed centers. They range in diameter from 30 m to approximately 1 km; the smaller mounds are most abundant. The mounds consist of a core of lacustrine silt and clay surrounded by a sandy rim. The lacustrine sediments overlie Woodfordian till and outwash, and are in turn overlain by Woodfordian loess.
The morphologic and stratigraphic characteristics of the mounds suggest that they are deposits formed within the lakes of pingo craters. These characteristics include (1) a high degree of symmetry, (2) overlapping and superpositional relationships, (3) occurrence over both till and outwash, (4) surrounding annular depression, (5) confinement to a low-relief inter-morainic area, (6) mineralogical identity with the underlying materials, and (7) the association of the mounds with indicators of permafrost. The pingo lake hypothesis is also consistent with implied groundwater conditions and ice margin locations during the Woodfordian.