Pingos are large frost mounds which develop in permafrost as the result of the segregation of massive ground-ice lenses. At least two genetic varieties of pingos, open- and closed-systems, form under differing conditions of climate, topography, and groundwater occurrence. Active pingos are known to occur in many high latitude regions. Pingo scars are the degeneration products of pingos. Ideally they are ramparted, circular depressions, although they may be expressed in a variety of divergent forms due to differing conditions of topography, substrate materials, degree of thermokarst overprint, and erosional/depositional histories. Pingo scars occur in many modern permafrost regions. Presumed pingo scars have also been identified in many regions beyond the present permafrost limit and therefore may have utility in reconstructing former permafrost environments.
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 stratigraphie 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 229 on March 15, 2015 memoirs.gsapubs.org Downloaded from 230 r. c. flemal and others superpositional relationships, (3) occurrence over both till and outwash, (4) surrounding annular depression, (5) confinement to a low-relief intermorainic 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.
The paha of northwestern Illinois are longitudinal dunes developed in toto during Woodfordian time. Cores of noneolian materials, such as are typical in the paha of northeastern Iowa, are either not present or much reduced in numbers in the paha of Illinois. Variation in the thickness of the surface blanket of Peoria Loess accounts for the total topographic expression of the Illinois paha.
Underlying the Peoria Loess is a sequence of Pleistocene deposits consisting of two tills of Illinoian age, and the Plano Silt and Roxana Loess of Altonian age. Radiocarbon ages obtained from the Plano Silt range from 41,900 ± 1300 to 34,630 ± 710 radiocarbon years B.P.
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