Ice‐wedge casts and relict polygonal patterned ground in North‐East Iowa, USA

Abstract

AbstractSediment‐filled wedges and associated polygonal patterned ground occur in pre‐Illinoian sediments in north‐east Iowa. A total of 183 vertical to near‐vertical wedge‐shaped bodies of sand have been observed at 12 sites in Black Hawk County, Iowa, and 80 of them were examined in detail. The wedges average 105cm in width at their tops and about 190cm in depth. Infilling material is mostly sand, with pockets of silt, silty sand, and gravelly sand. Slump and subsidence features were also noted in some wedges. Loamy sediment about 1 m thick, with a stone line at its base, overlies the sediment‐filled wedges and usually descends slightly into their upper parts.Based on their characteristics, these sediment‐filled wedges are interpreted as ice‐wedge casts. Supportive palaeoenvironmental studies in Iowa and adjacent states indicate that tundra conditions probably existed in places between 21,000 and 16,500 years BP, the coldest part of late Wisconsinan time. Mean annual air temperatures at this time may have been at least 14°C colder than at present. The degradation of permafrost and formation of ice‐wedge casts must have occurred near the end of this episode of cold climate, which also promoted extreme erosion of the landscape in north‐east Iowa. Aeolian and slopewash deposits later buried the erosion surface and the ice‐wedge casts.