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FOSSIL ICE WEDGES AND LATE PLEISTOCENE ENVIRONMENT IN DATONG BASIN,SHANXI PROVINCE

Yang Jingchun Li Shude (Department of Geography,Peking University) Sun Jianzhong (Institute of Geolegy,State Seismological Bureau) Wang Yixun (Department of Geography,Beijing Teachers College)  
In the east of Datong basin (about 1000m above sea level),we found some fossil ice wedges of late Pleistocene.The surface polygons were composed of ice wedges with meshes measured as large as 3m across.The ice wedges themse-lves were dominantly vertical and extend to a depth of 120 cm and widened to a width of 20~30cm.Upon the thawing of ice,the cavity of the ice wedge left was filled by fineroundish,eolian sand,forming a cast of sand wedge. Because of the lateral growth and squeeze of the wedge,adjacent lacustrine clay sediments usually developed an upward bend.This fossil ice wedge indicated a rather arid and windy periglacial environment with its mean annual temperatures estimated as some 11.5° to 16℃ colder than those of the present.About 26000 years ago,the southern boundary of late Pleistocene perma-frost extended to the latitudes 40°N.
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