北海道東部のice-wedge cast

概要

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Recently some ice-wedge casts or pseudo structures have been reported in the lowlands of Hokkaido. In present paper ice-wedge casts in three localities (Fig. 1) are surveyed and described in detail. Ice-wedge casts consisting of fine materials driven from superficial deposits cut through stratified volcanic ash or terrace gravel. They have V-shaped structure normally and it's variations in some cases as much as about 2 m long and as much as 1-2 m wide at the top. Along the side walls of wedges there are found clay flaps and distortion, especially up-turning of strata, and within the walls aligned stones. Large scale polygons and white patches are interpreted from air photos at Shimosahoro and Kamishunbetsu. At Chiraibetsu the wedges are supposed to form polygonal patterns on the densely vegetated ground surface judging from strikes of the wedges in vertical exposures. Erimo Volcanic Sand, which had been the uppermost active layer of frozen ground and filled up wedges when ice melt, dates approximately 32, 000 14C years. No ice-wedge cast has yet been found in another horizon of air-laid volcanic deposits. Thus the coldest phase of climate during the Last Glacial is thirties thousand years BP. Based on Péwé's observations (1966), decreasing of temperature at the time should be estimated as 12°C-14°C in the lowlands of eastern Hokkaido where present mean annual temperature is nearly + 6°C.
社団法人東京地学協会の論文