フィンランド北部の岩海と礫質多角形土

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  • Block Fields and Sorted Polygons in Finnish Lapland
  • フィンランド ホクブ ノ ガンカイ ト レキシツ タカッケイド

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The authors discuss the development of block fields and sorted polygons on the flat surfaces at the summits of fells (monadnocks; tunturi in Finnish) in Finnish Lapland in relation to the retreat of the ice sheet during the last glaciation.<br> There are many fells in the study area (Fig. 1) with their summits elevated several hundred meters; the highest peak is 806 m. Block fields and inactive sorted polygons with erratic boulders in their forming materials were developed on the flat surfaces at the summits of fells above about 600 m. Only block fields with erratics were developed on the summits between about 500 m and 600 m in elevation. Furthermore, on the summits below about 500 m, neither block fields nor sorted polygons were developed but glacial striaes and erratics existed.<br> All of the summits in the study area were covered with ice sheet during the late glaciation. However, because the higher summits with block fields appeared above the ice sheet like nunatak forms in the early stages of deglaciation, it was cold enough to produce block field-forming materials. On the other hand, the lower summits without block fields were covered with ice sheet for a longer period than the higher ones. It was not cold enough to produce block field-forming materials on the lower summits (Fig. 3, Table 1). Therefore, in the present study area, the authors define a boundary zone, which is a transitive zone of freeze-thaw weathering with an altitudinal range below the linear weathering limit (Nesje et al., 1987).<br> Inactive sorted polygons are commonly developed on the summits of fells higher than 600 m in their elevation. On the other hand, active sorted polygons are developed on the river floors and lower river terraces at the bottoms of the shallow valleys, mostly about 100 m in elevation (Fig. 2). Very severe temperature inversion was observed at Kevo (Fig. 1) in one of the valleys where active sorted polygons were developed. This is a very common phenomenon in Finnish Lapland. The values for freezing index, thawing index, and freeze-thaw days and cycles (Fig. 4, Table 2) show that valley bottoms have a more suitable climatic environment for periglacial processes than mountain tops. The authors think that severe temperature inversion is one of the important factors for active sorted polygon forms.

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