昭和基地付近,露岩地域の地形

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The bare rock areas along the eastern coast of Lutzow-Holm Bay are topographically classified into (1) strandflat, (2) hilly lands, (3) mountains, and (4) recessional moraines, which had been described as divisible into two: hilly lands and mountains (YOSHIKAWA and TOYA, 1957), or flat surface and mountains including hilly lands (TATSUMI and KIKUCHI, 1959a). (1) The marginal belt of Langhovde and Skarvsnes areas, the Ongul Islands and many other coastal islets, formerly described as flat surface are as a whole regarded as strandflat. It rarely attains to 50 metres and in most cases to less than 30 metres in height above sea level. Judging from its topographical characteristics it is considered to be a glaciated piedmont plain (Plate 1). (2) The hilly lands attaining to 50-300 metres in height and showing as a whole an accordance of summit level are regarded as roches moutonnees of a rather large scale, among which there are many cirque-like hollows, small-scale U-shaped valleys and steep-sided inlets formed by the differential erosion of ice sheet or by the erosion of ice falls and or ice streams (Plates 1 and 2). (3) The mountains, 300-500 metres high above sea level, which dominate over the surrounding hilly lands have an appearance of so-called "giant roche moutonnee" or glaciated monadnock (Plate 3). In some places, mountain flanks are almost vertically truncated by the lateral erosion of ice streams as in the northern face of Mt. Langhovde, the southwestern wall of Mt. Skjegget and the southwestern part of Breidvognippa. (4) A series of recessional moraines, 5-30 metres wide and 100-1,000 metres long, occur at a distance of 10-100 metres landward from the inner margins of the actual bare rock areas (Plate 4). At present, these moraines indicate the position of the boundary between the "dead" and the "active" glacier ice. No morainic hillock other than these and no meltwater drainage channel could thus far be found on the bare rock areas. These facts imply that the front of the former ice sheet retreated on the present bare rock areas without any halting to the position above-mentioned, and that the ablation of ice sheet was caused not by melting but mainly by evaporation. It has been known that the ice front began to retreat from the Ongul Islands at least 30,000 years ago (NAGATA and YOSHIDA, 1962). The Ongul Islands have been located in a periglacial morphogenetic area since that time, yet the development of patterned grounds, nivation features and the deposits of cryoturbate are poor and limited. In East Ongul Island, patterned grounds were found at four spots. They are nonsorted nets on the ground moraines in shallow hollows (Plate 6), and monosorted stripes on the raised beaches (Plate 7). They are nothing but mere frost cracks without any marked segregation by frost heaving of the facial coarser materials from the finer materials to be found beneath them. Sorted polygons are found at five localities, three on the veneer of ground moraines at the foot of the plucking side of roches moutonnees in Skallen area, and other two on the recessional moraines underlain by dead ice in Breidvog area. In the latter cases, the sorted polygons are considered to be formed by the differential melting of ice beneath them (Plate 8). As the air temperature is usually low in these areas, opportunities for air temperature of its alternate rise and fall above and below freezing point, and of its abrupt rise above that point are very rare (Fig. 2). Both the duration for the occurrence of the feezethaw cycles of water in soils and rock joints and the snow melting period in a year are very short. Moreover, low humidity, lack of rainfall and strong wind tend to stimulate the evaporation of ground water. Hence the inactive cryoplanation and the poor or indefinite development of microtopographies such as patterned grounds of the bare rock areas under consideration. It was impressive to the present writer to find the facts that the cryoplanation in these areas is inactive compared with that in the Japanese Alps and the Central Highlands of Hokkaido.
国立極地研究所の論文
1964-02-00

昭和基地付近,露岩地域の地形

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