やまと山脈の地形

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The fourth wintering party of JARE made a survey trip to the Yamato (Queen Fabiola) Mountains. The fifth wintering party traversed the inland of Lutzow-Holm Bay south to 75° in Latitude, and on the way back to Syowa Station they visited the mountains for the geomorphological survey. A part of the results obtained in 1960 was already reported (YOSHIDA, 1961). In this parer the geomorphology of the mountains is described, based on the observations made in 1960 and in 1961. The Yamato Mountains consist of seven relatively small massifs and several nunataks and form an arcuate mountain chain which extends over 50 kilometers from north to south. These massifs and nunataks are composed of plutonics and metamorphics such as gneisses and granites, and strikes of their foliations are conformable to the arrangement of the massifs. The inland ice differs in its elevation on both sides of the mountain arc, being dammed up by the mountains and detected subglacial rises. This difference of ice levels continues in a form of rather gentle ice cliff south to the Belgica Mountains and suggests the existence of a great structural feature such as fault block mountains under the vast inland ice. A few reversed faults were found at the foot of the D Massif. These were presumably related to the development of the mountains. Shear zones and joint systems rather perpendicular to the strikes of foliations are also considered to have conditioned the formation of limbs of the massifs. It is quite possible to conceive that the courses of outlet glaciers which drain the inland ice through the mountains from ESE to NW were subsequent to depressions of tectonic origin. That is also suggested by the fact that the geological structures of the massifs slightly differ from each other. Therefore, the mountain is a kind of arcuate fault blocks. The mountains have been subjected to glacial erosion by inland ice and small detached ice masses. The glaciation of inland ice, however, has produced rather flat and gentle slopes, depositting morainic debris in some places. On the other hand, glacial erosion by outlet glaciers, cirque glaciers, or drift-snow ice has been acting so as to sculpture steep slopes and cirques. The massifs were once covered by the inland, ice nearly up to there summits, because erratic boulders and glacially polished surfaces are found at and near the summits. It has been often advocated that the formation of cirques in the Antarctic antedates the maximum ice flood period (TAYLOR, 1922; GOULD, 1940). In the Yamato Mountains, however, some cirques were gouged on surfaces eroded by former inland ice or outlets. It is reasonable, therefore, to assume that at least some of them were formed after the retreat of inland ice, though it is impossible to discern preglacial features of the mountains. The former inland ice, at least by 400 meters on the east side and by 600 meters on the west side of the mountains higher than the present level, commenced to retreat at an unknown time and, then, were transformed into characteristic shapes of outlet glaciers in consequence of cropping out of bare rocks in the mountain region. As stated before, the tectonic depressions might be chosen as the courses of the outlets, and glacial corrasion of the outlets scooped out parts of rather flat surfaces formed by the inland ice, producing precipitous landforms discordant to them. During the shrinking period of the inland ice, ice masses composed of drifting snow and relics of inland ice; occupying locations on mountain slopes favourable to deposition of snow and ice, scoopad out cirques. The cirques have not so peculiar features as typical ones in temparate regions, though polished surfaces and ground moraines on their bottoms show that glacial scouring took part in the formation of the cirques. No bargschrund could be found anywhere. This fact appears to indicate that in the Antarctic nivation and sapping are rather weaker in the formation of cirques than glacial corrasion. Many types of cirque glaciers are found on the mountains. Three cirque glaciers an located at positions favourable to be nourished by drift snow and the accumulations are now exceeding over whole surfaces of the glaciers But two cirque glaciers composed of blue ice on almost flat surfaces show negative regimen The others have both accumulation and ablation areas, but two of them have shrinking ice tongues already detached from the inland ice. The regimen of each ice mass is affected by locations and features of ice masses in such a manner as mentioned above, and might have been changed also with the progress of lowering of the ice level. There are some vacant cirques, surrounded by ragged peaks, where the nourishments of ice have ceased in consequence of the lowering of the ice level. Small outlets have also been abandoned by ice. Most of the surfaces of the inland ice on the east side of the mountains are covered with drifted snow layer, but on the west-leeward-side there is a rather narrow blue ice area. In other words, snow accumulation exceeds on the windward side of the mountains and the leeward side is the ablation region. It is not clear whether stagnant stages existed during the shrinking period of the inland ice. The flat surfaces in two different heights, interrupted by steep cliffs, in the D Massifs have been developed apparently by glacial corrasion of the inland ice. Two cirques have been formed in the heights between these two surfaces, and they have slightly scooped out the steep cliffs and the lower flat surface. This fact is explained as follows. The inland ice had receded to some extent after the formation of the upper flat surface and, then, the recession had ceased for a short time. At this interval, cirque glaciers were formed by drifted snow onto the steep cliffs and the cirques were gouged. Then, the inland ice began to recede again and the lower flat surface exposed above the ice level. The cirque glaciers survived a little later after the recession of the inland ice, and slightly scoured the lower surface. Therefore, at least one stagnant stage existed during the shrinkingperiod. The characteristics of moraines which cover ridges of the mountains differ more or less according to their locations. Thin moraines which directly cover bed rocks are often found in the northern part of the mountains. Rather thick moraines with ice cores cover ridges of the massifs in the middle part. Patterned grounds have developed on them. The southernmost massif is covered with thin moraines with ice cores. The difference of the development of moraines would be due to the difference of the durations since they have been exposed above ice the level. The fact that landforms are more precipitous in the northern part than in the southern can be attributed to some extent to the difference of time elapsed since the regions have been exposed. It is concluded, therefore, that the deglacierization of the northern part of the mountains advanced slightly previous to the southern. It is quite difficult to verify whether the recession of the inland ice has been taking place in the recent decades or not. Difficulty is increased by the lack of lichen on exposed rock surfaces (SWITHINBANK, 1958). But many of the cirque glaciers appear rather inactive. Especially, two of them have receding ice tongues, and freshly polished roches moutonnees are exposed in front of the tongues. The striated bed rocks on bottoms of a glacial trough are also fresh. Dead ice under moraines which cover a ridge of the southernmost massif is now melting. Bare rocks which have rather fresh surfaces are found at several places on foots of the massifs and nunataks. Considering these facts, it is tentatively concluded that the recent ice recession is somewhat conspicuous in this area, apart from the ice budget of more extensive region inland of Prince Harald Land.
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やまと山脈の地形

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