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Three localities with marginal moraines deposited by former cirque glaciers are investigated in east-central southern Norway. The wet-based (erosive) cirque glaciers with aspects towards S-SW and N-NE are mapped at altitudes above 1100 m, and have a mean equilibrium-line altitude of 1275 m. With a suggested mean annual winter precipitation close to the average for the modern accumulation season (1 October-30 April) when the cirque glaciers existed, the mean air-temperature depression during the ablation season (1 May-30 September) is calculated to be 6–7°C lower than at present. The high-altitude cirques of central Rondane were still covered by ice when the low-altitude cirque glaciers developed in distal position for this massif in eastern Rondane and on isolated mountains. Hence, the cirque glaciers are suggested to have existed during the deglaciation after the Late Weichselian maximum, and most likely during the Younger Dryas (11000–10000 BP). The cirque glaciers indicate a downwasting ice-sheet surface well below an altitude of 1100 m prior to the Younger Dryas, and this supports a limited (small) vertical extent for the Late Weichselian ice sheet in this region. With the contemporaneous level for instantaneous glacierization (glaciation threshold) just below the highest elevated peaks in east-central southern Norway, this fits with the idea of a continuous downwasting of the Late Weichselian ice sheet since the 'first' nunataks appeared. The occurrence of the cirque glaciers indicates a multidomed Scandinavian ice-sheet geometry during the Late Weichselian. 相似文献

18.

THE LAST ICE SHEET IN NORTH-WEST SCOTLAND: RECONSTRUCTION AND IMPLICATIONS

COLIN K. BALLANTYNE DANNY McCARROLL ATLE NESJE SVEIN OLAF DAHL JOHN O. STONE 《Quaternary Science Reviews》1998,17(12):1149-1184

Recent models of the last Scottish ice sheet suggest that nunataks remained above the ice surface in areas peripheral to the main centres of accumulation. This proposition has been investigated on 140 mountains over an area of 10,000 km² in NW Scotland. Outside the limits of the later Loch Lomond Readvance in this area there is evidence for a single high-level weathering limit that separates glacially eroded terrain from higher areas of in situ frost debris. This limit occurs at altitudes ranging from 425 to 450 m in the Outer Hebrides to >950 m on the mainland, and is best developed on lithologies that resisted breakdown after ice-sheet downwastage. Interpretation of this weathering limit as a periglacial trimline cut by the last ice sheet at its maximum thickness is supported by: (1) joint-depth and Schmidt hammer measurements that indicate significantly more advanced rock breakdown above the weathering limit; (2) a much greater representation of gibbsite (a pre-Late Devensian weathering product) in the clay fraction of soils above the limit; (3) cosmogenic isotope dating of the exposure ages of rock outcrops above and below the limit; (4) the sharpness of the limit at some sites and its regular decline along former ice flowlines; and (5) shear stress calculations based on the inferred altitude and gradient of the former ice surface. Reconstruction of the ice surface based on trimline evidence indicates that the mainland ice shed lay near or slightly east of the present watershed and descended northwards from >900 m to ca. 550 m at the north coast. Independent dispersion centres fed broad ice streams that occupied major troughs. On Skye an ice dome >800 m deflected the northwestwards movement of mainland ice, but the mountains of Rum were over-ridden by mainland ice up to an altitude of ca. 700 m. The Outer Hebrides supported an independent ice cap that was confluent with mainland ice in the Minches. Extrapolation of the trimline evidence indicates that most reconstructions of ice extent are too conservative, and suggests that low-gradient ice streams extended across the Hebridean Shelf offshore. Wider implications of this research are: (1) that blockfields and other periglacial weathering covers are not all of the same age or significance, depending on the resistance of different lithologies to frost weathering; (2) that the contrasting degree of glacial modification in the Western and Eastern Highlands of Scotland may reflect a former cover of predominantly warm-based ice in the former and predominantly cold-based ice in the latter; and (3) that the approach and techniques developed in this study have potential application for constraining ice-sheet models, not only in areas peripheral to the main centres of ice accumulation in Britain and Ireland, but also in other mountain areas where nunataks protruded through warm-based Late Pleistocene ice masses. 相似文献