Jökull


Jökull - 01.12.1987, Síða 54

Jökull - 01.12.1987, Síða 54
the hollow and the transition to the feature described as a rock glacier. The rock glacier The main characteristics of the rock glacier can be identified in figures 2 and 3. All basic topographical features of a rock glacier can be found: Iateral furrows, medial furrow, flow ridges and steep snout, (as de- scribed by Wahrhafting and Co.x 1959)\ a spoon- shaped depression at its head (Outcalt and Benedict 1965); and it has the general resemblance of a glacier with a rocky surface (Capps 1910). Some moraine fea- tures are also visible, such as ‘horns’ (figure 2), but the landform undoubtedly possesses the characteristics necessary to define it as a rock glacier (Martin and Whalley, in press, present photographs of rock gla- ciers very similar to Nautadalur). The detailed structure of the rock glacier surface is complex. The general topography is shown in figure 5, which illustrates a plan, cross-profiles and the already described long profile of the main body of the rock glacier. Ridges and furrows are most notable, with steep lateral margins leading up to the main sur- face. From a comparison of profiles A—B and C—D it can be seen that the ridges and furrows do not match up, instead they exhibit displacements and wavering courses. This is most clearly seen from ana- lysis of the medial furrow. At the head medial this furrow is located on the western side of the rock gla- cier and near the terminus it swings to the eastern side (see figure 2). The medial furrow is the deepest fur- row, and for all but two months of the year contains a snow patch. It is worth noting that whilst walking along the Iength of this furrow, water can be heard flowing beneath the surface. The underground stream appears to be substantially the same water derived from the melt of the glacier surface. Close to the snout the water emerges and flows at the surface. This sug- gests that beneath the open matrix of rocks there exists an impermeable structure reasonably close to the surface. Evidence of this impermeable layer is also found in the presence of pools of standing water on the rock glacier. Given that an impermeable layer exists beneath the open matrix of the rock glacier surface, the nature of this layer needs to be considered. In the main body of the rock glacier, glacier ice is often exposed along the line of the medial furrow. At the point where the stream emerges onto the surface, a mass of ice, rich in debris can be seen to lie below. It was estimated that 50—60% of the mass was debris. In 1977 an exposure in the form of a hole, sounded to be over 30m deep, Figure 6: Glacial ice exposed in the central furrow showing sedimentary bedding of debris. 6. mynd. Sprunga þar sem í Ijós kemur jökulis og lagskipt möl. was found in the median furrow. The sides of this hole showed ice with sedimentary bandings of debris (fig- ure 6). This ice appears to be of glacial origin. From the presence of the stream, and from a line of melt-out debris cones along the Iength of the median furrow it is suggested that the glacial ice which forms an impermeable layer within the rock glacier is melt- ing when it is in close proximity to the surface. The melt is concentrated in these areas and surface insta- bility (as judged by ease of climbing over the rock sur- face) is generally compatable with this interpretation. In other areas, especially near the snout, colonisation of the surface by lichens, mosses, grasses and a few alpine flowers indicate a very stable surface. Figure 5 illustrates the profiles of the rock glacier, 52
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