Jökull


Jökull - 01.12.1987, Page 53

Jökull - 01.12.1987, Page 53
Figure 4: Nautadalur glacier — rock glacier transit- ion. 4. mynd. Grjótjökullinn í Nautadal — mörk hans og hvilftarjökuls. and Benedict, 1965) and is typical of those found else- where in the peninsula. The Nautadalur rock glacier was examined in 1977 by an expedition from Devizes School, England and again by us in 1985. nautadalur rock glacier The corrie glacier The main features of the glacier can be seen in fig- ures 2 and 3. It is clear that the ice is active even though downwasting is evident both at the margins and at the head wall. The bergschrund is frequently covered by snow blocks though some rock debris does fall into it. Many debris falls do however pass over it and lie on the old snow surface. Aerial photographs of the glacier taken in 1982 show that these debris paths have been predominantly the same since the earliest coverage in 1946. Also consistent over this time is the emergence of debris in the lower portion of the gla- cier, (figure 4). These debris emergence zones are as- sociated with the dirt lines of yearly ablation and ac- cumulation surfaces, (McCall 1960) and can be corre- lated with zones of high rockfall activity at the back- wall. The density of these lines on the glacier surface is high, with a surface area ratio of 1 : 6, debris to ice. Debris, once released from these surfaces on the lower part of the glacier, accumulates and forms a relatively continuous cover on the ice. It is in this zone that the distinction between rock glacier and debris covered glacier becomes difficult (figure 4). Figure 5 shows a long profile surveyed from the rock glacier surface onto the corrie glacier and it shows a significant hollow in this zone of transition. The lower, debris free area of ice in this concavity (figure 4) is clearly the main ablation area of the corrie glacier, as shown by the origin of a number of supraglacial streams in this locality. Melting is likely to be increased by a very thin layer of debris but rapidly decreased as it thickens downvalley. This thickening of debris cover corresponds to a rise out of Figure 5: Plan, long profile and cross profiles of 5. mynd. Langsnið og þversnið af grjótjökli í Nauta- Nautadalur rock glacier, also illustrating rock glacier dal. Mynd til hcegri sýnir legu sniðanna. movement along lines A—B and C—D, 1977 — 1985. 51
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