Jökull - 01.12.1987, Blaðsíða 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