Jökull - 01.12.1987, Blaðsíða 56

Figure 8: Nautadalur rock glacier snout.
8. mynd. Sporður grjótjökuls i Nautadal.
Figure 9: Glacier ice exposure at rock glacier snout.
9. mynd. Jökulís við sporð grjótjökulsins.
backwall. It is evident that debris input from the
corrie backwall must account for some of the debris
input to the rock glacier surface.
From the observations of the debris relationship to
the corrie glacier at the rock glacier‘s head it appears
that the debris constituting the rock glacier surface is
the direct result of debris release and transfer from the
glacier, directly or indirectly, as a consequence of de-
bris production from the corrie backwall.
The snout
The rock glacier snout is the only part of the feature
visible from the bottom of Nautadalur. It is at an alti-
tude of 850m a.s.l. and has a height of 41m. Surveys
of the limit of the snout were made in 1977 and again
in 1985 (see figure 7). The results stongly suggest that
the rock glacier snout is at a standstill and that minor
boulder movements recorded are the result of the melt
of ice contained close to the rock glacier surface.
Figure 8 illustrates the profile of the snout as ob-
served in 1985. The most notable characteristic is the
‘pinnacle’ of debris rising above the profile line on the
left hand side. Below this pinnacle, approximately 5m
from the top of the snout, a meltstream discharges.
This meltstream is that already described which flows
both under and along the surface of the main body of
the rock glacier. Like many rock glaciers this stream
contains very little sediment (Corte 1976) and con-
trasts markedly with characteristically sediment-rich
glacial streams.
The area of the snout where the stream issues is un-
stable, with rocks slipping downslope. Because of this
instability, internal ice can be exposed. In 1977 such
an exposure was seen (figure 9). Examination showed
this ice to be true glacier ice with crystal size of about
2cm diameter. Debris was emerging from the ice in
several places; the subhorizontal banding and the in-
clination of stones within them suggested that this was
indeed a section of glacier ice from which debris was
emerging along former accumulation surfaces.
SUMMARY OF FIELD EVIDENCE
1. Glacier ice can be traced from the head of the
rock glacier to its snout.
2. The debris superimposed on this glacier ice has
predominantly been derived from the corrie backwall
by means of transference through or on top of the gla-
cier located at its head.
3. The rock glacier is virtually static as shown from
boulder movements recorded over an 8 year period
and vegetation colonisation of the bouldery surface.
4. The glacier ice when exposed either at the head
of the glacier or in the rock glacier is seen to experi-
ence ablation during the summer months.
DISCUSSION
Examination of the Nautadalur rock glacier clearly
illustrates the relationship which exists between this
feature and glacier ice. Direct evidence is found of
rock glacier development from glacier ice superim-
posed by rock debris. This model of rock glacier
development has been suggested as a possible origin
since the beginnings of rock glacier studies (Capps
1910; Cross and Howe 1905; Brown 1925).
The association of rock glaciers in Tröllaskagi Pen-
insula with small corrie glaciers and corries suggests
that a glacial origin (backed up in this case of field
evidence of glacial ice being traced along the profile of
54