Jökull - 01.12.1987, Page 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,
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