Jökull - 01.12.1985, Síða 67

greater distance away from the erosional part of the
feature. The size of the largest clasts, after the occur-
rence of some much larger boulders close to the end of
the gully also showed no subsequent pattern of decreas-
ing size with distance from the gully (Table 2).
From all the observations and analyses that were
made the following points can be made:
1. The gully, which was dry during the period of
survey, was not part of the sub-glacial/pro-glacial
stream system mapped in earlier years although water
had previously been observed flowing along some
points of the glacier margin, especially on the western
side. At the head of the gully there was no evidence for
water to have originated from under the ice at that
point. Partial collapse of the snow cover at the head of
the gully possibly pointed to an origin further around
the margin. There is therefore no direct evidence to
define the water source as englacial.
2. AIl the material deposited below the gully is com-
parable to that derived from the till in the gully. While
some of this material has been sorted there had been
virtually no rounding of clasts, only partial preferential
loss of different size fractions and there is no evidence
for a source of sediment outside the gully.
3. The deposit lies on top of snow, probably at
thicknesses in excess of 1.5 m above the main river, the
Gljúfurá, but snow had also probably once covered the
deposit, the snow which marked the edge of the deposit
showing no signs of incorporated sediment despite lying
below the level of the ridges. Deposition must therefore
have taken place after a period of thick snow cover but
prior to further snow fall.
From all the evidence it would appear that the overall
feature of the gully and associated spread of material
derived from a burst of water at or just outside the
glacier margin. The origin of the water is unclear, it may
have been englacial, released due to a realignment of
water stored within the glacier finding an outlet at a
point along the glacier margin, or it may have been
water from sources outside the glacier dammed up and
then released by movement of the glacier or some
failure on the surrounding slopes. This release of water
must have been sudden and of high energy, sufficient to
cut into and erode till to a depth of 2 m. As erosion
continued along the side of the glacier much of this
energy dissipated as more and more material was
eroded and deposition began, generally in a weakly
braided form with the largest boulders quickly being
deposited. The lateral spread of deposition appears to
have been very limited with sediment eventually con-
centrated into one main channel, possibly partly
restricted by the snow into which the debris-charged
Fig. 3. Survey of features associated with the "burst“.
— 3. mynd. Kort sem sýnir framburð í hlaupinu veturinn
1982-83.
water must have been cutting. The depth and velocity of
the water did allow some sorting of the sediment to take
place but eventually deposition became more diffuse,
presumably as energy was lost and no further deposition
was possible. It is difficult to judge just how deep the
water would have been, at present much of the deposit
lies above the surrounding snow cover and there is no
evidence of deposition or erosion outside the immediate
confines of the feature. At first sight the whole feature
looked very much like a flow but the bedding and
overall nature of the landform, especially the ridges of
material, strongly support the importance of water
borne transport of eroded material and it does therefore
JÖKULL 35. ÁR 65