Jökull - 01.12.1975, Blaðsíða 46

Fig. 6. Suggested recon-
struction of ice-front at
time of cutting of escarp-
ments shown in Fig. 4.
Mynd 6. Líkleg staða
jökuljaðars, þegar rof-
brúnir i Mynd 4 mynd-
uðust.
tain but there appears to have been a flow
from the location of the present ice-cap to tlie
north-west, some of the ice from tliis source
turning west into the main stream over the
nortli of Arnarvatnsheidi.
CONCLUSIONS
The major relief of Arnarvatnsheidi, for ex-
ample the arrangement of plateaus falling in a
series of steep slopes towards the south-west,
may be pre-glacial and partly controlled by
structure. However the minor relief features,
escarpments, channels, and arrangement of sur-
face deposits are all consonant witli evidence of
main ice-sheet moving towards the south-west.
This ice-sheet may have originated at first to
the north of the area, with a later centre form-
ing over Tvídægra. The present site of Lang-
jökull may have been occupied by another ice
centre which appears to have become separated
from the main stream during deglaciation, ex-
cept at its northern end.
Some of the escarpments and channels may
have been formed sub-glacially, either in early
stages of deglaciation or in jökulhlaups, when
water was flowing through or under the ice.
There is little evidence about the dip of basalts
in the area, but observation suggests that this
is slight and towards the south-east. Tlius the
ice, and the water flowing under it, seem, to
some extent at least, to have been guided along
the strike of the rocks thus giving rise to the
north-east to south-west trend of the escarp-
ments.
At a later stage of deglaciation the ice shrank
towards the nortli-west across Arnarvatnsheidi.
At various stages in the retreat melt-water,
streaming along the ice edge, was trapped be-
tween the ice and the fronts of the escarpments,
forming ice-dammed lakes connected by mar-
ginal channels across spurs in the escarpments
(Fig. 6). Arnarvatn stóra, with channels cut over
the spurs and the ash deposits at the moutli of
Skammá, is an example of the process. As the
ice retreated, the lakes lowered in level as na-
tural drainage developed. Any local post-glacial
rebound in the Tvídægra area would tend to
form lakes against the north-west facing sides
of the escarpments.
During the ice retreat streaming water would
cut into the escarpment faces, dislodging the
basalt capping to form angular boulder fields
below them as the less well-consolidated deposits
below the basalts were removed. The finer de-
bris would be carried downstream to form the
thicker cleposits in the west of the area.
44 JÖKULL 25. ÁR