Jökull - 01.12.1985, Page 68

Plate 1. Upper section of the feature
seen from Ll, showing crevassed
snow above the head of the gulley
and the narrow neck between the
gulley and the deposit. - Efri hluti
hlaupsets, séð frá stað Ll.
seem likely that flow must have been channelled within
a snow cover.
The whole feature probably formed in a single event
of short duration, possibly as little as a matter of hours
in view of its limited extent and integrity, almost cer-
tainly within the winter season of 1982/83, late enough
for there to have been a good snow cover over which to
deposit material. During the 1983 ablation period prior
to July any thinner snow cover over the deposit melted
quickly but after this the deposit acted to protect the
underlying snow. It is however unlikely that much of the
deposit would last the summer as it was already being
broken up by undercutting and melt of the snow from
below where the main river flows down the valley. All
that may well remain at the end of the summer would be
the gully system but this could be quickly modified due
to the unstable nature of the slopes and the continued
advance of Gljúfurárjökull.
Overall the feature bore many elements of a minor
jökulhlaup, caused not by geothermal processes but by
a realignment or temporary damming of the drainage
system associated with the glacier. It is not unlikely that
this happened before in the valley but that all traces
have been lost due to the ephemeral nature of the
deposits produced. Processes such as those associated
with such bursts could account for isolated elements of
coarse yet bedded sediments occasionally found within
predominantly unsorted till ridges, and also possibly
account for the dead ice features found in front of the
glacier snout in 1979, and which had disappeared by
1981. Cover of snow and firn very close to the ice front
may not have been so rapidly destroyed by melting and
hence features comparable to ice-cored moraines would
have survived.
At the present rate of advance it is likely that the
gully will largely disappear by 1984 and that due to its
location directly overlying the main river system the
spread of sediment will also be unlikely to survive. It
Plate 2. The deposit at the point where it first overri-
des the main river showing thin spread of angular
debris and finer sediments overlying more than a
metre of snow, now being undercut by the river. —
Hlaupsetið þar sem það hefur lagst þvert yfir farveg
Gljúfurár.
66 JÖKULL 35. ÁR