Jökull - 01.12.1953, Side 29

dirtlayered ice surface and one on a high mora-
ine.
Fig. 4 shows the comparative ablation dur-
ing the second half of the summer. The moraine
had such a protective layer of grit, pebbles and
boulders that ablation was much less on the
moraine than elsewhere until the end of the
summer when, due to steepening sides of the
moraine and long periods of rain, the moraine
ablated at a much increased rate. The surround-
ing glacier surface had by the end of the summer
become more dirty and was losing its dirt cones
by the slumping of dirt from the sides which
had become too steep to retain their dirt cover.
The three stakes at the different areas of
detailed observation showed little difference in
ablation recorded during the mid-summer, yet
the ablation per hour plotted from the micro
tests reveals a marked increase in ablation of
clean ice and decrease in ablation of dirt cones
during periods of bright sunshine.
The near coincidence of the three ablation
graphs is probably due to inevitable smoothing
of marked differences in ablation by recording
at the stakes only every few days. The stake at
the dirt-cone site was not itself in a dirt cone
and was thus little different from the clean ice.
The stake in the dirty ice was at a surface
which was a little steep and thus had melt
water passing over it constantly. This caused
a slight mudflow but was replenished by the
dirt sludging down from ice farther up the
slope of the extensive glacier snout.
The clean ice surface melted 20 cms. more
than the dirt-covered ice, 60 cms. more than the
dirt-coned ice and 140 cms. more than the
moraine-covered ice in eight weeks of late sum-
mer. Rain following sunshine caused clean ice
to melt most rapidly. See 27th July to lst Aug.:
21st August to 25th August; 6th September to
lOth September. Sunshine following rain caused
dirt covered ice to melt most rapidly. See 5th
to 8th August; 9th to 14th August; 3rd to 6th
September. Dirt-coned ice was rarely dominant
in ablation. Those broad generalisations can-
not be applied rigidly. Indeed the graph shows
various anomalies but on the observations made
the above conclusions were valid on this first
analysis. A mathematical corroboration or con-
tradiction should evolve from the calculations
on heat transfer to be made in conjunction
with the micro observations on ablation.
Fig. 4. The comparative ablation of clean and
dirt- or moraine covered ice.
Samanburður á leysingu á hreinum og
aurbornum is.
ABLATION OF DEAD ICE.
Three stakes were drilled into debris covered
ice, one of which was very recently part of the
glacier snout and had only a very thin dirt cover.
The change in thickness of the dead ice was
also measured at each point of observation on
ablation. When the upper surface ablation was
subtracted from the decrease in total thickness
the amount of bottom melting was found. The
surface ablation was 1, 13 and 7 cms. while the
bottom melting was 1.5, 19 and 18 cms. re-
spectively. In each case the bottom melting
was greater than the top melting. The smallest
difference being at the very thinly mantled
ice mentioned above. In both the thickly mant-
led areas the debris cover was nearly 50 cms.
thick but at each site, if access could be made to
permit measurement, then certainly the at-
mosphere had sufficient access for some form
of circulation. Much water was present under
the dead ice and cauldron slumping on a small
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