Jökull - 01.12.1970, Síða 58

2500 H
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1894 1900
1910
Fig. 6. The variations in length of the front of Breidamerkurjökull according to J. Eythors-
son’s measurements. E indicates the glacier front to the east of Jökulsá, but W to the west of it.
Mynd 6. Breytingar á jöknljaðri Breiðamerkurjökuls samkvcemt mœlingum Jóns Eyþórssonar. E
sýnir jökulsporðinn austan Jökulsár, en W er vestan hennar.
Fig. 8 shows the longitudinal section from the
estuary of Jökulsá river to Nordlingalaegd and
Fig. 9 the cross section from Rákartindur in
Breidamerkurfjall to Fellsfjall. The sections do
not include the lowering that occurred in the
years 1894—1904 and 1965—68. For these inter-
vals the lowering was estimated as proportional
to the reduction in area. The sections were
then used to find the average lowering for
each altitude interval of 100 meters up to 500
m elevation. For the altitude interval 500—
1000 m the lowering was estimated in accord-
ance with the lowering measurements repre-
sented in Table 1. In that estimation regard
was payed to the local conditions at each
measuring point (Table 3). Above 1000 m
elevation there are very scanty indications to
which the thinning of the glacier can be relat-
ed. Yet the following attempt was made. Ac-
cording to lowering measurements the average
lowering for the 8 measuring points in the
altitude interval of 850—960 m proved to be
39 m and the three measurements closest to
the glaciation limit gave 36 m as average lower-
ing (Table 1). The thinning at the glaciation
limit is therefore very near to 30 m. The
thinning above the firnline is rather caused by
the flow of the glacier than ablation. It is
therefore quite probable that the thinning at
the firnline is representative for that of the
glacier above it as long as the thickness of the
glacier is many times greater than the lowering,
which is the case for a greater part of Vatna-
jökull (Eythorsson 1951). The highest parts of
Vatnajökull (> 1600 m) are most likely due to
underlying mountains and mountain ridges and
it is quite uncertain whether any thinning has
occurred there. They are therefore excluded
56 JÖKULL 20. ÁR