Jökull - 01.12.1967, Blaðsíða 34

Fig. 6: The Sanddalur Col seen from the NW.
Brekknafjöll to the left and Fagradalsfjall to
the right. The abandoned shore lines in the
western slopes of Brekknafjöll represent the
changing elevation of the Hagavatn lake level.
1: 22 m shore line, 2: 16—18 m shore line zone
and 3: 10—12 m shore line zone.
Sanddalsvarp séð frá NV. Brekknafjöll til
vinstri, en Fagradalsfjall til hcegri. Strandlinurn-
ar i vesturhlíðum Brekknafjalla sýna vatns-
borðsbreytingar Hagavatns. 1 er 22 m strand-
línan, 2 er 16—18 m strándlínan og 3 er 10—
12 m strandlínan.
has 'escaped subglacially through the Leynifoss
Col or even the Nýifoss Col. The drainage has
most probably íollowed both these courses from
the former Hagavatn.
A great glacier stream has issuecl from the
maximum ice front about 350 m west of
Brekknafjöll, where it has built a 300—400 m
wicle marginal delta into the former Hagavatn.
This delta reaches right up to the slopes of
Brekknafjöll, where a stream channel shows
drainage toward the NE along the slopes. These
channels and delta terrace are at a height of
21—22 m above the present lake level and are
thus in good correlation with the highest shore
line (level 1 Fig. 5).
Near tlie NW corner of Brekknafjöll there
are big kettle holes and a stream channel trends
west from there, cutting through some more
kettle holes before reaching Hagavatn. This
channel seerns to have been eroded during the
formation of the kettle holes. Another channel,
somewhat higher, trends NE along the north
slopes of Brekknafjöll to the Leynifoss Col and
it shows that in some places there has been
some water erosion, but in other places it is
covered with ground moraine. The present
water divide of this channel is 18 m higher
than the present lake level, i.e. it is approxi-
mately at the same elevation as the 16—18 m
shore line zone (level 2 Fig. 5). A moraine-
covered ridge trends east-west on the north side
of the channels and kettle holes, which separate
the ridge from the delta terrace. The riclge is
up to 470 m in elevation and it reaches about
250 m farther to west than the NW corner of
Brekknafjöll. The direct outlet frorn Hagavatn
through the Leynifoss Col was blocked when
the advancing glacier reachecl this ridge and
the lake level was graduallv raised until the
water could escape via the 18 m channel (level
2 Fig. 5).
The advancing glacier also crossed the ridge
and the channel up to 500 m elevation in the
northern slopes of Brekknafjöll. Yet the height
of the delta terrace at the NW corner of
Brekknafjöll seems to have controlled the
maximum elevation of Hagavatn (level 1 Fig.
5). The drainage from the lake has therefore
probably been in a subglacial tunnel through
this channel to the Leynifoss Col, but the kettle
holes and the ablation moraine in the channel
are more recent glacial features.
The 16—18 m shore line zone, which re-
presents the lake level before 1929, is to be
found on the north side of the delta terrace,
thus proving that the ice front hacl retreated
somewhat from its maximum extent. When the
ice front receded, the water table was no longer
controlled by the height of the delta terrace,
but the 16—18 m shore line zone seems to be
controlled by the 18 m water divide in the
channel. In this way the Hagavatn has probably
drained subglacially through the Leynifoss Col
at least most of the time it was overridden by
the glacier, but the discharge has not been com-
parable with the present one from Hagavatn
through the Nýifoss Col for two reasons: There
has been much more leakage through the perme-
able bedrocks of the Hagavatn basin during the
highstand of the lake level and also a great
deal of the drainage from the glacier escaped
through the glacier lobes in the Nýifoss Col
268 JÖKULL 17. ÁR