Jökull - 01.12.1952, Blaðsíða 15

Fig. 3. Section of two E-
W channels in lake sedi-
ments. Heavy silts of
lower channel have sunk
down into the fluid var-
ved clays (See text below).
Snið af tveim rdsum i
botnseti Hagavatns og
röskuðum leirhvörfum.
ning to block col. No. 4. Often there were evi-
dences of erosion of the upper parts of clay
bands: this was especially noticeable where clays
and fine sands had been laid down on a rippled
surface of sands. Coarse sand beds deposited on
these ripples tended to cut down into the clays
at the tops of ripples. Rounded clay balls
were noted in tlie coarse sandy layers, suggest-
ing that often considerable erosion of the lower
clays had taken place, i. e. of clays that had been
deposited a sufficient time to have become fairlv
compact.
Further up in the succession, where finer
sediment predominated, and where the clays
approached true varved sediments, there were
often grey silt bands present. Typically, these
silts occurred in U-shaped channels, running
generally east-west, and in which the silts were
bedded parallel to the sides (Fig. 3). The upper
surfaces of the sediments in the channels were
often irregular, and the laminated clays above
lay unconformably on the silts. Below these
channels the laminated clays were usually con-
torted and compressed, suggesting that the clays
were in a very fluid condition when the silty
material was laid down. Contortion was evi-
dently due to weight of these heavy silts, and
thus the U-shaped channels were probably
formed by a downward slumping of the silts
into the clays (Fig. 3).
A shallower type of channel was also noted,
in which the silts truncated the varves below
without any undue disturbance. These silts
extended over considerable distances beyond
the confines of these channels, eventually thin-
ning out laterally.
Occasionally current-bedding was noticeable
in the silts, and this was always from the west,
as it was for the sands below, showing that the
main sedimentation for the western end of the
lake was from the west.
The succession rose northeastwards, along the
river section, and was measurable to within
about one hundred and fifty yards of the mor-
aines, which marked the maximum extension
of the ice. Approaching the moraines, the varves
became strongly folded and thrust, showing
structures reminiscent of Alpine folds. It was
impossible to measure the succession of this
area.
The sands and silts tended to die out to the
north and east; and so towards the moraines,
the succession comprised mainly laminated
clays. A typical cycle of these comprised: very
fine black and brown current bedded sands at
the base, with fine grey clay bands between sand
lenses. These are followed by grey, and fawn
and grey clays, finely laminated, with between
twenty-five and fifty laminations to the inch.
The grey clays tended to be silty, and occasion-
ally current bedded. Often a pale flesh coloured
band of clay, usually thin, followed the lami-
nated clays, overlain by fine grey clay. These in
turn were followed by several bands of yellow
clay, which tended to be rather thick, and which
were separated from each other only by fine
grey bands. This whole sequence appeared to
be a rhythm, and was repeated several times,
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