Jökull - 01.12.1952, Page 16

Fig. 4. Section through varved sediments in a
gully (II on the map) S of Hagafell. White poin-
ters indicate ash layers. Note strong rippling of
fine sands.
Hvörf i botnseti Hagavatns snður af Hagafelli.
Hvítu örvarnar benda á öskulög.
though occasionally incompletely. A total rhyth-
mic sequence was between six inches and a
foot in thickness, the variation being due mainly
to the thickness of the sands at the base. Often
the yellow bands of the upper part appeared
to have been removed by the currents deposit-
ing the sands of the succeeding rhythm. It seems
improbable that such a rhythm is an annual
deposit, yet similar sediments occur at the south
eastern side of the lake bed, the individual
rhythms being only about an inch thick in this
case. It is suggested by Thorarinsson (1940, p.
238) that much of the minor lamination of the
sediments is due to cyclonic weather effects.
Tliese may at times be sufficiently large to be
confused with seasonal deposits.
In a section dug to the east of the river, two
varves were noted to be lying on boulder clay
which in turn lay on the contorted sediments
of the same type as those seen in the river
section. These upper varves were probably of
the 1925—1939 succession, and the boulder clay
on which they lay appeared to be a solifluction
layer from the moraines. The latest members
of the varve succession were removed by water
erosion when the lake was drained, and at the
present time rapid deflation is taking place.
A gully section (II) from the south side of the
lake bed towards the river showed black volcanic
sands intercalated between the varves. The indi-
vidual beds of sand were over a foot in thick-
ness on the southern margin of the lake bed,
but lensed out northwards, most not reaching
as far as the river section. Obviously, these
sands were washed in from the Lambahraun,
probably during the snow melt in the spring,
though possibly also during occasional storms.
The bases of most of the sand layers were
unconformable on the varves below and the
upper surfaces of the sand layers were rippled.
These 'appeared to be oscillation ripples (on
which the laminated clays were laid down).
This being so, then it is possible that in many
cases non-deposition of clays occurred, or ero-
sion of clays when waves produced by the wind
vfrere sufficiently large. Disturbed clay layers
noted at different levels along the river section
may also be due to the effects of waves, though
there seemed to be slump-structures present in
some cases.
Layers of volcanic ash were found at three
horizons in the succession, one quite near the
base of the sediments. These were composed of
black volcanic glass, and were widespread in
extent. N. B. Trial pits were dug at one or two
points on the lake bed: the ash layers were
noted, but the sediment was in too fluid a con-
dition to allow any measurement of thickness
to be made — slumping tended to occur almost
as soon as the pits were dug. In two cases, above
each ash layer was a layer of impure ash
evidenty washed into the lake later; these
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