Jökull - 01.12.1952, Page 13

over Leynifoss. Another „hlaup“ followed in
1939, over col No. 4, when the southwest basin
of the lake was practically drained. Since then
the river draining into Hagavatn from Sand-
vatn efra has cut a section through the sedi-
nients of the western basin, which has made an
examination possible.
It was originally intended to determine the
age of the south west basin by counting the
varves. However, owing to the difficulty of
determining what was one year’s sediment, and
to the presence of several erosion levels, it was
found more preferable to use Thorarinsson’s
method of dating by study of volcanic ash
layers.
The standard section was set up by Thorarins-
son at Skallakot 20 km NW of Hekla (cf. Fig. 1),
whilst another soil profile was measured at
Hagavatn in 1948. Most of the ash layers noted
in this section by Thorarinsson were also found
in the lake sediments, the earliest identified
being pre-colonization.
The sediments of the south western basin
were examined along above mentioned river
from Sandvatn, and along a gully running north-
wards into the river from the southern margin
of the lake bed. Three main types or grades of
material were noted: fine clays, silts and sands
of distinctly different types. The clays were of
thíee colours: a fawn-yellow, a pale flesh-pink
and a grey clay, the latter being of the coar-
sest texture. The silt was rather sandy, grey in
colour, often containing bands of pebbles, and
was the heaviest and most compact of the sedi-
ments. The sands were of mainly volcanic
origin,. black in colour when pure, but often
containing yellowish tuff fragments: these were
generally coarse sands, except where they occur-
red as thin bands between the clays. The sedi-
ments were not typical lake sediments, as little
gradation between types of materials occurred,
e. g. coarse sands were intercalated between very
fine clays. Also, the almost machine-like lami-
nation of the fine clays is comparable with the
„sub-glacial shear-clays" of Carruthers. It is
thought that the depositional conditions were
somewliat unusual, though it may be explicable
in that sedimentation took place from three
directions.
The lowest sediments exposed were at the
western end of the river section; these compri-
sed coarse current-bedded black sands, with
SKALLAKOT HAGAVATN
LEGEND:
Black
______Portlde dlom.
HBll< 0.2 mm
Llghf
Fr-plV^ 0.2 mm
rrrFii) 0-2-10 mm
> 2.0 mm
Brown
nnninniiin) < 0.2 mm
iim.iÍi:ii:ib)0-2-2.0 mm
. > 2,0 mm
Mlnerol sollj
|l~L | Claylsh
— Loamy-sllfy
— I Flne-sandy
Sondy
|a a a|
I A A I
Morolne
Fig. 1. Two soil profiles measured by S. Thor-
arinsson (as explained in text).
Jarðvegssnið úr Þjórsárdal og úr moldarbarði
austan undir Fagradalsfjalli.
occasional brown silty bands on the upper sur-
faces of individual beds. The sands rested on
the lavas of Lambahraun, which had suffered
some weathering and erosion before sendimen-
tation began. These sands formed the delta of
the river flowing in from the Sandvatn, and
thus thinned out to the east. No clay bands
were seen towards the base, but higher, begin-
ning about six feet above the lowest sands seen,
narrow clay bands were present, lying on ripp-
led surfaces of the sands. Rising in the succes-
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