Jökull - 01.12.1967, Síða 50

ernmost and biggest o£ alíj very indented and
perhaps consisting of many small lakes. It ex-
tends farther north than has been explored.
To the east and north it is bounded by an
amphitheatre of high mountain ranges cul-
minating in the farthest north in a towering
cone-shaped peak . . .” This lofty cone-shaped
peak is in all probability the mountain Sydri-
Kerling, which looking north-east lies open be-
fore the eye, Fig. 2.
At the present time there are only a few
small lakes in the Tungnaá depression ancl
surely no one would think of calling them
Stórisjór. All these lakes, altogether four, are
anonymous except Jökulvatn, which is situated
at Hraunsskard. Prior to the drilling in the
Tungnaá depression it was considered probable
that the small lakes were remains of a bigger lake
previously occupying the valley, having graduallv
silted up with the sediment load in Tungnaá
cluring the last thousands of years. Two jour-
neys have been undertaken for an investigation
of the thickness and nature of the sediments
on the proposed dam site in the Tungnaá de-
pression against the mouth of Lónakvísf with
driffing. The forrner took place in November
1965, but a sufficient result could not be ob-
tained then. Therefore, a new journey was
undertaken on the 19th to 21st of October in
1966. The main emphasis was put on the
effort of obtaining samples from the sediments
of the Tungnaá depression, and also on an
exploration of the depth to bedrock with
sounding as time would allow. The drilling
was performed by a group of six men under
the leadership of G. Sigurdsson foreman and
E. G. Vilmundardóttir geologist. As mentioned
above the main purpose of the investigation
was to obtain samples. They were intended to
be taken with a sampler, which is a liollow
cylinder open at the lower end wherein the
samples are pressed, but the sediments proved
to be so incoherent that the cylinder became
emptied on the way up and only one sample
was obtained by this method, from 2 m depth.
Then the alternative of washing the samples up
with water was turned to. It was done by press-
ing down a casing and washing frorn inside it
by the pumping down of water. Generally
samples were taken at intervals of one m. Initi-
ally material from an approximately 0.5 m
interval was washed up and made to flow away,
then the material from the next 0.5 m wTas
collected into a 20 1 pail and left for a short
time for sedimentation. Now the water was
poured from above and the sample put into
a plastic bag for preserving. The casing easily
pierced down to a depth of 12 m simultane-
ously with the washing out of it, but then it
could not penetrate further down. Therefore,
the samples from below that point were washed
up without the protection of the casing.
When 17 m depth was reached the sampling
was given up because the hole began to cave
in; yet bedrock had not been reached. Surely
this method of sampling is rather rough, but
it was the only applicable one. It is to be
expected that by this method a considerable
part of each sample got lost, especially the most
coarse and also the most fine-grained material.
Probably the samples therefore indicate a rather
greater sand content than the sediment actu-
ally contains. On the other hand it is likely
that variation between individual samples is
rather caused by different composition than by
what was lost in their collecting, because the
same method was used with all of them except
the first sample. The samples do not seem to
have become mixed with material from the
walls of the hole as they were washed up. Such
could not have taken place in the first 12 m
protected by the casing, but the lowest sample
clearly shows that no intermixing has taken
place below that point either, as both white
pumice and diatoms, which abound in the next
sample above, are totally lacking here.
The investigation of the samples can be divid-
ed into three stages:
1) The samples were dried and sieved and a
part of them gluecl to a special sheet of
paste-board in order to get a general view
of the stratigraphy of the sediments.
2) A petrographical and teprochronological
analysis.
3) Investigation of diatoms.
1) This part of the investigations was per-
formed in the Laboratory at Keldnaholt by S.
Pálsson and G. Gudmundsson. A part of the
sieved samples was glued to a sheet of paste-
board, cf. Fig. 3, where the size groups of each
284 JÖKULL 17. ÁR