Jökull - 01.12.1967, Page 58

to flow to the east directly into the Skaftá
river. As the glacier has retreated the pass has
become wider ancl deeper and the glacier river
there has continuously been moving its source
farther into the pass. In 1966 that river had
captured all drainage frorn the Skaftárjökull
glacier west of Fögrufjöll. No other water drain-
ed into Langisjór than rivulets of clear water
oozing fronr the stagnant ice at the uppermost
terminal moraine.
Briefly, the history of Langisjór appeared to
G. Kjartansson to have been like this: Some
centuries ago and even recurrently since “the
warm climate period ended nearly 2500 years
ago” the glacier has reached the north-eastern
end of Fögrufjöll and blocked the drainage
from Langisjór through there, causing a raising
of the lake level and the formation of the
shorelines. He considers the upper one to be
formed by stagnant lake level during a long
time perhaps many centuries.
We on the other hand consider all changes
at Langisjór to have taken place within much
shorter time than Ivjartansson maintains in his
report. Our arguments are the following:
1) The shorelines at Langisjór are cut in
pumice from the Laki eruption of 1783 and
therefore must be definitely younger than this
eruption. 2) The rapid increase in land at the
north-eastern encl of the lake, as testified on
maps ancl aerial photographs, has for the most
part taken place in the present century. 3) Ut-
fall, the outlet, has according to sources been
formed in the period 1878—1894. Therefore,
we conclude tliat the glacier did not extencl
to the slopes of Fögrufjöll until in the latter
part of the nineteenth century. Before that
Langisjór has either been drained between the
glacier and Fögrufjöll or been without surface
drainage.
Fig. 8 presents a diagram illustrating those
changes in the water level of Langisjór we con-
sider to have occurred during the last 100
years. It is divided into five stages. Stage I
shows the lake level before the lake began to
be afíected by the glacier shortly after 1850.
The lake level is supposed to have been in
650 m elevation in this stage. II began as the
glacier started to aftect the lake level. Glacier
rivers began to drain into the lake resulting in
a gradual raising of the water level. The glacier
also blocked the dramage via the passage be-
tween the glacier and Fögrufjöll. This stage
cante to an end about 1890 as the glacier
extended into the lake ancl the lake level reach-
ed its maximum elevation forming the upper
shoreline. Stage III began as water started to
overflow the rock barrier at the present Utfall,
rapidly eroding the fractured ancl loose surface
rock down for aboul 2 m. At that point the
erosion slowecl down and the lower shoreline
was formed. This lowering is likely to have
occurred about 1890. During stage IV the rock
barrier at the Útfall was gradually lowered by
erosion and the lake level clropped at the same
rate. In the present century the glacier has
been retreating. Since about 1960 the passage
between Fögrufjöll and the Skaftárjiikull glacier
has captured more and more of the glacier
water constantly reducing the inflow into
Langisjór. This stage came to an encl in 1966,
when both the glacier rivers previously flowing
into Langisjór had been captured by the
drainage between Fögrufjöll and Skaftárjökull.
The present staage, V, began at this point. Al-
most immediately the lake level clroppecl about
half a meter and it is probable tliat in the
future the lake level will remain in the same
elevation as the rock barrier in the Útfall or
even be lower.
6. ON GLAGIAL AND CLIMATIC
VARIATIONS
In this chapter we will discuss glacial and
climatic variations in the light of tlie investiga-
hœð - elevation
my s.- mojs.
Fig. 8. Changes in the water-level of lake Langi-
sjór during the last 100 years.
Myncl 8. Vatnsborðsbreytingar í Langasjó siðast-
liðin 100 ár.
292 JÖKULL 17. ÁR