Jökull - 01.12.1967, Síða 59

tions at Stórisjór ancl Langisjór discussed above.
As already stated in chapter 2, mountains and
valleys in the Tungnaá area have a NE—SW
lineament. The water divide between Tungnaá
and Skaftá lies along the Tungnaárfjöll moun-
tains, which disappear under glacier against
Tungnaárbotnar, cf. Fig. 1. As a continuation
of them the mountain Sydri-Kerling rises north-
west of Tungnaárjökull. Very likely Tungnaár-
fjöll are continuous co Sydri-Kerling below the
glacier. Thus, the Tungnaárjökull was formed
as the Vatnajökull ice cap had increased to
such a degree that an outlet glacier discharged
over Tungnaárfjöll turning Tungnaá into a
glacier-fed river. But when this happened can-
not be stated with accuracy. From A. Magnús-
son’s description dating from shortly after 1700
it appears that Tungnaá had by then become
glacier-fed, but the sediments in Stórisjór in-
dicate that this had not yet occurred in 1362.
At a certain time in this interval Vatnjökull
has overflowed the Tungnaárfjöll. The loess
layers in the hut ruins at Stórisjór indicate that
the huts were abandonned about or just before
1600, probably as a result of the contamination
of the lake water by glacier water.
As far as we know two attempts have been
made at reconstructing climate into the past.
One was made by thc meteorologist P. Berg-
thórsson (Bergthórsson 1967). For this purpose
he makes use of records on sea ice and years
of famine in Iceland and finds their correla-
tion to temperature. His reconstruction reaches
back to the colonization of Iceland in the 9th
century. The other attempt was made by an
English meteorologist H.H. Lamb (Lamb 1966).
He bases liis reconstruction on various sources
of information such as history and natural
sciences. Lamb’s attempt reaches far behincl the
settlement period of Iceland, even back to the
Ice Age. Bergthórsson and Lamb are in good
agreement as to the changes in climate. Both
reach the conclusion that the period from
about 870 up to the year 1200 had a compara-
tively warm climate. During the following two
centuries the climate was colder, then came a
short mild period from just before 1400 to
about 1450, when it began to grow colder again
approaching the cold climatic period. About
1600 a marked decrease in temperature occurr-
ed. The cold climatic period then lastecl up to
the last turn of a cenlury witli only short inter-
vals of warmer climate.
In connection with these variations in climate
it is of interest to note the oscillations in size
and volume of glaciers. The investigations of
Stórisjór indicate that there are strong prob-
abilities as to Vatnajökull liaving overflowed
the Tungnaárfjiill mountains as the climate
began to deteriorate markedly about 1600.
Tungnaárjökull seems to have reached its
maximum size shortly before 1890. The glacier
at Langisjór, the Skaftárjökull is somewhat
different from Tungnaárjökull since it creeps
along a valley whereas Tungnaárjökull creeps
right across a mountain ridge. Skaftárjökull
seems to reach its maximum advance about
1890.
The authors have estimated the pre-1600
position and thickness of Skaftárjökull to the
east of the Tungnaárfjöll. In this connection
Tungnaárfjöll are considered to continue in a
minimum elevation of 800—900 m to the Kerl-
ingar mountains. For the estimation of the in-
crease in thickness of glaciers, use lias been
made of Nye’s formula (Matheivs 1967), which
is adapted to a level l'loor and therefore fits
fairly well to the relevant conditions. l'he for-
mula is:
li
sin a =----,
K
wliere a = the slope of tlie ice surface, h = the
thickness of the ice and
T
h =-------= constant ~ 10.
Og
(t shear stress per unit area; e mean density
of ice; g the acceleration of gravity).
According to Nye’s formula a glacier with its
margin lying in 700—750 m elevation east of
Tungnaárfjöll due east of Jökulheimar would
have been in 1100 m elevation at Kerlingar.
The same result is obtained by using for com-
parison the slope of the flat outlet glaciers of
Vatnajökull as it is shown on maps. It can
hardly have been higher at Kerlingar without
overflowing Tungnaárfjöll.
From this premise the advance of the glaciers
from 1600 to 1890 can be supposed to have
been as follows:
JÖKULL 17. ÁR 293