Jökull - 01.12.1952, Blaðsíða 12

SEDIMENTARY SEQUENCE IN THE HAGAVATN BASIN
By R. GREEN
Introductory:
As the following article refers to some papers
011 tephrochronology which cannot be suppo-
sed to be known by many readers of this jour-
nal some introductory tuords may be desirable
for a better understanding of the article.
Fig. 1 shows the soil profiles at Skallakot and
Hagavatn referred to by Green. The Skallakot
profile was measured in Þjórsárdalur, ab. 15 km
WNW of Hekla. The situation of the Haga-
vatn profile is shown by a cross on the map
Fig. 2. The layers III-V in these profiles are
very easily recognisible in the districts NW of
Hekla and so is the characteristic layer series
VHa, b, c and d. Layer Vlla is greyish black.
layer Vllb light, and the layers VIIc and Vlld
black.
The layers X and Y are light, rhyolitic layers
and they were both erupted from Hekla, layer
X 2500-3000 years ago, layer Y probably 4000-
5000 years ago. The source of the rhyolitic
layer VHb is a crater just NE of lake Frosta-
staðavatn 30 km E of Hekla. Pollenanalytic
studies of soil profiles near old farmruins in
Þjórsárdalur have proved that layer Vlla was
deposited shortly before the colonization, pro-
bably ab. 850 A. D.
Greens paper does not only confirm the re-
sults obtained by the study of the changing
outlet of lake Hagavatn; it also adds interesting
facts such as the statement that the 1693 tephra
layer is found in the SW-basin of Hagavatn,
underlain by ten or twelve varves which means
that already 1680 or even somewhat earlier had
Hagafellsjökull eystri advanced so far as to
block col 4. Thus the glacier has now an exten-
sion which is considerably less than its extension
ab. 1680.
Sigurdur Thorarinsson.
The Hagavatn — an ice-dammed lake along
the southern margin o£ the Langjökull — is
of geologically recent date. According to Thor-
arinsson (1949), Hagavatn was formed by the
closing of a post-glacial valley running north
east to south west, by the lava cupola Lamba-
hraun. This is of recent date, but precoloni-
zation in age.
This particular study was mainly concerned
with an examination of the lake sediments in
order to determine the age of the two lake
basins, the south west and the north east, by
counting varves or by using Thorarinsson’s
tephrochronological technique. The latter met-
hod was found to be of most use.
Thorarinsson (1939, 1949) has made detailed
studies of this district from the glacial-morp-
hological aspect. The results achieved here only
serve to confirm Thorarinsson’s conclusions
concerning the fluctuation of climate in Iceland.
The southwestern margin of the lake, as stated
above, is formed by Lambahraun, which blocks
part of the valley excavated in tuffs with in-
terbedded lava flows. The northern boundary is
the Hagafellsjökull eystri, on the extent of
which depends the size of the lake.
Quoting from Thorarinsson (1949): „The
size and extension of the lake, is determined
by the extension and thickness of the Haga-
fellsjökull eystri and through which of the cols
(marked on the map by numbered arrows, cf.
Fig. 2) the lake has its outlets. The sill of the
north-easternmost col (No. 4) is the lowest
(433 m), and thus the lake normally has its
outlet there. If that col is dammed by the gla-
cier, the water can escape through col No. 3,
the Leynifoss col, the sill of which is 9.5 m
higher than sill No. 4. If that col is also dammed
by the glacier, the lake would find an outlet
over col No. 2 in Fagradalsfjall, the sill of
which is 15 m above sill No. 3.“
In Thorarinsson (1939) it is shown from
historical records that a flood (jökulhlaup)
probably occurred in 1708, as the ice advanced
to block col No. 4 and reached a maximum
post-glacial extension about 1850. From a map
made by Gunnlaugsson in 1834—’'35, it is shown
that drainage was tlien through col No. 2, i.e. the
ice was at its maximum extent. A hlaup occurred
through Leynifoss in 1884 as the ice retreated.
An advance until ab. 1892 followed this and
then retreats caused floods in 1902 and 1929
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