Jökull - 01.12.1983, Blaðsíða 97

Fig. 9. Glacial cols at just over 400 m.a.s.l. on Há-
degisfjall (right) and Skessa (left) opposite the
town of Búdareyri in Reydarfiördur. (cf. Fig. 1). —
Photo G. Sigbj.
Mynd 9. Jökulaxlir í rúmlega 400 my.s. í HádegisJjalli
(t.h.) og Skessu (t.v) á móti Búðareyrarkauptúni við
Reyðarjj'órð. (Sbr. 1. mynd) —Ljósm. G. Sigbj.
spite of the fact that the coast there is directly
connected to the central highlands. This feature is
especially poorly developed west of Hornafjördur,
where it is hardly possible to talk about well deve-
loped glacially eroded valleys, except where the
present glaciers, Hoffellsjökull, Breidamerkurjökull
and Skeidarárjökull flow today. On the other hand
marine eroded features, wave cut cliffs and promon-
tories are very well developed and little eroded by
encroaching glaciers. The age of strata does not
seem to really matter, except for the very youngest.
This points without doubt, to the fact that in pre-
vious glaciations the glaciers here have not attained
great thickness or extent. The glaciation limits have
hardly been lower than 400-600 m a. s. 1. and glac-
iers have either melted on the slopes beneath the
moutains or in the case of piedmont glaciers at the
valley or fjord mouths, where calving could also
have played a part in the ablation. The eastern
fjords and southeastern Iceland have therefore not
been unlike the coast ofSpitsbergen and many other
arctic islands today. I consider that a greatly reduc-
ed precipitation from that of the present could be a
satisfactory explanation for this.
Observations on the geomorphology of south-
eastern Iceland and the eastern fjords suggest de-
cidedly that marine erosion as well as alpine glaciers
are mainly responsible for the landscape which we
find there today.
GLACIER THICKNESS AND ICE
FREE AREAS
Glacier thickness may be estimated from the geo-
morphological forms left by the glacier, which again
to some extent depend on the rate of flow of the ice.
The thickness of those glaciers which formed the
alpine landforms, covering a quarter ofthe country,
has been touched upon (Figs. 3 and 4). The land-
forms only give an estimation of the mean thickness
of the glaciers which formed them, but I do not
consider that there is any reason to conclude that
those glaciers have ever been much thicker. It is
possible that they have at some time reached a
greater ice thickness, when conditions of glacier
formation were at a maximum, but erosional feat-
ures left by such glaciers should be found since
erosive capability rapidly increases with ice thick-
ness.
Ifone considers the distributionofalpine areas in
Fig. 3, it can be seen that their position around the
country suggests that glacier thickness in the centr-
al highlands has never been, at least in late Quater-
nary time, so great that the outflowing glaciers
from there have buried these areas, but rather that
the glacial valleys and fjords have always managed
to transport the glacial ice away. It suggests further
therefore that glacier flow from the central high-
lands has continually adapted to the landscape and
this is true of the ice centres too as pointed out by
Trausti Einarsson (1977). In other words, there have
therefore continuously been many ice centres and
ice ridges in the main highland areas and glacial
flow has been from there in all directions or similar
to what can be seen in the present glaciers.
The height of móberg mountains has been used
as an indication of the minimum thickness of the
glaciers (Guðmundur Kjartansson 1943, Kristján Ste-
mundsson 1979). I think it is likely that this gives not
only an idea of minimum thickness but also the
approximate maximum thickness. A superficial ob-
servation of the structure of móberg mountains in
the southern higlands of the country south of the
Tungnaá river and Thjórsá river show that nearly
all those eruptions they formed in, reached above
the glaciers, since they are first and foremost built of
hyaloclastics, pillow lava formations being infre-
quent. On the other hand the proportion of pillows
and cubic jointed basalts increases greatly, to the
north of the T ungnaá river and elsewhere within the
central highlands. This suggests a considerably
JÖKULL 33. ÁR 95