Jökull - 01.12.1975, Page 41

Arnarvatnsheidi and its Regional Geomorphology
IAN ASHWELL,
UNIVERSITY OF SALFORD, ENGLAND
ABSTRACT
The landscape of Arnarvatnsheidi, here taken
to mean that area of larger lalies running south-
westwards from Arnarvatn stóra to the Nord-
lingafljót, consists of rather level plateaus bro-
ken by rock escarpments facing generally north-
west and in some cases by channels through
liills. Deposits in the area are thin in the north
and east, thicker in the south and west.
It is suggested that tliis landscape results
from an ice-sheet moving south-westwards over
the area, witli sub-glacial drainage forming the
escarpments in the overlying basalts, underlain
by less well consolidated rocks. The ice-sheet
retreated totuards t.he north-west, forming ice-
dammed lakes between the ice front and the
escarpments luith marginal channels over the
spurs, some evidence for a higher lake level on
tlie south shore of Arnarvatn stóra being dis-
cussed. Finer deposits were carried south-west
and deposited on tlie south and west of the
lieath, but some of the material previously
classified as deposits in this area may be hyalo-
clastite or sedimentary rocks previously covered
by basalts and exposed by the stripping off of
the basalt cover.
INTRODUCTION
„There are three innumerable things in Ice-
land, the islands in Breidafjördur, the hills in
Vatnsdalur and the lakes on Arnarvatnsheidi."
In the folk saying quoted above Arnarvatns-
heidi must cover the whole broad area of up-
land heath lying between the settled districts
of the Borgarfjördur and Húnavatn counties in
Western Iceland, since it is largely covered
with lakes. The naming of the different parts,
however, is most uncertain, and has been dis-
cussed at some length elsewhere (Thorsteinsson,
1962), in a publication which is very valuable
to the researcher in that area. For the purposes
of this paper I have defined the Arnarvatns-
heidi as that area shown in Fig. 1, including
the two Arnarvatn lakes. The area is bounded
on its south side by the Hallmundarhraun
lava, on the east side by the Langijörfi desert
plateau, on the north side by the gently un-
dulating plateau covered with innumerable
small lakes referred to here as Tvídægra, al-
though this name properly refers to the routes
across that area, and on the west by the Fljóts-
tunguháls area where the rivers of western Ice-
land, for example Lambá and Kjarrá, form
youthful valleys in the Plateau basalts, valleys
whicli are not found on the open heatli.
STRUCTURE AND RELIEF
The underlying lithology of the entire area
is most uncertain, due to the thick covering of
drift deposits. From outcrops shown on the geo-
logical map, (Kjartansson, 1965), it appears that
along a southeast to northwest section the
youngest rocks, Móberg and Young Grey basalts,
to the north of Langjökull give way further
west to Old Grey basalts, occurring as out-
crops on Arnarvatnsheidi, and then to Plateau
basalts in the wide space covered by drift be-
tween Arnarvatnsheidi and Holtavörduheidi.
On an east-west section, however, the junction
between Old Grey and Plateau basalts must
occur near the western edge of Fig. 1.
Einarsson (1962) describes sections some dis-
tance to the north of Fig. 1 on Hrútafjardará
in which his Young Plateau basalts overlie fair-
ly thick layers of sedimentary rocks which rest,
in turn, on the older Plateau basalts.
Atliins (1971), whose investigations were
carried out simultaneously with the author’s,
shows that hyaloclastite rocks of the Móberg
formation extend further west than indicated
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