Jökull - 01.12.1985, Page 33

Geomorphology of Fljótsdalshérað,
Eastern Iceland, and its Implications
IAN ASHWELL
Dept. of Geography, University of Salford,
Salford, Lancs., M5 4WT, England
abstract
Field examination of the Fljótsdalur area, with an
associated part of Jökuldalur, aided by air photographs
and the mechanical analysis of samples of deposits,
indicates that the features found, mainly rock-cut chann-
els and shelves and water-deposited eskers, (Malarásar),
can be explained by a comparatively simple model.
Within a thick overall ice cover, an original ice move-
ment down valley towards the NE was diverted to the N
and even a little W by large amounts of ice flowing from a
centre above the E side of the area and by local folding
and a closely-spaced pattern of dykes. Many of the
crosional and depositional features ofthe whole area can
be interpreted in terms of subglacial or englacial water
flow from geothermal centres S of the area, associated
with the building ofthe Móberg formation during Glacial
times. The associated jökulhlaups would have tended to
remove any stadial moraines from the valleys during
deglaciation, accounting for the absence of such features
°f appropriate size from these valleys, and suggesting
that glaciofluvial, rather than glacial, deposition is the
key to the subsoil in the lowlands, and that evidence for
deglaciation timescales is more likely to be found higher
up.
INTRODUCTION
Structure and process are both vital components in
the geomorphological development of landscapes, and
an attempt is made to integrate them in this paper. No
overall geological map in the Iceland 1:250000 series
was available for the area, (Fig. 1), although useful
published sources exist. The other important aim was to
try to establish the processes involved in landscape
sculpture without any preconceptions.
STRUCTURE
Fljótsdalur lies near the boundary between Tertiary
and more recent rock systems, but the location and
nature of the actual boundary is a matter of some
discussion. However Walker (1982) has summarised the
situation in terms of erosion surfaces on bedrock
varying in age from about 12 m.y. in the northeast to
less than 5 m.y. in the southwest of the area, where
building of volcanic structures has continued into the
present, with migration in that direction of the locus of
volcanic activity and the imposition of the Glacial ice
cover.
The rocks of Fljótsdalur are mainly Tertiary volca-
nics, with greater or smaller intercalations, such as the
so-called „red partings“ dipping gradually, on average,
towards the WSW. The more recent rocks towards the
southwest tend to include increasing amounts of glacial
deposit, while outside the immediate area the large
structures of the Móberg formation, such as the Snaefell
massif, rising to 1833 m and of Glacial age, lie to the
south and west.
The normal pattern of the Tertiary basalt plateau is,
however, interrupted in the Fljótsdalur area by the
existence of the Thingmúli central volcano core imme-
diately to the E and the associated Lagarfljót flexure
zome stretching to the NW and a corresponding high
intensity of dyke formation, (Walker 1974), as shown in
Fig. 2, enlarged from a rather small original. The flex-
ure zone shows maximum values of dip, about 25° under
Lagarfljót itself, decreasing gradually both to SSW and
NE. The general direction and intensity of dykes is
shown in Fig. 3, from the same source, with maximum
intensity closest to the volcano core.
Walker (1982) shows the area to the W of Fljótsdalur
as an erosion surface at about 650 m but the more
mountainous area to the E, including the relics of the
Thingmúli volcano, as part of a much higher surface at
1100 m. The initial construction of both these upland
surfaces, however, pre-dated the onset of the Glacial
period and thus the higher plateau surface shows many
signs of glacial activity, especially the central area of the
volcano core, where even today permanent snow and
ice remain on the peaks and plateaus rising to 1200 m.
JÖKULL 35. ÁR 31