Jökull - 01.12.1990, Page 54

LANDFORMS REFLECTING THE
DIRECTION OF GLACIAL MOVEMENT
Glacial striae
Two sets of striae directions are often found on the
same rock and three different directions at the same lo-
cality are not uncommon. Usually the youngest striae
are parallel to the streamline forms of the glacially
sculptured bedrock, while an older direction is found
on the lee sides. Not as common is the situation where
the older direction is dominant and the youngest striae
only form vague scratches on the stoss sides. In such
cases the glacier action has not lasted long enough to
reshape the rock according to the new direction of ice
flow. Correlation between localities is difficult, and
one must bear in mind, that observations indicating
the same direction of movement, are not necessarily
of the same age. The oldest striae direction commonly
found is likely to indicate the movement pattem of the
glaciers at their Weichselian maximum extent.
Fluted moraine
In some areas the surface has lineations approx-
imately parallel to the former ice movement. These
are called fluted moraine. The flutes are subglacially
formed and always show the last direction of the ice
movement. Of more than 1300 mapped flutes 80% are
between 300 and 800 m long. In Central Iceland flutes
are often very distinct because of lack of vegetation.
Drumlins are not shown on the maps, because
quick consolidation of the glass- rich till makes it pos-
sible for drumlins to last for more than one glaciation.
Consequently their directional pattern can be com-
plex. Furthermore they can easily be confused with
volcanic ridges where the ice flow direction is similar
to the direction of volcanic fissures.
End moraines
Many end moraines have been found in the re-
search area. Most of them are low and narrow ridges
of boulders and stones and there is no indication of a
major halt in the retreat.
Sandur plains formed in front of the end moraines
are usually small. The meltwater runoff in the retreat
period was no less than at present so the reason for the
small sandar must be a very short standstill of the ice
front. Huge deposits of this kind do occur but only
where the general ground surface slopes towards the
retreating ice front.
As most of the broad and flat inland outlets of
Vatnajökull are known to be affected by periodic
surges (Thorarinsson, 1964), one can expect the inland
glaciers of the retreating Weichselian ice to have acted
in a similar way. Brúarjökull, a flat and lobe-shaped
outlet on the northern side of Vatnajökull, surged 1890
and 1964 (Thorarinsson, 1969). End moraines and
other features resulting from these surges are very
much alike moraines described in this paper.
It is therefore our opinion that the end moraines
we are dealing with show the maximum extent of in-
dividual surges of the glacier. For expanding glaciers
one can expect such moraines to form close together
or to form belts of parallel moraines. Where a series
of end moraines with considerable distance between
them have been formed, as in the research area, it in-
dicates that the climate was warming up and that each
surge did not reach as far as the previous one. In other
words the surges were superimposed on a general re-
treating trend.
It can be risky to correlate end moraines from two
different ice lobes without any stratigraphical evidence
(Punkari, 1980). Data on surges of various ice lobes
of the present glaciers in Iceland (Thorarinsson, 1964;
1969) show that concurrent surges of two adjacent ice
lobes would be a mere coincidence.
Eskers
Eskers are long curveous ridges of glaciofluvial
material formed where glacial meltwater looses its
transport capacity near a glacier snout. They are
formed in a subglacial or submarginal environment.
The direction of eskers is likely to reflect the sub-
glacial runoff pattem which depends primarily on the
direction of the slope of the upper surface of the ice
(Björnsson, 1988), which in turn determines the direc-
tion of ice movement. Thus, eskers are ice-directed
features and indirect indicators of the last direction of
regional ice movement.
52 JÖKULL, No. 40, 1990