Jökull - 01.12.1957, Side 32

Fig. 6. Dirt band outcropping on the forward
slope of a ridge on Svínafellsjökull, below the
icefall. On the crest of the ridge can be seen
one of the ice arches.
Aurborfíi á Svinafellsjökli.
new winter snow and ice could be turned
through an angle to outcrop at the surface
along only a comparatively thin line. A further
suggestion has been made that the inclined
dirt layers may be due to dirt which has lodged
in crevasses on the icefall in the summer. This
might be expected to produce a great number
of dirt bands of no great length of density,
but on Svínafellsjökull we found that the dirt
bands were spaced well apart with no sign of
dirt lines in between, and we found them
quite high up the foot of the icefall, where, if
they had originated in the bottoms of crevasses,
the ablation had not been great enough to
expose the dirt. Yet it must be admitted that
dirt lodges in crevasses on the icefall would
produce many of the necessary features of
multiple ogives — the faint but wide dirt
distribution, the seasonal rhythm due to chang-
es of ablation on the fast moving iceíall and
the striped nature of the ogive.
The second solution which suggested itself
was that ogives are in fact thrust planes, to
which they are outwardly so similar. Chamber-
lin (10), Lewis (11) and Miller (12) have all
noted the three-dimensional nafure of ogives,
and related them to possible zones of thrusting.
In their study of Morsárjökull’s single ogives
Ives and King noted that “the presence of the
discrete surface at the top of the dirty band
and in some cases also at its base may suggest
that overthrusting or rotational slipping is an
important process in the formation of the
ogives”. At periods of maximum ice supply
down the icefall the inertia of the glacier at
the bottom may be such that pressure builds
up in the icefall, and is released by shearing
and overthrusting of ice, along zones of max-
imum shear stress. A number of difficulties
arise in connection with this theorv, although
it can be shown to satisfy most of the require-
ments.
The first difficulty is that of differentiating
between a thrust plane in which dirt has been
brought up from the glacier bed, and a thrust
plane that has developed along a dirty layer
already present in the ice. This difficulty is
equally evident when studying apparent thrust
planes at glacier snouts. Granted that stresses
in the ice may occasionally rise to a point
where shearing could take place (which is
doubtful), such shearing woulcl occur at the
weakest point. It might be along the theoreti-
cal line of maximum shear stress, or it might
take advantage of a tectonic band or dirt layer
already present and constituting a line of weak-
ness.The latter may be the case on Morsárjökull’s
avalanche fan, where the regenerated stratifica-
tion layers provide excellent surface of weak-
ness. Thus in one case a dirt layer may be the
cause of shearing occurring at that point, in
another it may be the result.
According to Nye’s classification (7) the cre-
vasse pattern below the icefalls of the three
glaciers studied is longitudinal, suggesting a
zone of compressive flow. This may be as-
sociated with curved surface of maximum shear
stress within the ice along which thrusting
could develop, carrying up dirt from the bed
of the glacier. The next question is to decide
what order of magnitude the thrusting must
be to originate ogives that outcrop at the sur-
face. If one assumes that the main activity
occurs below the foot of the icefall a very
large vertical movement is necessary. However,
the highest evidence of a single ogive found
on Svínafellsjökull was high up the lower part
of the icefall, and the single ogive bands were
already well developed at its foot. This sug-
gests that thrusting might occur in the icefall
itself, at the change of slope below the main
avalanche zone.
The diagram (fig. 7) shows the sort of pro-
30