Jökull - 01.12.1957, Blaðsíða 30

single ogives are inclined dirt layers, like the
thrust planes, though not necessarily caused by
thrusting, and that the distinctive profile of
both is the product of ablation on the steeply
dipping dirt band. Ives and King consider that
the ogives could be formed by regenerated
stratification in the avalanche material.
A third glacier in Öræfi which possesses un-
usual ogives is Falljökull. Like Morsárjökull it
is a double glacier fed by two icefalls, and
each strearn has a different type of ogive (see
fig. 4). The northern side is fed both by a
narrow icefall and an avalanche fan; below
the latter are very thin and dirty multiple
bands with almost no white space between
them, outcropping at the surface some distance
from the foot of the avalanche fan. Below the
narrow icefall are four gentle waves succeeded
by single ogives, which are narrow and indistinct,
irregularly spaced and often merging or broken
up. They are generally far too close to each
other to be annual in origin. In width and
profile these resemble the single ogives on
Morsárjökull. The southern stream of the
double glacier is fed solely by a very steep,
wide icefall, at one side of which apparent
thrust planes were seen, bringing up dirt to
the lateral moraine; however, these were fol-
lowed round into the centre of the glacier
where they had all the appearance of true
ogives. In the middle of the glacier very large
ogives were found (fig 5), also of the “single”
Fig. 3. One of the single ogives below the
avalanche fan of Morsárjökull. Medial moraine
and multiple ogives in the background.
Einföld svigða á Morsárjökli
28
Fig. 4. A sketch of Falljökull. The lines repre-
senting the ogives show only their type and
direction.They are not actual surveyed positions.
Riss af Falljökli.
type, but very wide (8 m) and dirty, with a
most pronounced break of slope and raised
white bluff up glacier of the ogives. At the foot
of this icefall there are a number of longi-
tudinal crevasses, in which one could see cur-
ved dirt lines coming to the surface along the
lines suggested by the theoretical lines of
maximum shear stress (7).
At this point the information we hacl glean-
ecl was as follows: there were two main types
of ogives — the “single” type, which was nar-
row (often only one or two metres) and very
dirty, and which was in profile similar to the
thrust planes identified at glacier snouts, and
there were the “multiple” ogives that were
wide (30—130 m on Svínafellsjökull) ancl faint,
and were made up of five or more narrow
bands that were almost invisible from the sur-
face of the glacier. An explanation (regenera-
ted stratification) was available for the ogives
below the avalanche fan of Morsárjökull, but
would not satisfactorily explain the presence
of either single or multiple ogives below ice-
falls, furthermore, the avalanche fan on Fall-
jökull was producing multiple ogives.
Several writers (8, 9) have remarked on the
coexistence of ogives and “pressure waves” be-
neath an icefall, and waves were found beneath
the icefalls of Svínafellsjökull, the western
stream of Morsárjökull and the northern ice-