Jökull - 01.12.1957, Blaðsíða 31

fall of Falljökull. The waves on Svínafells-
jökull are at first sight most confused and ir-
regular in amplitude and wavelength. From
the trim line at the head of the glacier they
can be seen in profile against the opposite
valley wall as several long low undulations of
a wavelength of the order of 30 m, on which
smaller crests are superimposed. The first three
ogives lie on the crests of the long, low undula-
tions. The smaller ridges vary greatly in size,
and many showed evidence of a dirt band on
their forward slope (fig. 6). The dirt band was
of “single” ogive type, and the average number
of these small crests and their associated bands
on the top of each undulation was similar to
the number of bands in the “multiple” ogives
further down the glacier. This suggests that the
multiple ogive is formed of half a dozen or
more single ogives.
Any theory put forward to explain the origin
of the ogives on the glaciers we studied would
have to account for a number of observed
phenomena:
1) The ogive systems usually follow an annu-
al or seasonal rhythm. This is visible in
the single ogives of Morsárjökull or the
multiple systems, as on Svínafellsjökull.
2) The theory must explain why some glaciers
liave single ogives, others multiple ogives
of several single bands close together and
in order as on Svínafellsjökull or irregular-
ly spaced ogives as on Falljökull. ít must
explain the change that occurred on Mors-
árjökull.
3) ít must explain the heavy concentrations
of dirt and stones found on some single
ogives, and the very faint nature of others.
4) ft must explain the extension of the mo-
raines out along the black bands of the
ogives, which was noticed on Svínafells-
jökull, and to a lesser extent on Auster-
dalsbreen in Norway, which í visited in
1956.
5) ft must explain why there was a change
in the structure of the ice on parts of
Svínafellsjökull, wliere the white spaces
between the ogives were predominantly
white, bubbly compact ice, and the black
bands coincided with more transparent ice.
6) It must account for the fact that on some
glaciers ogives appear in conjunction with
Fig. 5. Large single ogive on Falljökull, south-
ern stream.
Stór, einföld. svigða á Falljökli.
waves of ice below the icefall. On Svína-
fellsjökull there appeared to be two sets
of waves, the smaller series of which were
connected with the individual bands of
the multiple ogives. On Morsárjökull’s
west side there were poorly developed
waves and multiple ogives, on Falljökull’s
north stream there were waves and single
ogives. If there is a connection between
waves and ogives, the theory must explain
how some glaciers do not develop ogives
until some distance below the wave system,
on others they are superimposed.
Two possible processes of ogive formation
were suggested by our observation. The first
process that was considered was that of re-
generated stratification because of avalanche
activity. Ives and King have shown that on
Morsárjökull’s eastern stream the ogives can
be formed by new stratification on the debris
cone itself: the summer’s ablation dirt being
covered by new winter ice. It then seerns likely
that rotational movement of the fan occurs
against the back wall, as new weight of ice is
added to the upper parts, and this carries the
dirt layer underneath the glacier so that it out-
crops at the surface as a thin dirt line — the
single ogive.
A similar process might occur on an icefall
in winter, when there is often avalanching
right down the face of the glacier, but it is
difficult to see how any dirt trapped under
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