Jökull - 01.12.1957, Blaðsíða 29

Fig. 2. Svínafellsjökull from
near the south side of the ice-
fall. The multiple ogives are
faint, but the extension of the
moraine along the black bancl
of the ogive can be seen clearly.
Suðurjaðar Svínafellsjökuls.
Svigður.
opinion that these are not separate features
but are part of the annual ogives. Ogives of
this type I shall refer to as “multiple ogives”.
The darkest ogives near the glacier snout
coincide with concentrations of fine dirt seen
on the surface of the glacier, and at the sides
of the glacier, all along its length, long streaks
of dirt and rubble, in cones and ridges, stretch
out from the lateral moraine along the ogives.
í his extension of the moraine into the ogive
is very noticeable when walking down the
moraine edge, and is shown in the photograph
(fig. 2). This suggests that there may be a sea-
sonal rhythm in the supply of debris to the
moraine.
To try to get more information on the
formation of ogives visits were paid to Morsár-
jökull. It is a double glacier formed by the
junction of two ice streams, — the west side
fed by a narrow icefall and the east side by a
disconnected avalanche fan. Ogives were seen
on both streams: those on the west side under
the icefall were “multiple” and in all respects
similar to those on Svínafellsjökull. Those on
the east side were of an entirely different na-
ture, and were studied in detail by Ives and
Iving. They are very dark, narrow bands, from
1 m to 3 m in width, and separated by about
120 m. The ogives proper are very dirty, often
Wlth considerable concentrations of gravel and
rounded stones, including a few quite large
boulders (fig. 3). On the white spaces boulders
are rare.
However, lower down the east side of Mors-
árjökull the character of the ogives changes.
The change is from “single” type to “multiple”
type ogives and seems to occur between the
22nd and 23rd ogives. Since the ogives are
annual this means that a change in the type
of ogive formation took place about 1935, and
it is known that the icefall which fed the east
side of Morsárjökull became completely dis-
connected between 1937 and 1938, but for
several years previously was fed predominantly
by avalanche ice. Since the change over from
multiple to single ogives coincides with the
increased dirtiness of the whole surface, this
suggests that the multiple ogives were the pro-
duct of the icefall and the single ogives are
the product of the avalanche fan.
Many comparisons were found between the
profile of the single ogives and the profiles of
thrustplanes seen at the snouts of several glaci-
ers in the vicinity. In each case the thrust
planes displayed the same characteristic raised
bluff of clean white ice, below which the dirt
began with a sharply defined upper edge and
faded away in a down glacier direction.
In many cases it was possible to follow the
dirt and boulders down along the discrete sur-
faces of both the thrust planes and the ogives
in the sides of crevasses. It seems that the
27