Jökull - 01.12.1953, Síða 32

Four stagesinthe formation ot an end moraine
Accumulation of dirt on snout
Recession due to melting
** Isolated ice blocks leftunderdcbris. Snout recedes
Fig. 6. The formation of an end moraine at a
receding glacier snout.
Jökulgarður myndast við jökultotu, sem er að
bráðna.
This is not in accordance with Carruthers (1943).
The lack of much englacial material is contrary
to the impression of Sharp (1948).
MORAINES.
Medial Moraines examined by the expedition
are described in another section of this expedi-
tion report. Moraines were traced to the source
and development noted as far as possible. Ice
was sampled for dirt content in, near and below
different moraines. The moraines were princi-
pally a surface feature which is contrary to pres-
ent concepts (de Martonne and Flint), and had
little form until the surface cover of grit was
sufficiently thick to protect the ice under it.
Mr. D. E. St. A. Harney made the only real
contribution at this stage in that he found a
vertical dirt band beneath a medial moraine
and suggested that the few cms. wide dirt band
was the boundary between two glaciers and the
dirt was collected when the glaciers were each in
contact with a valley side or nunataks. The
melting down of the surface permits the dirt to
spread out and become a moraine.
End Moraines were of all sizes some of which
had wet, steep sides on the hottest days, the
water coming from the dead ice within the
moraine. Much morainic deposit had been re-
sorted by the glacial streams and rivers which
braided and changed their beds with amazing
frequency. Along the glacier snout the angle
of steepness of the ice varied considerably. It
was only at the thinner, less steeply sloping
parts that there were definite linear deposits
close to the ice edge. Long wedge shaped slabs
of ice were deposited a few metres from the
glacier snout along a stretch of its northern
margin. The ice observed for bottom-melting
appeared to have been recently broken off the
glacier snout. The underside of the glacier was
vaulted as far as one could creep in to see.
Each point between the arcs seemed to be
supported by the rocks under the glacier but
fluvial action sometimes removed these supports
and large pieces of ice slumped to break off
from the main body of the glacier snout.
A suggested sequence of events in the förma-
tion of a terminal moraine is outlined in Fig. 6:
(I) Rock debris of all sizes, particularly the
coarse grit, accumulates on the lower slopes
of the snout. The main drainage being in
channels and crevasses leaves most of the
debris between these channels relatively
undisturbed.
(II) The meltwater sorts the debris at the ice
edge and under the ice and accelerates
undermelting of the snout edge. The water
under the snout is added to from tension
cracks, moulins, etc. The debris on the
ice edge is relatively undisturbed between
the gaps where the meltwater emerges.
(III) The decrease in surface melt leaves under-
melt more dominant which causes cont-
inued thinning. Removal of the supports
under the ice edge by water action or by
forward movement of the glacier (particu-
larly during winter) causes parts of the ice
edge to fracture due to shear of the canti-
lever.
(IV) The ice blocks which are broken off sink
into the waterloggod debris. Some debris
may be added to the top from off the snout
until the snout recedes due to melting.
CONCLUSION.
The results indicate that much more quantita-
tive data is required on ablation during periods
of good weather and on quantity and forrn of
rock debris present in moraines. The apparatus
used has been modified in the light of experi-
ence gained yet still keeping expense within
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