Jökull - 01.12.1953, Blaðsíða 34

Some Notes on the Formation
of Medial Moraines
By r. a. young
The processes responsible for the formation
of medial moraines on glaciers have long been
the subject of discussion. Briefly, the argument
turns on two theories of formation. The first
is that medial moraines are a purely surface feat-
ure formed by the deposition of detritus from a
nunatak on to the surface of the ice, whence it
is carried to the snout. In much the same way
the junction of two glaciers would form one
medial moraine from two lateral moraines
(diagram a).
The second theory postulates a continuous
moraine reaching down through the ice to the
valley floor and in effect forming a boundary
between two independent ice masses. In section
it would appear as in diagram b.
The prominent medial moraines of Breiða-
merkurjökull offer an excellent opportunity for
the examination of these two concepts.
The two most distinct moraines rise from
the nunataks of Máfabyggðir and Esjufjöll. In
the latter case both the source and the snout of
the moraine were carefully mapped. In ad-
clition the nunatak of Esjufjöll has a complica-
ted system of moraines which was also exa-
mined.
The Esjufjöll moraine has three main sources,
Skálabjörg, Austurbjörg and Esjubjörg. These
three points in association with lateral mora-
ines give rise to three medial moraines which
converge at a point some 1.5 km south of
Skálabjörg. The rock types are distinet so
that the material can be readily identified.
At the source the moraines rise to a height of
up to 60 metres being here a thick covering of
debris over ice. They are up to 100 metres wide.
As the level of the ice rises away from the
nunatak so the thickness of debris lessens and
the moraine becomes level with the ice surface.
Towards the snout the moraine again grows in
height, width and thickness in dirt cover. This
co-incidence of thickness of dirt cover and
maximum wastage of the ice leads one at once
to suppose that the supply of debris is from
Fig. 1. Cf. text.
particles contained in the ice itself and brought
to the surface by melting out. With this in mind
examinations were made of the moraine from
crevasses walls. At no point on the Esjufjöll
moraine, however, were there any signs that
there was a continuation of the moraine down
through the ice to the valley floor. The floor
itself was reached from crevasses in the moraine
near Skálabjörg but the ice under the surface
coating of debris was perfectly clean and samples
taken showed no marked grit content. It is not
suggested that the examination was exhaustive
but the work done did not indicate a pronoun-
ced „morainic core“.
Further, if the height of the moraine above
the glacier is a function of ablation then it is
difficult to see why it peters out to an insignifi-
cant feature on its journey to the snout. For a
rise of level only a 100 metres (or less) above
the source, it is inconceivable that the ablation
can diffice as much as the ratio of the moraine
heights, 0 m. to 60 m., suggests.
If then these moraines are purely surface
features they would not form distinct lines but
mingle from their point of convergence. This
does not happen. The different lines are clearly
discernible even at the snout,so that it ispossible
to trace the Skálabjörg, Austurbjörg, and Esju-
björg material for some 10 to 20 km. Moreover,
the character of the ice on each side of the
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