Jökull - 01.12.1988, Blaðsíða 25

both pushing and squeezing mechanisms.
A third process, which can be seen in operation
along much of the present Svínafellsjökull ice-front,
is that of ridge formation by "dumping" mechanisms
(Goldthwait, 1951; Price, 1977; Boulton and Eyles,
1979). Englacial and supraglacial debris, released by
ablation at the glacier snout may accumulate at the
base of the ice front, forming a ridge once the ice
has retreated. Whilst in contact with the ice, the
form of such ridges may be extensively modified by
saturated flow slides, triggered by the abundance of
meltwater and the low permeability of the sedi-
ments.
The "dumping" process is clearly most effective
when the debris content of the ice is high, and where
rates of forward movement at the snout are balanced
by the horizontal component of ablation, so that the
ice-front remains in more or less the same position
for several years. Both of these conditions are met at
Svínafellsjökull, where the sediment content of ice
in the terminal zone is enriched by the elevation of
debris-laden basal layers along inclined shear
planes, as a result of severe compression of the gla-
cier snout against the barrier of steep end moraines.
The moraines themselves have developed by posi-
tive feedback from the superimposition and bulldoz-
ing of successive ridges, but may originally have
formed from a nucleus of outwash sediment, as in
the model proposed by Boulton (1986) for
Breiðamerkurjökull. Sections cut by meltwater
channels through the highest moraines at Svínafells-
jökull reveal a substantial proportion of well-
rounded cobbles and boulders, comparable with
those of the modem outwash fans.
Supraglacial debris can also be deposited more
extensively by the widespread downwasting of stag-
nant ice within the terminal zone, to produce a dis-
tinctive facies of "hummocky moraine". Such depo-
sits are well developed in front of many of the steep
Öræfajökull outlet glaciers (Eyles, 1978, 1979,
1983a), and are seen to a limited extent at Svína-
fellsjökull, especially at the northem and southem
margins of the glacier snout (Fig. 2), where elevated
basal debris combines with laterally derived
material to produce a continuous veneer of
supraglacial sediment. Similar conditions exist on
Skaftafellsjökull, where supraglacial debris is con-
veyed along major lateral and medial moraines (Fig.
2). In this case however, the deposits are rarely
preserved within the proglacial zone, as the debris
becomes widely dispersed at the snout by splaying
crevasses, and is largely removed by powerful melt-
water streams emerging from beneath the glacier
(Eyles, 1978).
It can be seen therefore, that Skaftafellsjökull and
Svínafellsjökull are characterized by two quite
different assemblages of glacial landforms and depo-
sits: The moraines of Skaftafellsjökull are dom-
inated by subglacial lodgement till, moulded by
minor oscillations of the retreating ice-front into a
series of broad concentric ridges. At Svínafellsjökull
by contrast, the altemate episodes of slow recession
and readvance have produced a more complex series
of higher push-moraines, composed largely of eng-
lacial and supraglacial debris, together with
reworked outwash material.
HISTORICAL DEVELOPMENT
1904 -1930
The earliest detailed topographic survey of the
glaciers and their associated moraines was under-
taken in 1904 by the Danish General Staff, at a scale
of 1:50,000, and this provides a valuable point of
reference which can be tied-in with more recent sur-
veys and with the earlier Neoglacial moraines (Fig.
2).
On this map a clear distinction is made between
the older, stable moraines such as Stóralda, and a
zone of younger moraines at the glacier margin. The
latter are shown to be continuous with both medial
and lateral features on the surface of the ice, and
may therefore have been ice-cored at the time of the
survey. If this suggestion is correct then the distal
rather than proximal margin of these moraines as
depicted on the 1904 map should mark the true posi-
tion of the ice-front at this time.
At Skaftafellsjökull, the position of this distal
margin is found to correspond with that of the outer-
most moraines (Fig. 2). Thompson andJones (1986)
JÖKULL, No. 38, 1988 23