Jökull - 01.12.1988, Blaðsíða 30

described by Thompson and Jones (1986), but here
the wider development of the outwash fan and the
changing positions and pattems of its major chan-
nels are considered. From the lichenometric dating
study (Thompson and Jones ibid.), it is known that
the isolated remnants of the highest terrace (units B
and C in Fig. 5), — representing the maximum
aggradation of the sandur at the time of the 1870 ice
advance — were finally abandoned during the phase
of incision which accompanied the strong glacier
recession of the 1890s. By examination of the relict
channel pattems on these abandoned surfaces, it is
apparent that they had been formed by meltwater
issuing from numerous sources along the ice-front,
building up an apron of small coalescing fans in
front of the glacier, partially burying or cutting
through the 1870 moraines.
Subsequently, with the continued retreat of the
ice, meltwater feeding the Svínafellsá fan became
concentrated into two major streams, issuing from
the extreme southem tip of the glacier tongue and
cutting through the late nineteenth century
moraines. This pattem is indicated on the 1904 map
(Fig. 2) and seems to have persisted until the onset
of accelerated glacier recession in the mid 1930s, as
can be seen in the distribution of outwash for this
period (units D and E in Fig. 5).
As the glacier retreated from the high crest of the
1930 moraines, the associated incision of the melt-
water streams resulted in the abandonment of wide
areas of former outwash deposition, (units D and E),
transforming them into terraces. Eventually, at some
time between 1934 and 1945, the continued retreat
of the ice-front and reduction in source elevation of
the meltwater streams resulted in a complete aban-
donment of the southemmost stream, concentrating
the entire flow through a single gap in the moraines.
Since 1960, there has been little net recession of
the glacier, and consequently little, if any incision
by the river (Thompson and Jones, 1986). As a
result, lateral migration of the channel has predom-
inated, especially downstream of the modem road
bridge where the channel begins to free itself from
the restrictions of the confining terraces and adopts a
wider and more complex braided pattem.
Neskvísl — The Neskvísl is the second largest of the
Svínafellsjökull meltwater streams, issuing from the
northem side of the glacier. Its present course,
confined between the 1934 and 1939 moraine ridges
of Skaftafellsjökull, came into existence only after
the two glaciers separated in 1935. Prior to this,
meltwater from the same source escaped in a south-
erly direction, to join the Skráma and Stóralda
streams of Svínafellsjökull, as shown on the map of
1904 (Fig. 2).
In the aerial photographs of 1945, this southerly
course was still occupied by meltwater issuing from
a temporary lake, held up between the glacier snout
and a complex dead-ice area beyond (Fig. 2). At the
same time, the low ground between the Skaftafells-
jökull moraines, which was to become the new
course of the Neskvísl, was occupied by a series of
abandoned channels, initiated at an earlier stage as
an ice-marginal drainage system of Skaftafells-
jökull.
By 1954, the continued decay of the dead-ice area
resulted in a new outlet being formed on the
northwestem side of the lake, allowing meltwater to
escape into one of these abandoned channels (Fig.
2).
Subsequently, the stream has cut down slightly
below the 1934-1945 outwash surface (unit E in Fig.
5), and has continued to evolve between these
confining terraces as a ’wandering’ gravel-bed
stream — a transitional form between meandering
and braided pattems (Ferguson and Werritty, 1983,
Thompson, 1987).
CONCLUSIONS
Detailed analysis of the evolution of proglacial
features at Svínafellsjökull and Skaftafellsjökull has
highlighted important contrasts between the two
areas, which are ultimately related to the behaviour
of the glaciers themselves, and their differential
response to recent climatic change.
At Skaftafellsj ökull, the rapid and almost continu-
ous retreat of the ice-front has revealed an exten-
sive, predominantly lodgement till plain, locally
modified by a combination of pushing and squeez-
ing mechanisms to produce a series of well-defined
28 JÖKULL, No. 38, 1988