Jökull - 01.12.1988, Page 29

outwash surfaces. The various units can be tenta-
tively placed in chronological order by noting the
geomorphic relationships of one surface to another,
and to the sequence of independently dated
moraines: for example, where one surface is
dissected or buried by another, it is clearly the older
of the two. Similarly, a channel cutting through, or
formed on the proximal side of a particular moraine
ridge, is clearly a later feature than the ridge itself;
whereas a channel formed entirely on the distal side
of an unbroken moraine, and ’sealed off by the
moraine at its source, may either pre-date or be con-
temporaneous with the formation of the moraine.
Such arguments have recently been used by Maizels
and Petch (1985), in dating the intermoraine areas
of Austerdalen in Norway.
The evidence provided by such geomorphic rela-
tionships can be supplemented by that of aerial pho-
tographs and historical maps: a minimum age for the
abandonment of a particular outwash surface is
given by the date of the earliest survey on which the
features of that surface can be recognized in their
present form. A maximum age on the other hand, is
given by the date of the most recent survey prior to
the feature’s formation.
For the proximal outwash terraces of the
Svínafellsá, these approximate ages have been
refined using lichenometric dating (Thompson and
Jones, 1986). By detailed inspection of the aerial
photographs it is possible to trace these dated sur-
faces downstream from the initial sampling areas,
the junctions between successively younger units
being clearly marked by the abrupt truncation of
older channel pattems. For the present purposes,
however, the subdivision of these surfaces has been
simplified, to correspond with the units identified in
other parts of the proglacial area.
A total of eight different age zones of outwash
deposition can thus be identified, (Fig. 5), with two
quite different pattems emerging for the two glacier
forelands, each of which are now discussed in tum.
SKAFTAFELLSJÖKULL
The position of each of the major meltwater
streams at Skaftafellsjökull has repeatedly changed
in response to the rapid recession of the glacier
snout, old channels being successively abandoned in
favour of newer courses closer to the retreating ice-
front (Fig. 2). Increasingly older outwash deposits
are therefore generally found with increasing dis-
tance from the glacier snout, and are frequently
separated by more or less continuous bands of
moraine.
The initial stages in the abandonment of a particu-
lar course may involve incision of the main channel
below the general level of the adjacent outwash sur-
face, in response to the upstream displacement of
the river’s equilibrium profile (Thompson and Jones,
1986). Two stages in the incision of the westem arm
of the Skaftafellsá, between 1934 and 1968, are
clearly displayed in the sequence of deposits
preserved between the 1934 and 1939 moraines
(Fig. 5).
In the final stages, the flow of meltwater is
diverted by the formation of a completely new chan-
nel, usually along the margin of the retreating ice-
front, or through a series of connected proglacial
lakes, as occurred between 1960 and 1968 to form
the present course of the westem Skaftafellsá. (Fig.
2).
The recent advance of Skaftafellsjökull, since
1973, has been accompanied by the development of
increasingly complex braided pattems in the proxi-
mal reaches of its meltwater streams, suggesting that
glacier advance may be associated with conditions
of aggradation, in contrast with the incision which
appears to accompany ice-front retreat (Thompson
andJones, 1986).
SVÍNAFELLSJÖKULL
At Svínafellsjökull, where the position of each of
the major meltwater outlets has remained essentially
constant since at least 1904, successively older
outwash surfaces are preserved as a series of ter-
races on either side of the modem channels. This is
especially tme of the main Svínafellsá deposits,
which form an extensive, low-angled, dissected
outwash fan.
Svínafellsá — The historical incision and formation
of terraces on the proximal part of the Svínafellsá is
JÖKULL, No. 38, 1988 27