Jökull - 01.12.1981, Síða 21

the lithological differences betvveen the two
areas, the morainic ridge samples have very
similar roundness distributions to those of tills
m Wensleydale, Yorkshire (Gregory and Cul-
hngford 1974; Cullingford and Gregory 1978),
whilst the proglacial stream bedload sample
had very similar roundness properties to
fluvioglacial deposits in Wensleydale.
MORAINE chronology
As regards dating of the ice-marginal
posttions marked by the ridge complexes there
ls> as yet, little hard evidence. Lichenometric
dating of the ridges is to be attempted in 1981
but a possible chronology can be put forward
using a combination of photogrammetric, ob-
servational and phytometric data, the latter
collected in 1979. There was a net retreat of at
least 265 m between 1939 and 1972 (Eythórsson
1963; Rist 1977), which places the 1972 mar-
gm very close to the 1979 snout position. The
position of the ice margin in 1946 as deter-
mmed by aerial photography is given in Fig. 1.
Aerial photographs taken in August 1960
show the downvalley extremity of the snout to
he close to the position of L2. This suggests a
uet recession of c. 50 m between 1946 and
1960, but since Eythórsson (1963) suggests a
retreat of 151 m between 1953 and 1960 it
would appear that an advance had occurred
between 1946 and 1953. On this basis it would
appear that the minimal extent occurred bet-
ween 1972 and 1977. Thus all the ground
between ridge 2 and the present (1979)
position of the ice front had been uncovered
for not more than 19 years in 1979. The cha-
racter of the vegetation communities within
this area tends to confirm this age with the
appearance of Saxifraga caespitosa and S.
oppositifolia (Figs. 4 and 5), species noted by
Persson (1964) in front of Skaftafellsjökull on
land uncovered for 4 to 9 years, and by Stork
(1963) in Sweden on land cleared of ice for less
than 20—30 years.
Within the major ridge complex defined by
ridges 4/5 and 53 the plant species found on
both sides of the valley are very similar with
the presence of Saxifraga hypnoides, Alchemilla
alpina, Bartsia alpina, Oxyria digyna, Cerastium sp.
and Arabis alpina, as well as those already
present on ground nearer to the ice. Moving
from the ice margin, it is noticeable that when
this series of ridges is reached almost 75% of
the species found within the outermost
moraine are already present. The main dif-
ference between the two sides of the valley at
this point is that Salix herbacea occurs within
the ridges on the eastern side but does not
appear until outside ridges 4/15/5 on the
western side. Gnaphalium nowegicum character-
istically first appears outside the 4/15/5 ridges
as does Polygonum vivaparum but the latter can
also be found occasionally on younger ground.
Both Persson and Stork consider Polygonum
viviparum to be indicative of a surface ice-free
for more than 30 years. The age of the mar-
ginal position marked by the major cross-
valley ridge system incorporating 4/5 and 53
can therefore be estimated. The complexity of
the system suggests a period of relatively slow
retreat with many ridges being formed and in
view of the vegetation evidence this need not
have occurred more than 30—40 years ago.
This was the time when Eythórsson (1956)
visited the glacier but none of his survey marks
were unambiguously identified. It is assumed
therefore that the ridges mark the extent of the
glacier during the late 1930s/early 1940s.
There is very little change in the vegetation
cover in terms of the introduction of new
species between ridges 50 and 41 in the east
and between 22/20 and 38 in the west. Within
this area there are relatively few continuous
ridges and the topography is more subdued,
related rather to local slope conditions than to
glacial deposition. It is suggested that the
morphological and biogeographical evidence
points to the existence of an area with a gra-
dual variation in age indicating a uniform and
relatively rapid rate of retreat. This period of
retreat would have taken place during the
early decades of this century.
The outer clearly defined moraine ridges,
40/36/35, prominently mark the outer mar-
gin of the glacier foreland. On the basis of the
JÖKULL 31. ÁR 19