Jökull - 01.12.1983, Síða 118

position denoted by the clearly defined outer mor-
ainic ridge and only started to retreat from this
position during the last decade of the 19th century.
Studies of the slope forms in Gljúfurárdalur either
side of the moraine also tend to support this view (/.
Jarvis in preparation).
During the 1981 expedition a less pronounced
ridge was noted outside what was earlier described
as the outer moraine on the westem side of the
valley. This had no obvious counterpart on the
eastern side and was less continuous and less well
developed than the mapped outer ridges as defined
in Fig. 1. It is cut by debris flows and although it has
a poorly developed soil cover it is extensively vege-
tated, especially by Salix herbacea. The origin and
age of this feature are uncertain as it is not signifi-
cantly different in many respects from the mapped
outer moraine. It may represent a slightly more
advanced marginal position of Gljúfurárjökull dur-
ing the earlier part of the “Little Ice Age” for it is
not known when, or even perhaps how often, the
glacier reached the position originally marked as its
maximum extent during the entire period defined as
the “Little Ice Age”, i.e. from the 16th century to
1900.
Within the outer moraine lichen sizes show a
decreasing pattern as sites are located nearer to the
ice with the exception of F. This anomaly is not
caused by any inconsistency in the lichen measure-
ments, the values for “largest lichen” and “mean of
five largest lichens” falling close to the expected
relationship, and is thought to be a result of delayed
colonisation as the site lies close to the gorge in
which the Gljúfurá flows and may have been affect-
ed by excessive snow lie. Sites M, N and G show
that the ridges on which EuB and Station 5 stand
were probably formed between 1915 and 1917 with
up to 250 m of overall retreat in the previous 20
years, an average value per year very close to that
found by Eythórsson (1963) between 1939 and 1960.
Between these sites and L and E which lie close to
the ridge on which EuA stands there is a significant
change in lichen sizes. E shows the highest discrep-
ancy between the size ofthe “largest lichen” and the
“mean size of the five largest lichens” and a date of
1934-35 may be a very young estimate of the date of
deglaciation of this surface. Indeed it is possible
that at this site the “largest lichen” more closely
reflects the date of colonisation, implying that the
ice had retreated several years earlier in keeping
with the morphological evidence. The difference
between L/E and the G, M and N dates means
either that the rate of retreat slowed between the
middle of the second decade ofthe 20th century and
1930, or that there was a phase of stillstand or small
advance perhaps marked by the ridges between the
sites.
The date of 1935 for the deglaciation of site K is
given by both lichen curves and suggests rapid glac-
ial retreat during the early 1930’s with considerable
oscillations allowing formation of the closely spaced
ridges down valley from K. Site D, dated to 1940-41,
indicates the time at which icemoved away from the
ridge immediately in front of the most continuous
series of ridges close to the ice front. Hence this
important ridge system probably marks the position
of Gljúfurárjökull in the early 1940’s although it
may not necessarily have been formed at that time.
There is a noticeable variation in lichen sizes either
side of this ridge system (incorporating 3/4/5/53 see
Caseldine and Cullingford 1981, Fig. 2) with lichens at
J inside the ridge only half the size of those at D
outside the ridge. The date fbr J is 1954, and H,
separated from J by three small but distinct ridges,
is 1955. On the morphological evidence these dates
should be further apart, at least by three or four
years. The similarity of the lichen sizes can be ex-
plained by the local topography, with J being locat-
ed in an inter-ridge depression and H being along a
more exposed ridge crest. Thus greater reliance
should be placed on the date from H with lichen
colonisation delayed more at J. The small size of the
lichens at these sites also makes accurate separation
of the populations a problem.
The dates given by J and H appear to be in
conflict with the evidence for the 1946 margin given
by the aerial photographs but this conflict can be
resolved by looking at other evidence for the posit-
ion of Gljúfurárjökull during the last thirty years.
Given the figures for the retreat of Gljúfurárjökull
between 1939 and 1960 (Eythórsson 1963), the posit-
ion of the ice in 1963 when visited by a Leeds
University party (lying close to L2 and L3 on Fig. 1)
and the rates of movement observed since 1952 by
Vignir Sveinsson of Thverá (Escritt 1974), a period
ofuninterrupted retreat between the 1930’s and the
1970’s cannot be envisaged. The clear form and
number of the ridges in this part of the valley imply
active forward movement to produce the contin-
uous ridges and there must have been some förm of
advance during this overall period. It is suggested
that following 1940 ice retreated to, or just beyond,
the 1946 position before readvancing eventually to
produce the continuous ridge sequence inside D, at
116 JÖKULL 33. ÁR