Jökull - 01.12.1983, Blaðsíða 117

at Sólheimajökull and Fláajökull (Jaksch 1970,
1975).
The major problem found with the use ofaband-
oned farmsteads to determine a growth curve has
been the long time lag of twenty years between
nudation and lichen colonisation of the intemal
wall footings. This lag is therefore considered longer
than would have been applicable for rock surfaces
emerging from below the ice in Gljúfurárdalur. Est-
imation of the applicable time lag for the higher
valley proved difficult but with the prevalence of
many late-lying snowbeds in the glacier foreland,
expecially immediately in front of the ice, a relative-
ly long time lag, in particular for lichen develop-
ment on land between ridges, would not be sur-
prising.
In view of this general problem ages were gen-
erated for surfaces using the regression lines incorp-
orating lags of 5, 10 and 15 years to give a range of
estimated dates. These were producedfor the“larg-
est lichen” and the “mean of five largest lichens”
curves. Applying the lichen growth curves to the
sequence of morainic ridges in Gljúfurárdalur mea-
surements were made at 12 sites (Fig. 1 and Table
2). At each site, whether along a ridge or between
ridges, as large an area as possible was searched
excluding sites that were thought to be susceptible
to recent scree activity or instability which could
cause the redeposition of older material and hence
older and larger lichens. In such a steep-sided and
active area as this the area available for search was
therefore very variable. Examination of supra-glac-
ial debris on Gljúfurárjökull did not reveal any
problem from lichen growth on such debris preced-
ing deposition (Matthews 1973). The results of the
lichen survey are summarised in Table 2 where
sizes and estimated ages given diíferent lags are
presented. The location of the sites, together with
the lichen sizes, is given in Fig. 1. Only one of the
sites was not accurately surveyed, L, but its position
is thought to be quite accurately known. After an
examination of the different implications of the re-
sults using various lags the 15yearlagwas thought
to be most applicable for the valley (similar to the
lag observed by Jaksch (1975) in front of Sólheima-
jökull), especially in view of the cover of the inter-
ridge areas by snowbeds for much of the year and
the dates used in the discussion are based on this set
of figures. In many cases the estimated ages using
both "largest licnen” and ”mean offive larges lich-
ens” were very similar (at M, F, G, E, L D, J and
H). Where the two dates varied by more than one
year an average of the two dates was used unless
there were grounds for regarding either of the re-
sults as unreliable.
II) Dating of the ridge sequence
The sizes of the lichens either side of the main
outer morainic ridge at A and B show little vari-
ation. Limited searches carried out further down-
valley outside the ridge (Table 2) similarly showed
no noticeable increase in lichen sizes away from the
recently deglaciated terrain. More extensive
searches associated with analysis and dating of
debris flows in Gljúfurárdalur (J. Jarvis pers.
comm., in preparation) also showed no evidence for
the presence of lichens of greater size than those
found at A and B. On the basis of the sizes at A and
B it would appear that ice began to retreat from its
“Little Ice Age” maximum extent at the tum of the
century as suggested earlier (Caseldine and Culling-
jord 1981). This date compares well with other ob-
servations at better documented Icelandic glaciers
(Thórarinsson 1943), especially the outlet glaciers
from Vatnajökull. The agreement is found despite
the difierence in type between the glaciers and indi-
cates a response to the rising temperatures experi-
enced in Iceland at the turn of the century (Eythórs-
son 1949). In view of the resistance of lichens to
extreme conditions in arctic and alpine environ-
ments (Kappen 1973) it is not clear why larger lich-
ens are not found outside the glacier foreland. Large
areas of Gljúfurárdalur would have been covered by
snow during the coldest period of the “Little Ice
Age” but, although not able to respire or photo-
synthesise, lichens can survive in a frozen state
(Larsson 1978). They may however have been de-
stroyed after such a period of inactivity as, for in-
stance, is now happening to lichens which have
recently emerged from below the Golden Eagle
Glacier in Alaska (Calkin and Ellis 1981). It could
be argued that in the absence of larger lichens the
largest lichens found only date a period ofwarming
when there was decreasing snow cover and does not
indicate a “Little Ice Age” date for the outer mor-
aine, but the pattem of lichen sizes does show a
regular decrease with proximity to the ice front and
studies of vegetation and soil cover have shown
distinct differences in character and hence age, bet-
ween the land surfaces either side of the moraine
(Caseldine and Cullingford 1981). In view of the evid-
ence available at present it is believed that during
the “Little Ice Age” Gljúfuráijökull did extend to a
JÖKULL 33. ÁR 115