Fróðskaparrit - 01.01.1998, Blaðsíða 319

VEÐURLAGSELVDAR JØKULBROYTINGAR í TJÚGUNDU ØLD
Á S UÐURLANDINUM f ÍSLANDI
325
However, this record can now be complet-
ed with relative confidence given the
lichenometric evidence collected from the
foreland in 1996.
The following maps (fig.4) show the po-
sition of the ice margin at Virkisjokull-Fall-
jokull at eight time intervals during the last
115 years. Evidence has been compiled
from aerial photographs in the case of
1945, 1960, and 1980, lichenometric data
in the case of 1880, 1918 and 1928, carto-
graphic data in 1904 and by direct field ob-
servation in the case of 1996. These time
slice reconstructions enable a complete pic-
ture of the recent chronology of the glacier
to be generated.
Results: A glacial chronology 1880-1996
The dating of the landforms on the pro-
glacial foreland at Virkisjokull-Falljokull
has allowed a detailed chronology of the
glacier’s movement over the last 115 years
to be compiled. The use of lichenometry as
a relative dating technique has enabled any
gaps in the glacier’s history to be restored
with relative confidence.
The 50-year period following the glaci-
er’s retreat from the limits of maximum re-
cent glaciation in 1880, appear to have been
one of complex ice-marginal fluctuations.
Evidence gathered by Thorarinsson (1943,
1956) suggests this period was punctuated
by three episodes of stillstand or advance.
The geomorphology at Virkisjokull-Fall-
jokull is complex and requires careful
study, but good evidence can be found on
the eastem half of the foreland for two such
advance events. The story on the western
section of the margin is more complicated
and throws some doubt on the lichenomet-
ric dating method adopted. A plausible ex-
planation for the source of misinterpreta-
tion may be due to the presence of a large
body of un-nourished or ‘dead ice’ which
was still connected to the active margin.
The absence of any discrete, recessional,
moraine ridges and the presence of a large
area of hummocky, ablation moraine on the
western half of the foreland suggest that the
snout dynamics were greatly influenced by
the dead-ice mass. The later 1928 re-ad-
vance is manifest in the imposing terminal
moraine complex with its oversteepened
proximal slope. It is probable that both ear-
ly twentieth-century re-advances returned
successively to occupy the same position
on the western foreland. This scenario
would explain the lichenometry anomaly
encountered when surveying ice-proximal
slopes in this region of hummocky topogra-
phy. Reactivation of the moraine flank but
not the crest would lead to unrepresenta-
tively young dates being returned for this
slope. A secondary break of slope was ob-
served on the proximal side of the terminal
moraine, which may represent the height of
re-activation by later episodes of advancing
ice. The impressive height of this moraine
ridge may also be due in part to the occu-
pation of this position by numerous ice
fluctuations. Increased dumping of
supraglacial debris caused by successive
margin occupations may have given this
moraine complex a constructional and de-
positional element to its formation. The
lichenometric data confirms the multi-
phase construction of moraine complex 1.
However, the exact age of the feature is hid-