Jökull - 01.12.1983, Side 115

TABLE 2. Lichen sizes and estimated ages for locations in front of Gljúfurárjökull
TAFLA 2. Stœrb flétta og mat á aldri viðjaðar Gljúfurárjökuls.
Site Staður Size of area surveyed Flatarmál kannaðs svœðis (m2) Lichen size Largest Lichen (mean of 5) Pvermál stœrstu Jlétta (mm) Age of site (5 year lag) Aldur Age of site (10 year lag) Aldur Age of site (15 year lag) Aldur
Outside the foreland 39 (34) 1908 (1909) 1903 (1904) 1898 (1899)
M 304 27 (23-2) 1929 (1930) 1924 (1925) 1919 (1920)
N 238 28 (26-5) 1927 (1924) 1922 (1919) 1917 (1914)
A 400 36 (29-7) 1913 (1917) 1908 (1912) 1903 (1907)
B 400 38 (32) 1909 (1913) 1904 (1908) 1899 (1903)
F 280 24-5 (21-3) 1933 (1934) 1928 (1929) 1923 (1924)
G 210 28 (25-3) 1927 (1926) 1922 (1921) 1917 (1916)
E 247 22 (15-9) 1937 (1945) 1932 (1940) 1927 (1935)
L 195 18 (15-6) 1944 (1945) 1939 (1940) 1934 (1935)
K 140 17-5 (15-4) 1945 (1945) 1940 (1940) 1935 (1935)
D 80 15 (13-1) 1950 (1951) 1945 (1946) 1940 (1941)
J 210 7(5-9) 1964 (1964) 1959 (1959) 1954 (1954)
H 690 6 (5-4) 1965 (1965) 1960 (1960) 1955 (1955)
ably into the crevasses which cover the margins.
Only those stations utilising large boulders appear-
ed to have survived. The general similarity in move-
ment shown by all these stations between 1979 and
1981 (Table 1) strongly suggests that their move-
ment is representative of the movement of the glaci-
er surface and not due to sliding over the glacier
surface during periods of ablation.
The rate of movement of the glacier suríace in
areas comparable to those examined between 1977
and 1979 has slightly slowed between 1979 and
1981, from c26m/year to c21 - 22m/year. In the
absence of annual observations it is not possible to
discover whether this represents a gradual change
or hides more noticeable year to year variadons.
Fastest movement was recorded at Station IV in the
centre of the dip above the snout and a slower rate
was found at Station I immediately above the steep
frontal slope of the glacier. This diíference is prob-
ably a result of the coníiguratíon of the ice surface
over the snout area. There is a significant difference
in the direction of ice flow between I and the other
sites suggesting that flow is not simply parallel to
the general trend of the glacier throughout the
snout. Station II, although located in 1979, was not
surveyed and could not be used to indicate ice
movement between 1979 and 1981.
CHRONOLOGY OF REC ENT
DEGLACIATION
I) Lichenometry and the derivation of a growth curve
Following the 1979 survey of the morainic ridges
in the glacier foreland of Gljúfurárjökull an ex-
tremely tentative chronology for recent deglaciation
was proposed (Caseldine and Cullingford 1981). Due
to the lack of unequivocal documentary or observ-
ational evidence for former ice-marginal positions it
was necessary to use lichenometry of the morainic
ridges and inter-ridge areas to develop a temporal
sequence for the recent deglaciation of Gljúfurár-
dalur. Lichenometry has been used for this purpose
in many areas of the world (e.g. Beschel 1961, And-
rews and Webber 1964, Karlén and Denton 1976, Malt-
hews 1977) and is now a well attested dating techn-
ique (Locke, Andrews and Webber 1979, Mottershead
1980). As yet however little published evidence has
been produced for Iceland (Jaksch 1970, 1975, Gord-
on 1980). Forthis study in Gljúfurárdalurthelichen
Rhigocarpon geographicum agg. was chosen as this was
present throughout the valley and also occurred on
lower ground in Skíðadalur. There has been some
considerable debate over which parameter of the
lichen thallus should be measured to determine the
“size” of the lichen (Locke et al. 1979) and here the
JÖKULL 33. ÁR 113