Jökull - 01.12.1988, Síða 27

TABLE 1. Lichenometric dating of the outer moraines at Skaftafellsjökull.
TAFLAl. Aldursgreiningar á ystu jökulgörðum Skaftafellsjökuls samkvœmt mælingum á skófum.
Moraine ridge Thallus diameters (short axis) of Rhizocarpon sub-genus lichens (mm) Calculated date
Largest 5 specimens Mean
A (outermost) 50.7 47.7 46.0 42.6 42.2 45.8* 1909*
B 56.1 48.4 44.9 44.9 43.9 47.6 1906
C 44.2 43.0 41.4 41.3 38.5 41.7 1917
D 35.5 33.8 32.6 31.1 30.4 32.7 1930
Notes: Dates calculated from calibration curve developed from independently dated landforms (Fig. 6 in
Thompson and Jones, 1986).
* Lichen sizes anomalously small due to limited sampling area and disturbance of the ground by road con-
struction: derived date is therefore likely to underestimate the true age of this moraine.
associated with the outermost lateral and terminal
moraines, marks the height of the glacier at the time
of its maximum historical advance (ca. 1870). On
the basis of field measurements, King and Ives
(ibid.) determined that the lower trimline was at a
height of 58 metres above the 1954 glacier surface,
implying that average rates of net downwasting in
this area, between 1930 and 1954, were 2.42 metres
per year.
1945 -1984
After 1945 the overall rates of retreat of both gla-
ciers were considerably reduced, and episodes of
readvance became more frequent (Fig. 3). At Skafta-
fellsjökull, significant advances occurred during the
periods 1951 to 1953, 1957 to 1958, 1967 to 1969
and 1973 to 1982. Prominent moraine ridges were
formed on each of these occasions, but also it seems
during several other minor readvances during the
same period. The latter cannot be identified in the
annual records and must therefore represent sea-
sonal fluctuations of the glacier, comparable to, but
less regular than the annual push moraines at
Breiðamerkurjökull, formed by winter readvances of
the snout (Boulton, 1986).
Svínafellsjökull has also been subject to a number
of important readvances during the last 40 years, the
most significant of which occurred in 1951 to 1955,
1961 to 1964 and 1975 to 1984. The aerial photo-
graphs of 1954 and 1960 conveniently allow the
identification of ridges corresponding to the two ear-
lier advances, whilst the most recent advance, which
in places has seen the glacier approach to within 20
metres of its 1964 position, has destroyed all evi-
dence of the minor fluctuations of the late 1960’s
and early 1970’s.
Considerable variations in overall rates of retreat
are evident in different parts of the Svínafellsjökull
ice front (Figs. 2 and 3A). These are most clearly
demonstrated in the contrast between the northem
half of the glacier, where up to 230 metres of net
recession has taken place since 1945, and the south-
em part which appears to have been downwasting in
situ for many years with little frontal retreat (ca. 30
metres), becoming heavily charged with supragla-
cial debris. The stagnation of this area was noted by
King and Ives (1955), and is clearly evident in the
aerial photographs of 1945 and 1954. The latter
show a distinctive area of decaying dead-ice ridges
at the extreme southem tip of the glacier, which
eventually became detached from the snout, forming
a spectacular series of ice-cored moraines (Ives,
1956, Illus. 16). Subsequently, the relief of these
features has been progressively subdued, as the ice
JÖKULL, No. 38, 1988 25