Jökull - 01.12.1981, Page 15

TABLE2. Net movement of the glacier surface August 1977 — July 1979 and altitudes of
ablation stakes. — TAFLA 2. Heildarhreyfing áyfirborði jökuls frá ágúst 1977 til júlí 1979.
Marker Merki Direction of movement Hreyfingarstefna Distance moved fœrsla m 1977 altitude hœð m 1979 altitude hæð m Altitude change Hœðarbreyting m
Stn. 7 N18°E 52.5 763 750.9 -12
Stn. 8 N25°E 53.0 772 761.0 - 11
Ab (east) — — — 656.8 —
Ab (west) — ~ — 658.9 —
stations were set up for future use (I and III to
IX, Fig. 1, Table 1).
ii) Ridge Survey — The complex pattern of
moraines and other ridges in the kilometre
downvalley from the snout was mapped by
surveying the ridge crests from stations Ll,
EuA and EuB. The staff was positioned and
read at c. 15—25 m intervals along each crest,
the closeness of the spacing varying with the
sinuosity of the crest. A reading was always
taken where ridges join and where they
change direction. Only ridges whose crests
could be identified and followed unequivo-
cally were mapped. Since the purpose was to
map relict moraines no attempt was made to
survey the small ice-cored ridges in the im-
mediate vicinity of the snout which were
manifestly undergoing rapid morphological
change during the survey period.
INTERPRETATION OF
SURVEY RESULTS
i) The Glacier — Even taking the most
pessimistic view regarding the accuracy of the
survey results (as detailed above) it is quite
clear that the margin of the snout of Gljúfur-
arjökull has advanced downvalley by about
30—50 m as compared with the 1977 position.
The 1978 Bristol University Expedition
reported their suspicion that the ice edge had
advanced and its front steepened since the
previous year, but their surveying work was
very limited and did not permit an unequivo-
cal statement about either the realty or the
magnitude of the advance. In the light of the
1979 results it seems probable that a signifi-
cant forward movement of the glacier edge
had occurred by the summer of 1978. The
presence near the snout in 1979 of ice-cored
ridges consisting mostly of ice under a thin
and discontinuous veneer of debris suggests
that the ice edge may have occupied a slightly
more advanced position in 1978 than in 1979,
though the difference could only have been in
the order of 10 m or so. These ridges changed
noticeably during July 1979 both by ablation
and the consequent slippage of their morainic
cover, and by the addition of supraglacial
moraine material by sliding from the snout. As
there is no firm evidence for the presence or
absence of these ridges before 1979 the
possibility remains that they could predate
1978.
Determination of the 1979 positions of
Stations 7 and 8, established and surveyed in
1977, allowed the net amount of downglacier
movement and altitudinal change of the
glacier surface at these marks to be measured
(Table 2). The mean ice surface horizontal
velocities for the two-year period indicated by
the movement of these stations are 26.25 and
26.5 m/yr respectively. Two ablation stakes
established in 1977 were also surveyed (Ab,
Fig. 1, Table 2), but their identity on the 1977
map is uncertain, so their usefulness must
await future survey results.
This post-1977 advance of Gljúfurárjökull is
the first recorded forward movement in the
period since 1939 (Eythórsson 1963; Rist 1977).
JÖKULL 31. ÁR 13