Jökull - 01.12.1985, Blaðsíða 66

TABLE 1. Measurements of permanent lichen sites established in July 1983 in Gljúfurárdalur (in mm).
TAFLA 1. Mœlingar á fléttum (þvermál í mm).
Site — outer moraine
Number Longest axis 90° axis Number Longest axis 90° .axis
G1 39.2 35.4 G9 31.4 30.8
G2 37.8 26.4 G10 28.1 24.4
G3 28.2 20.8 Gll 23.4 19.3
G4 30.9 27.4 G12 42.7 28.7
G5 31.3 28.5 G13 41.8 28.2
G6 28.4 21.6 G14 23.3 16.9
G7 28.2 22.2 G15 29.5 27.3
G8 33.8 28.3
Site - RG2 Site — RG3
G16 36.0 29.2 G21 26.7 23.4
G17 44.0 31.5 G22 36.2 34.5
G18 36.2 32.4 G23 16.4 11.3
G19 33.2 24.9 G24 27.3 21.0
G20 26.0 17.4 G25 31.4 27.3
GLACIER BURST FEATURES
On first visiting the glacier in early July when there
was a continuous snow cover down the valley beyond
Leeds 1 a feature was observed on the eastern side of
the valley lying close to the ice front (Figs. 1,3, Plate 1).
This comprised a spread of boulders and finer material
reaching 250 m down valley from the snout of the
glacier. Close to the eastern margin of the glacier a gully
had been formed in the till cover immediately outside
the ice margin. The gully started rather abruptly, was up
to 2 m deep, up to 15 m across and showed some
sinuosity in the shape of the channel floor. For much of
its 65 m length it had produced a highly unstable cliff on
the non-glacier side, uncovering some extremeiy large
(> 2 m across) boulders, and on the glacier side the
channel was partly cut into old snow to a similar depth,
but not cut into glacier ice. At a height of ca 1 m on the
glacier side the snow had been partially planed and
fresh snow formed the upper margin.
After 65 m running parallel to the glacier margin the
gully, having gradually become shallower, reached a
narrow neck and there followed a continuous spread of
coarse cobbles and finer sediments (Plate 2) weakly
aligned in low discontinuous ridges lying on top of
snow. This cover was slightly breached as the axis of the
deposit turned down the valley at 90° to the ice front.
After this breach the deposit, while still producing a
broad cover, showed increasing alignment into 1 or 2
narrow (< 50 cm) ridge forms which then continued
sinuously along the valley floor, still on snow. With this
increasing alignment the ridge or ridges became slightly
thicker and showed evidence of bedding (Plate 3). At
times the thickness of the underlying snow could be
ascertained, reaching over 1.6 m at the thickest observ-
able location. The snow itself lay over the main river
system of the valley and for the remaining 250 m of
deposit it followed the same course as the underlying
river (Plate 4). Eventually after 250 m the deposit, again
quite abruptly, terminated, the single main ridge even-
tually dissipating into poorly aligned dumps of material.
The margins of the feature were well picked out
throughout by the surrounding snow which was up to 50
cm deep and the degree of sinuosity shown in the
deposit, although on too small a scale to be surveyed,
was also picked out in the snow margin.
Bulk samples were taken for particle size analysis
from a number of locations (Fig. 3) along the feature
and the results of these analyses demonstrate the
similarity between the sediments deposited on the snow
and the till previously deposited by Gljúfurárjökull in
the valley (Fig. 4). Although this material had at times
been sorted, the bulk samples show that there had only
been partial loss of the finest fraction during the sorting
process. Analysis of roundness characteristics of larger
clasts (> 10 cm long axis), using both Powers visual
roundness test and the slightly more objective Cailleux
roundness index (Briggs 1977) similarly showed very
little variation along the deposit. Although very angular
clasts were only found at the upper end of the spread
there was no increase in the subrounded category with
64 JÖKULL 35. ÁR