Jökull


Jökull - 01.12.1985, Page 66

Jökull - 01.12.1985, Page 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
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