Jökull

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Jökull - 01.12.1985, Qupperneq 67

Jökull - 01.12.1985, Qupperneq 67
greater distance away from the erosional part of the feature. The size of the largest clasts, after the occur- rence of some much larger boulders close to the end of the gully also showed no subsequent pattern of decreas- ing size with distance from the gully (Table 2). From all the observations and analyses that were made the following points can be made: 1. The gully, which was dry during the period of survey, was not part of the sub-glacial/pro-glacial stream system mapped in earlier years although water had previously been observed flowing along some points of the glacier margin, especially on the western side. At the head of the gully there was no evidence for water to have originated from under the ice at that point. Partial collapse of the snow cover at the head of the gully possibly pointed to an origin further around the margin. There is therefore no direct evidence to define the water source as englacial. 2. AIl the material deposited below the gully is com- parable to that derived from the till in the gully. While some of this material has been sorted there had been virtually no rounding of clasts, only partial preferential loss of different size fractions and there is no evidence for a source of sediment outside the gully. 3. The deposit lies on top of snow, probably at thicknesses in excess of 1.5 m above the main river, the Gljúfurá, but snow had also probably once covered the deposit, the snow which marked the edge of the deposit showing no signs of incorporated sediment despite lying below the level of the ridges. Deposition must therefore have taken place after a period of thick snow cover but prior to further snow fall. From all the evidence it would appear that the overall feature of the gully and associated spread of material derived from a burst of water at or just outside the glacier margin. The origin of the water is unclear, it may have been englacial, released due to a realignment of water stored within the glacier finding an outlet at a point along the glacier margin, or it may have been water from sources outside the glacier dammed up and then released by movement of the glacier or some failure on the surrounding slopes. This release of water must have been sudden and of high energy, sufficient to cut into and erode till to a depth of 2 m. As erosion continued along the side of the glacier much of this energy dissipated as more and more material was eroded and deposition began, generally in a weakly braided form with the largest boulders quickly being deposited. The lateral spread of deposition appears to have been very limited with sediment eventually con- centrated into one main channel, possibly partly restricted by the snow into which the debris-charged Fig. 3. Survey of features associated with the "burst“. — 3. mynd. Kort sem sýnir framburð í hlaupinu veturinn 1982-83. water must have been cutting. The depth and velocity of the water did allow some sorting of the sediment to take place but eventually deposition became more diffuse, presumably as energy was lost and no further deposition was possible. It is difficult to judge just how deep the water would have been, at present much of the deposit lies above the surrounding snow cover and there is no evidence of deposition or erosion outside the immediate confines of the feature. At first sight the whole feature looked very much like a flow but the bedding and overall nature of the landform, especially the ridges of material, strongly support the importance of water borne transport of eroded material and it does therefore JÖKULL 35. ÁR 65
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