Jökull - 01.12.1952, Blaðsíða 15

Jökull - 01.12.1952, Blaðsíða 15
Fig. 3. Section of two E- W channels in lake sedi- ments. Heavy silts of lower channel have sunk down into the fluid var- ved clays (See text below). Snið af tveim rdsum i botnseti Hagavatns og röskuðum leirhvörfum. ning to block col. No. 4. Often there were evi- dences of erosion of the upper parts of clay bands: this was especially noticeable where clays and fine sands had been laid down on a rippled surface of sands. Coarse sand beds deposited on these ripples tended to cut down into the clays at the tops of ripples. Rounded clay balls were noted in tlie coarse sandy layers, suggest- ing that often considerable erosion of the lower clays had taken place, i. e. of clays that had been deposited a sufficient time to have become fairlv compact. Further up in the succession, where finer sediment predominated, and where the clays approached true varved sediments, there were often grey silt bands present. Typically, these silts occurred in U-shaped channels, running generally east-west, and in which the silts were bedded parallel to the sides (Fig. 3). The upper surfaces of the sediments in the channels were often irregular, and the laminated clays above lay unconformably on the silts. Below these channels the laminated clays were usually con- torted and compressed, suggesting that the clays were in a very fluid condition when the silty material was laid down. Contortion was evi- dently due to weight of these heavy silts, and thus the U-shaped channels were probably formed by a downward slumping of the silts into the clays (Fig. 3). A shallower type of channel was also noted, in which the silts truncated the varves below without any undue disturbance. These silts extended over considerable distances beyond the confines of these channels, eventually thin- ning out laterally. Occasionally current-bedding was noticeable in the silts, and this was always from the west, as it was for the sands below, showing that the main sedimentation for the western end of the lake was from the west. The succession rose northeastwards, along the river section, and was measurable to within about one hundred and fifty yards of the mor- aines, which marked the maximum extension of the ice. Approaching the moraines, the varves became strongly folded and thrust, showing structures reminiscent of Alpine folds. It was impossible to measure the succession of this area. The sands and silts tended to die out to the north and east; and so towards the moraines, the succession comprised mainly laminated clays. A typical cycle of these comprised: very fine black and brown current bedded sands at the base, with fine grey clay bands between sand lenses. These are followed by grey, and fawn and grey clays, finely laminated, with between twenty-five and fifty laminations to the inch. The grey clays tended to be silty, and occasion- ally current bedded. Often a pale flesh coloured band of clay, usually thin, followed the lami- nated clays, overlain by fine grey clay. These in turn were followed by several bands of yellow clay, which tended to be rather thick, and which were separated from each other only by fine grey bands. This whole sequence appeared to be a rhythm, and was repeated several times, 13



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