Jökull - 01.12.1952, Blaðsíða 16

Jökull - 01.12.1952, Blaðsíða 16
Fig. 4. Section through varved sediments in a gully (II on the map) S of Hagafell. White poin- ters indicate ash layers. Note strong rippling of fine sands. Hvörf i botnseti Hagavatns snður af Hagafelli. Hvítu örvarnar benda á öskulög. though occasionally incompletely. A total rhyth- mic sequence was between six inches and a foot in thickness, the variation being due mainly to the thickness of the sands at the base. Often the yellow bands of the upper part appeared to have been removed by the currents deposit- ing the sands of the succeeding rhythm. It seems improbable that such a rhythm is an annual deposit, yet similar sediments occur at the south eastern side of the lake bed, the individual rhythms being only about an inch thick in this case. It is suggested by Thorarinsson (1940, p. 238) that much of the minor lamination of the sediments is due to cyclonic weather effects. Tliese may at times be sufficiently large to be confused with seasonal deposits. In a section dug to the east of the river, two varves were noted to be lying on boulder clay which in turn lay on the contorted sediments of the same type as those seen in the river section. These upper varves were probably of the 1925—1939 succession, and the boulder clay on which they lay appeared to be a solifluction layer from the moraines. The latest members of the varve succession were removed by water erosion when the lake was drained, and at the present time rapid deflation is taking place. A gully section (II) from the south side of the lake bed towards the river showed black volcanic sands intercalated between the varves. The indi- vidual beds of sand were over a foot in thick- ness on the southern margin of the lake bed, but lensed out northwards, most not reaching as far as the river section. Obviously, these sands were washed in from the Lambahraun, probably during the snow melt in the spring, though possibly also during occasional storms. The bases of most of the sand layers were unconformable on the varves below and the upper surfaces of the sand layers were rippled. These 'appeared to be oscillation ripples (on which the laminated clays were laid down). This being so, then it is possible that in many cases non-deposition of clays occurred, or ero- sion of clays when waves produced by the wind vfrere sufficiently large. Disturbed clay layers noted at different levels along the river section may also be due to the effects of waves, though there seemed to be slump-structures present in some cases. Layers of volcanic ash were found at three horizons in the succession, one quite near the base of the sediments. These were composed of black volcanic glass, and were widespread in extent. N. B. Trial pits were dug at one or two points on the lake bed: the ash layers were noted, but the sediment was in too fluid a con- dition to allow any measurement of thickness to be made — slumping tended to occur almost as soon as the pits were dug. In two cases, above each ash layer was a layer of impure ash evidenty washed into the lake later; these 14



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