Jökull - 01.12.1952, Blaðsíða 13

Jökull - 01.12.1952, Blaðsíða 13
over Leynifoss. Another „hlaup“ followed in 1939, over col No. 4, when the southwest basin of the lake was practically drained. Since then the river draining into Hagavatn from Sand- vatn efra has cut a section through the sedi- nients of the western basin, which has made an examination possible. It was originally intended to determine the age of the south west basin by counting the varves. However, owing to the difficulty of determining what was one year’s sediment, and to the presence of several erosion levels, it was found more preferable to use Thorarinsson’s method of dating by study of volcanic ash layers. The standard section was set up by Thorarins- son at Skallakot 20 km NW of Hekla (cf. Fig. 1), whilst another soil profile was measured at Hagavatn in 1948. Most of the ash layers noted in this section by Thorarinsson were also found in the lake sediments, the earliest identified being pre-colonization. The sediments of the south western basin were examined along above mentioned river from Sandvatn, and along a gully running north- wards into the river from the southern margin of the lake bed. Three main types or grades of material were noted: fine clays, silts and sands of distinctly different types. The clays were of thíee colours: a fawn-yellow, a pale flesh-pink and a grey clay, the latter being of the coar- sest texture. The silt was rather sandy, grey in colour, often containing bands of pebbles, and was the heaviest and most compact of the sedi- ments. The sands were of mainly volcanic origin,. black in colour when pure, but often containing yellowish tuff fragments: these were generally coarse sands, except where they occur- red as thin bands between the clays. The sedi- ments were not typical lake sediments, as little gradation between types of materials occurred, e. g. coarse sands were intercalated between very fine clays. Also, the almost machine-like lami- nation of the fine clays is comparable with the „sub-glacial shear-clays" of Carruthers. It is thought that the depositional conditions were somewliat unusual, though it may be explicable in that sedimentation took place from three directions. The lowest sediments exposed were at the western end of the river section; these compri- sed coarse current-bedded black sands, with IEGEND: Black Portlde dlom. < 0.2 mm 0.2-2.0 mm > 2.0 mm Llghf Fr-plV^ 0.2 mm rrrRo.2-10 mm > 2.0 mm Brown < 0.2 mm .2-2.0 mm > 2.0 mir Fig. 1. Two soil profiles measured by S. Thor- arinsson (as explained in text). Jarðvegssnið úr Þjórsárdal og úr moldarbarði austan undir Fagradalsfjalli. occasional brown silty bands on the upper sur- faces of individual beds. The sands rested on the lavas of Lambahraun, which had suffered some weathering and erosion before sendimen- tation began. These sands formed the delta of the river flowing in from the Sandvatn, and thus thinned out to the east. No clay bands were seen towards the base, but higher, begin- ning about six feet above the lowest sands seen, narrow clay bands were present, lying on ripp- led surfaces of the sands. Rising in the succes- 11



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