Jökull


Jökull - 01.12.1952, Blaðsíða 12

Jökull - 01.12.1952, Blaðsíða 12
SEDIMENTARY SEQUENCE IN THE HAGAVATN BASIN By R. GREEN Introductory: As the following article refers to some papers on tephrochronology which cannot be suppo- sed to be known by many readers of this jour- nal some introductory words may be desirable for a better understanding of the article. Fig. 1 shows the soil profiles at Skallakot and Hagavatn referred to by Green. The Skallakot profile was measured in Þjórsárdalur, ab. 15 km WNW of Hekla. The situation of the Haga- vatn profile is shown by a cross on tlie map Fig. 2. The layers III-V in ihese profiles are very easily recognisible in the districts NW of Hekla and so is the characteristic layer series Vlla, b, c and d. Layer Vlla is greyish black. layer Vllb light, and the layers VIIc and Vlld black. The layers X and Y are light, rhyolitic layers and they were both erupted from Hekla, layer X 2500-3000 years ago, layer Y probably 4000- 5000 years ago. The source of the rhyolitic layer Vllb is a crater just NE of lake Frosta- staðavatn 30 km E of Hekla. Pollenanalytic studies of soil profiles near old farmruins in Þjórsárdalur have proved that layer VHa was deposited shortly before the colonization, pro- bably ab. 850 A. D. Greens paper does not only confirm the re- sults obtained by the study of the changing outlet of lake Hagavatn; it also adds interesting facts such as the statement that the 1693 tephra layer is found in the SW-basin of Hagavatn, underlain by ten or twelve varves which means that already 1680 or even somewhat earlier had Hagafellsjökull eystri advanced so far as to block col 4. Thus the glacier has now an exten- sion which is considerably less than its extension ab. 1680. Sigurdur Thorarinsson. The Hagavatn — an ice-dammed lake along the southern margin of the Langjökull — is of geologically recent date. According to Thor- arinsson (1949), Hagavatn was formed by the closing of a post-glacial valley running north east to south west, by the lava cupola Lamba- liraun. This is of recent date, but precoloni- zation in age. This particular study was mainly concerned with an examination of the lake sediments in order to determine the age of the two lake basins, the south west and the north east, by counting varves or by using Thorarinsson’s tephroclironological technique. The latter met- hod was found to be of most use. Thorarinsson (1939, 1949) has made detailed studies of this district from the glacial-morp- hological aspect. The results achieved here only serve to confirm Thorarinsson’s conclusions concerning the fluctuation of climate in Iceland. The southwestern margin of the lake, as stated above, is formed by Lambahraun, which blocks part of the valley excavated in tuffs with in- terbedded lava flows. The northern boundary is the Hagafellsjökull eystri, on the extent of which depends the size of the lake. Quoting from Thorarinsson (1949): „The size and extension of the lake, is determined by the extension and thickness of the Haga- fellsjökull eystri and through which of the cols (marked on the map by numbered arrows, cf. Fig. 2) the lake has its outlets. The sill of the north-easternmost col (No. 4) is the lowest (433 m), and thus the lake normally has its outlet there. If that col is dammed by the gla- cier, the water can escape through col No. 3, the Leynifoss col, the sill of which is 9.5 m higher than sill No. 4. If that col is also dammed by the glacier, the lake would find an outlet over col No. 2 in Fagradalsfjall, the sill of which is 15 m above sill No. 3.“ In Thorarinsson (1939) it is shown from historical records that a flood (jökulhlaup) probably occurred in 1708, as the ice advanced to block col No. 4 and reached a maximum post-glacial extension about 1850. From a map made by Gunnlaugsson in 1834—35, it is shown that drainage was then through col No. 2, i.e. the ice was at its maximum extent. A hlaup occurred through Leynifoss in 1884 as the ice retreated. An advance until ab. 1892 followed this and then retreats caused floods in 1902 and 1929 10

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