Jökull


Jökull - 01.12.1981, Page 21

Jökull - 01.12.1981, Page 21
the lithological differences betvveen the two areas, the morainic ridge samples have very similar roundness distributions to those of tills m Wensleydale, Yorkshire (Gregory and Cul- hngford 1974; Cullingford and Gregory 1978), whilst the proglacial stream bedload sample had very similar roundness properties to fluvioglacial deposits in Wensleydale. MORAINE chronology As regards dating of the ice-marginal posttions marked by the ridge complexes there ls> as yet, little hard evidence. Lichenometric dating of the ridges is to be attempted in 1981 but a possible chronology can be put forward using a combination of photogrammetric, ob- servational and phytometric data, the latter collected in 1979. There was a net retreat of at least 265 m between 1939 and 1972 (Eythórsson 1963; Rist 1977), which places the 1972 mar- gm very close to the 1979 snout position. The position of the ice margin in 1946 as deter- mmed by aerial photography is given in Fig. 1. Aerial photographs taken in August 1960 show the downvalley extremity of the snout to he close to the position of L2. This suggests a uet recession of c. 50 m between 1946 and 1960, but since Eythórsson (1963) suggests a retreat of 151 m between 1953 and 1960 it would appear that an advance had occurred between 1946 and 1953. On this basis it would appear that the minimal extent occurred bet- ween 1972 and 1977. Thus all the ground between ridge 2 and the present (1979) position of the ice front had been uncovered for not more than 19 years in 1979. The cha- racter of the vegetation communities within this area tends to confirm this age with the appearance of Saxifraga caespitosa and S. oppositifolia (Figs. 4 and 5), species noted by Persson (1964) in front of Skaftafellsjökull on land uncovered for 4 to 9 years, and by Stork (1963) in Sweden on land cleared of ice for less than 20—30 years. Within the major ridge complex defined by ridges 4/5 and 53 the plant species found on both sides of the valley are very similar with the presence of Saxifraga hypnoides, Alchemilla alpina, Bartsia alpina, Oxyria digyna, Cerastium sp. and Arabis alpina, as well as those already present on ground nearer to the ice. Moving from the ice margin, it is noticeable that when this series of ridges is reached almost 75% of the species found within the outermost moraine are already present. The main dif- ference between the two sides of the valley at this point is that Salix herbacea occurs within the ridges on the eastern side but does not appear until outside ridges 4/15/5 on the western side. Gnaphalium nowegicum character- istically first appears outside the 4/15/5 ridges as does Polygonum vivaparum but the latter can also be found occasionally on younger ground. Both Persson and Stork consider Polygonum viviparum to be indicative of a surface ice-free for more than 30 years. The age of the mar- ginal position marked by the major cross- valley ridge system incorporating 4/5 and 53 can therefore be estimated. The complexity of the system suggests a period of relatively slow retreat with many ridges being formed and in view of the vegetation evidence this need not have occurred more than 30—40 years ago. This was the time when Eythórsson (1956) visited the glacier but none of his survey marks were unambiguously identified. It is assumed therefore that the ridges mark the extent of the glacier during the late 1930s/early 1940s. There is very little change in the vegetation cover in terms of the introduction of new species between ridges 50 and 41 in the east and between 22/20 and 38 in the west. Within this area there are relatively few continuous ridges and the topography is more subdued, related rather to local slope conditions than to glacial deposition. It is suggested that the morphological and biogeographical evidence points to the existence of an area with a gra- dual variation in age indicating a uniform and relatively rapid rate of retreat. This period of retreat would have taken place during the early decades of this century. The outer clearly defined moraine ridges, 40/36/35, prominently mark the outer mar- gin of the glacier foreland. On the basis of the JÖKULL 31. ÁR 19
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