PHYSICAL GEOGRAPHY 257 



downward-flowing water, during lhaw, tunnels below the ice-layer 

 and forms subterranean channels, causing the upper layer to col- 

 lapse and fall down into these hollows. In this way, after the vol- 

 canic eruptions of Askja in 1875, great damage was done to the 

 soil on Jokuldalur; its enormous moraine -terraces were covered 

 with pumice-gravel which froze into a thick layer that melted but 

 slowly, because the white, glistening gravel reflected the rays of the 

 sun ; below r this layer, the water dug out channels, 20 30 metres 

 deep, through earth, sand and gravel, and caused catastrophes such 

 as that mentioned above. 



In this connection may be mentioned the influence often exerted 

 upon the soil by the numerous avalanches and rock-slips of different 

 kinds, and the very slow, creeping movement which may be ob- 

 served in connection with gravel, stones and rocky blocks upon the 

 mountain sides which, in the course of years, may become of very 

 great morphological importance. During earthquakes it may happen 

 that mountain-sides clad with grass and coppice are suddenly de- 

 nuded of their surface soil, which slides down into the lowland 

 plain. During the earthquake of 1896 a piece of swampy soil, 10,000 

 sq. metres in area and 2 3 metres thick, at Thjorsa in the neigh- 

 bourhood of Krokur, thus slid down, being thrown into wavy folds 

 and hummocks, although the slope of the ground was only 1 2. 

 The mountain of SkarSsfjall, which rises 227 metres above the plain, 

 had, before the earthquake, a thick coating of soil, and was grass- 

 covered to its verge; but after the earthquake it resembled a fruit 

 which has been peeled. Thirteen landslips descended on the western 

 side, leaving behind them large surface depressions, and strewing 

 below mighty mounds of soil, clay, gravel and stones together with 

 larger and smaller fragments of torn greensward. These landslips 

 must, in the course of time, have had an enormous influence upon 

 the soil and the plant-growth of many districts where earthquakes 

 are very frequent. All over the island are often seen, along the 

 mountain-sides, marks of ancient huge earth- and rock-slips that 

 could only have occurred during earthquakes. 



Level tracts with their surfaces cracked into polygonal cakes 

 (rudemarks) are extremely common in Iceland, and fine specimens 

 of such may be seen as, for instance, in the neighbouring districts 

 of Reykjavik. They have a peculiar effect upon plant-distribution 

 on the rockv flat. - k Rudemarks" are usually formed only on flat 



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land where the soil consists of gravelly clay especially clay inter- 



