246 THORODDSEN 



often alternate with layers of wind-polished stones, gravel, scoriae 

 or pumice, sometimes with clay. Where the blown sand is conti- 

 nuously moving, no vegetation can thrive, but when the fine dust 

 and sand has blown away as far down as to the coarse gravel, 

 Icelanders say that the sand is "orfoka," i. e. it cannot drift any 

 longer (see Fig. 14). Then plants are again able to take root and 

 new soil is gradually formed - until that also is blown away. The 

 phenomenon of alternating periods of sand-drift and of vegetation, 

 which has lasted through centuries, is nowhere so distinctly trace- 

 able as in Rangarvellir. Here the substratum is exclusively formed 

 by "mohella," the thickness of which is unknown, but it must be 

 considerable, probably 100 metres or more. Here the lowland plain 

 abuts on the lava-fields of Hekla, whence quantities of volcanic 

 ashes are blown down into the cultivated land. The lowland plain 

 is intersected by deep, branching valleys, which are usually dry, 

 but during the thaws of winter and spring large quantities of water 

 have an outlet through these channels. From the plain a series of 

 small terraces leads down to the bottom of these valleys, which 



/ 



often consists of a grass-covered, level stretch of land. The valley- 

 sides offer favourable opportunities for studying the composition of 

 the mohella; fine bluish-grey layers of sand alternate with reddish 

 sand-layers penetrated by compounds of iron, and the embedded 

 stones of varying sizes bear testimony to the strong erosive action of 

 blown sand. In some layers soil and remains of plants occur, also 

 clay-tubes formed around haulms of grasses. Here and there layers 

 of pumice and scoriae are also seen. No inhabited district at the 

 present time is so exposed to being attacked and overwhelmed by 

 blown sand as Landsveit in the southern lowlands. Here, during the 

 nineteenth century, large stretches of grassland and many farmsteads 

 were overwhelmed by drifting sand, especially in the years 1836 and 

 1880 1881. The substratum consists of old lava which formerly had 

 a covering of mohella and greensward, now to a great extent torn up 

 and destroyed by the masses of blown sand from the north-east. 

 Sand storms cause deep channels and furrows in the soil, which 

 constantly enlarge and by combining with others, gradually destroy 

 the entire layer of soil, so that only a few massive fragments of 

 mohella with hollowed sides and covered with greensward traversed 

 by the fibres of plants, are left behind until they also succumb to 

 the universal destruction. In large stretches of this district all 

 greensward and soil have been torn off down to the naked lava-rock. 



