PHYSICAL GEOGRAPHY 



245 



Owing to the variability of the wind-conditions, the dunes in the 

 blown-sand districts are usually small and irregular in form; they 

 are rarely higher than 3- -4 metres, usually much less, and they 

 are bound together by lyme grass and a few creeping dwarf willows. 

 Sandy levels with low waves of sand are of general occurrence, and 

 when moisture comes into play, the surface is cracked into numerous 

 polygonal cakes by the action of desiccation or frost. The cracks are 

 filled with drifted sand, so that the surface resembles a kind of mosaic. 



Fig. 16. Soil torn up by the wind. Large tracts in Landsvcit are occupied by these loess-like 

 formations. Here several square miles of land, which were formerly wood-covered, are torn 

 up by the north-east wind. The district of Landsveit, west of Hekla. (Phot. Th. Thoroddsen.) 



Ill tuff districts proper older and younger seolian formations 

 are the thickest and most widely distributed, and often alternate 

 with volcanic and glacial formations; but the tuff-dust is also car- 

 ried to the basalt districts, where they initiate the formation of the 

 loess-like layers known in Iceland as "mohella." Smaller layers of 

 "mohella" occur everywhere in valleys and lowlands alternating 

 with older and more recent glacial formations, with peat and lava- 

 streams, but they decrease in thickness the further they are away 

 from the large stretches of blown sand in the tuff districts. "Mo- 

 hella" usually resembles a fine, easily crumbled, yellowish-brown 

 or grey tuff, which is often traversed by stems of plants and red 

 tuff-tubes which have been formed around the decayed stems; they 



