SHIFTING, OR WHITE, SAND DUNES 263 



The first three formations are halophytic communities and have been 

 studied (see p. 225). But the fourth formation, composed of large dune- 

 grasses, is much more psammophilous than halophilous ; for the dominant 

 and most prevalent grass, Psamma arenaria, thrives admirably at inland 

 spots far from the sea-coast ; other species, including Elymus arenarius, 

 Eryngium maritimum and Lathyrus maritimus, are more restricted to 

 the coast ; whilst still others are not in the least confined to the coast. 

 The influence of common salt vanishes at a short distance from the sea ; 

 dune-vegetation is by no means halophytic in constitution.^ 



CHAPTER LXIX. SHIFTING, OR WHITE, SAND-DUNES 



Essential to the building up of a dune are two main conditions 

 sand and wind and to these is added another, namely, that the sand must 

 be dry and bare, or, in other words, so free from vegetation that the wind 

 can act on it. Dunes mainly arise along sea-coasts and river-banks ; 

 waves and tides throw up on the shore sand which is usually quartz- 

 sand, whose grains generally average one-third of a millimetre in diameter ; 

 these are dried by the sun and transported by the wind. Shifting 

 sand, Hke snow drifted by wind, is deposited wherever it finds shelter 

 from wind behind stones, shells, fragments of wood, and, above all, 

 among plants. The sand accumulates in contact with the last-named, 

 among their shoots and leaves, and behind them on the lee side arises 

 the so-called 'tongue-like heap ',2 which is a long tongue-shaped accumu- 

 lation of sand sloping gradually downwards. 



Subsequently, these ' dune-embryos ' ^ themselves act as obstructions 

 to the drifting sand, and gradually change their forms until they give 

 rise to typical dune, whose angle of inchnation in typical cases is 5-io 

 on the windward side, and about 30 on the lee side ; the sand shps down 

 the steep lee side, on which the grains become arranged according to size, 

 power of cohesion, and so forth, quite uninfluenced by the wind. Dunes 

 become most regular where there is a prevailing wind and where vegetation 

 is quite scanty or lacking ; as the form of the dune is largely determined 

 by plant-growth.* 



Dunes also arise far removed from water, in deserts of Central Asia, 

 Africa, and elsewhere, where only the fundamental conditions for their 

 genesis prevail ; namely, that sand should be formed and not be com- 

 pletely clad with vegetation, and that a sufficiently strong wind should 

 blow. 



Dunes are described as ' white ' so long as the vegetation clothes the 

 soil to such a hmited extent that it is the colour of the sand which deter- 

 mines the prevailing tint and appearance of the dune : in fact, there are 

 some dunes entirely devoid of vegetation. White dunes are usually 

 at the same time ' shifting dunes ' which travel in the same direction as 

 the prevailing wind. 



' See Kearney, 1904 ; Warming, 1907-9. 



Sokolow, 1894 (' Zungenhiigcl '). * Cowlcs. 1899. 



* See Sokolow, loc. cit. ; Gerhardt, 1900; Cowles, loc. cit. ; Cornish, 1897; 

 Wessely, 1873; Warming, 1891, 1907; Massart, 1893, 1898a, 1908; Wery, 1906. 



