548 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



Dunes are accumulations of sand and are formed in much the same 

 manner as snow-drifts, being started by some obstruction which 

 causes an eddy in the sand-bearing wind. Once started, each dune 

 provides the obstacle which causes its further growth. Sand-dunes 

 are of very various form and size, ranging from a few square feet 

 up to several square miles in area, and from inches to perhaps 1,000 

 feet (304-8 metres) in height. Commonly the component sand is 

 well sorted as to size and rounded as to particle-shape. Dunes are 

 apt to be formed wherever there is a source of sand available — 

 for example on a sandy beach or river plain, or where sandstone 

 disintegrates in a dry climate. Unless ' bound ' and covered by 

 vegetation so that many surface particles are held while the wind is 

 slowed down and no longer has full access to the sand, dunes migrate, 

 though rarely at a rate of more than 25 feet a year. This migration 

 takes place through the transfer of sand from the windward to the 

 leeward side of the dune, and in its course may overwhelm roads, 

 farms, and forests. The most effective way of stopping the migration 

 of dunes is to plant on their windward side suitably hardy Grasses 

 and shrubs adapted to sandy soil and capable of binding the surface. 

 Such are Marram Grass {Ammophila arenarid), Lyme-grass {Elymiis 

 arenarius s.l.), Volga Giant-wildrye {E. giganteus), and certain mem- 

 bers of the Pea family, which are used for this purpose of stabilizing 

 dunes in various parts of the world and may be followed by suitable 

 coniferous and other trees. Even when a climax forest is attained 

 the dune origin is commonly evident — and irksome to the foot- 

 traveller — owing to the intimate topography of ups and downs caused 

 by the bodies of the stabilized dunes. Frequently sand-dunes 

 become stabilized naturally by similar means, the common sequence 

 being that one of the coarse rhizomatous Grasses will pioneer in 

 colonization {cf. Fig. 28, A, and Fig. 93), followed by Mosses or 

 Lichens or other ' secondary binders ' on the floor, followed in turn 

 by less xeromorphic higher plants (for the sand is usually damp 

 beneath the air-dried surface). These last include shrubs and 

 ultimately trees when the surface has become fully stabilized and 

 sufficient humus has accumulated to form a reasonably nutrient soil 

 (Fig. 177). 



Loess is wind-deposited dust ; its deposits are usually without 

 special form but may be of great extent and considerable thickness. 

 Its fine texture and content of food-salts make loess a fertile 

 substratum provided sufficient moisture is present and it is 

 not too compacted ; it gives an easily worked and agriculturally 



