140 PRELIMINARY WORKS. 



but occur also inluiul. In eitlier case, but especially near the 

 sea, they are capable of forming considerable accumulations 

 of sand, reaching a height of 200 feet and even more, which 

 are called dunes. Before such areas can be brought under 

 wood, it is necessary to fix them, so as to allow trees to 

 spring up and lay hold of the soil permanently. 



a. Coas/ Dunes. 



Along the sea coast the waves constantly throw up sand, 

 which, after drying, is carried inland by air currents, forming 

 a series of ridges and valleys in many cases, though not 

 necessarily, parallel to the sea shore. These sand hills move 

 steadily forward, being replaced behind by fresh sand thrown 

 up by the sea. The rate of progress varies considerably 

 according to circumstances. On the west coast of France it 

 is said to be about 14 feet a year, but as the process has gone 

 on for a long period of time, an enormous area comprising 

 millions of acres has become covered with sand. The further 

 progress of the evil has been checked only in comparatively 

 recent times by operations which it is useful to describe 

 shortly in this place. 



The measures which must be taken are : — 



(1.) Cutting off a further supply of sand from the sea. 



(2.) Fixing the sand temporarily, so as to allow sowing or 

 planting. 



(3.) Growing a crop of trees and bushes, which will per- 

 manently fix the sand. 



(4.) Maintaining permanently a crop of trees and shrubs. 



The first of these four measures is based on the fact that, 

 although air currents are capable of moving the sand along 

 level and gently sloping ground, they cannot lift it above a 

 certain height. Hence it is necessary, at a moderate distance 

 (100 — 300 feet) from high water level, to form an artificial 

 hill, which is high enough to arrest the forward movement 

 of the sand, and this is done by the construction of an artificial 

 dune, generally called the "littoral dune." With Ibis object 



