624 PROTECTION AGAINST SHIFTING SAND. 



by means of levers worked by one man against the next un- 

 raised plank, or by a stout rod passed through a chain fastened 

 round the plank and lifted by two men. This operation is 

 repeated until the proper height of the dune is attained, which 

 Grandjean places at 10 metres, or about 33 feet. 



The back fence is usually made of wattle-work and prevents 

 the sand which has passed through the front fence from being 

 carried inland, and gives a proper contour to the dune. This 

 fence is replaced by a new one when it has become covered 

 with sand. 



The preceding diagrams (Fig. 284) taken from Grandjeans' 

 work represent the ordinary sections of the littoral dune, and he 

 considers b and c preferable to a, as being much easier to 

 maintain. They have slopes of 26 or 27 facing the sea, which 

 corresponds with De R6gn6's natural slope of the dunes. 

 Grandjean, in the work already referred to, gives full and 

 satisfactory reasons for the height and gradient he prefers for 

 the littoral dune. 



3. Material for Fixing the Sand. 



The littoral dune can never be planted with trees, on account 

 of the salt spray of the waves and the strong sea-wind. In 

 certain places in Holland, masonry works and piles have been 

 erected to protect the dune, but as a rule flexible plants are 

 much more effective than rigid and costly works. Attention is 

 therefore directed to sand-grasses and sedges, and the best of 

 these for the purpose are : 



Sea marram, or matweed (Psamrna arenaiia, E. & S.) ; 



Baltic matweed (P. baltica, Schrad.) ; 



Sand lyme-grass (Elymus arenarius, L.) ; 



Sand sedge (Car ex arenaria, L.). 



All these plants are characterised by very long, much-divided 

 rhizomes, and can withstand being covered over and 'buried in 

 sand. The sea marram will grow only where it is continually 

 covered with fresh sand, and dies inland in the shelter of the 

 littoral dune, when no fresh sand covers it. It is termed 

 gourbet in France, and is exclusively used for fixing the littoral 

 dune along the Bay of Biscay. Next to the marram in repute 



