230 CONSOLIDATION OF DBIFTING SANDS. 



the wind and forms moving hills, that may attain a height of more 

 than 200 feet. These hills, as we would naturally expect, have a 

 gentle slope on the sea-face but descend abruptly landwards. 

 Sometimes they are long, continuous, and disposed in regular and 

 parallel lines ; at other times they are broken or run zigzag. This 

 depends on the form of the coast line and the direction and con- 

 stancy or fitfulness of the winds. It is to these moving sand-hills 

 that the name of dunes has been given. The valleys between the 

 dunes, when these latter are devoid of vegetation, are generally 

 marshy. 



On the western coast of France the rate of progression of the 

 dunes towards the interior has been ascertained to be 14 feet a 

 year, and the quantity of sand thrown up annually by the sea to 

 form them about 1,000 cubic feet for every yard of coast-line. 



The essence of all the work of fixation is the arresting of the 

 new sand-drifts as soon as they are thrown up by the sea and the 

 protection of the interior hill against the full brunt of the winds 

 blowing from the sea. This double object is secured by the raising 

 of a protective wall with the help of the new deposits brought in 

 by the tides. This wall is nothing more or less than a dune, that 

 is designedly formed, and which, from its position, may be called 

 the LITTORAL DUNE. 



The littoral dune is formed by erecting and constantly maintain- 

 ing, parallel to the coast line and about 100 yards from high-water 

 mark, a continuous line of dead fences or hurdles, against which 

 the drifting sands are arrested. The sand collects on either side of 

 it in the form of a long hill, whose backbone is the line of fences 

 itself. 



On the west coast of France the following is the procedure ; 

 The fences consist of a line of paling constructed of planks about 6 

 feet long, 1 inch thick, and about 6 inches wide, and pointed at the 

 lower end. The planks are put into a trench about 1^ feet deep 

 and then driven into the sand, so that when the trench is filled up, 

 only a little more then a length of 3 feet remains above ground. 

 An interval of about one inch should be left between two consecu- 

 tive planks. The sand is deposited up against the paling sloping 

 gently seawards ; but some of it passes through the spaces left be- 

 tween the planks and forms a sort of backing for them, thereby 

 given them increased stability. As often as the sand reaches the 

 top of the paling and begins to cover it, the planks are pulled up a 

 few feet by means of levers or differential pulleys. In this manner 

 the littoral dime rises gradually higher and higher. All tendency 



