SAND DUNES. 



623 



charge for new work and for maintaining the dunes which have 

 been already fixed having been 8,400 in 1893. In this way 

 vast areas have been saved for agriculture, and enormous 

 tracts of pine-forests created, which afford work to a large 

 number of people in the extraction of resin and turpentine 

 from the trees, and in timber-works, the pine-forests of the 

 Landes now exporting about 600,000 tons of pit-timber to 

 Great Britain annually, besides large supplies of timber and 

 firewood for local use. 



2. Construction of a Littoral Dune. 



As an embankment along the coast prevents the wind from 

 driving the sand inland, the chief point to be secured is to fix 



a. The Lion Dune. 



b. The Porge Dune. 



c. The Ferret Dune. 

 Fig. 284. 



the sand, so as to form what is termed the littoral dune, the 

 mode of construction of which is as follows : 



Two parallel fences are erected along the coast, between and 

 on both sides of which the sand accumulates, the fences being 

 gradually raised till the dune has attained such a height that 

 only inappreciable quantities of sand are blown over it. 



The fence facing the sea is a continuous line of paling, from 

 300 to 600 feet distant from high-water mark ; it is made of 

 inch planks 6 J feet long, 6 to 8 inches wide, which are pointed 

 below. They are inserted to f ths of their length into the sand, 

 and 1 inch apart, to allow sand to blow through. When the 

 sand has nearly covered the planks, they are raised three feet 



