rOEMATION OF DUNES. 201 



form of a descending talus (Fig. 90). It is due to the knowledge of 



Fig. 90.— Pormation of Sand Dunes. 



these facts, that we are able to force the elements to construct a pro- 

 tecting rampart of dunes on various points of the coast threatened 

 with erosion by the waves of the sea.* 



Such are always the commencement of dunes, whatever be the 

 object which opposes itself to the wind. It is easy to convince one- 

 self of this by the sight of the houses or huts which the Custom- 

 Ilouse officers and shepherds erect in the sandy hollows of the dunes 

 of the Landes, not yet fixed by seedling trees.- On the side to- 

 wards the sea^ which is also that from which the wind blows in 

 terrible gales, the dwelling remains separated from the talus of sand 

 by a ditch of defence, as regular as if it had been hollowed out by 

 the hand of man ; but on the side which fronts inland the sand is 

 gradually heaped up, and if it is not swept away, does not fail to rise 

 soon to the level of the roof. 



On the slightly-undulating plateau which extends at the foot of the 

 grand pyramids of Egypt we can also study the same phenomena. 

 The winds from the east and north-east, which strike against the 

 eastern face of the enormous masses of stone, rebounding and de- 

 veloping their reflected waves on the ground, do not allow the sand 

 to be deposited on the lower steps of the edifices. It is only at a 

 certain distance, at the precise spot where the current is neutral- 

 ized by the masses of air coming directly from the east, that the 

 dunes can form. To the west of the pyramids, on the other hand, 

 long mounds of sand, more or less inclined, support themselves 

 against the base of the monuments. In the same way, at the foot 

 of certain clifis of Liguria, w^here the sands accumulate in dunes, 

 there always exists a sort of trench between the rock and the mov- 

 ing heap. 



When the labour -^of man does not Intervene to arrest the progress 

 of the dunes formed on the sea- shore, the various obstacles which 

 have determined the accumulation of the sands disappear at first on 

 * See below the section entitled, The Work of Man. 



