PROTECTION AGAINST FIRES AND MOVING SAND 



33 



the different conditions encountered. The damage to oases has 

 resulted (i) from sand drifting from a distance, (2) from local 

 overgrazing, and (3) from erosion. 



Where the drifting sand is blown from a distance no perma- 

 nent relief can be hoped for unless the direction of the wind 

 changes. In this case the damage is uniformly from one direc- 

 tion and the drifting sand can be rendered harmless by proper 

 protection. When the sand appears merely as a result of local 

 overgrazing disintegrating a friable soil, it can be readily pre- 



FIG. 7. Typical sand wall topped with date-palm branches in use at 

 Tozeur, southern Tunisia. 



vented by excluding all stock from a protective zone, but until ( 

 the soil is healed by grass and weeds the damage is more dif- 

 ficult to control, because the direction of the local winds is not \ 

 uniform. The erosion is prevented or diminished by contour 

 ditches and by sand dams, where gullies have already been 

 washed. 



The typical desert oasis in southern Tunisia is not a mere 

 spring in the desert, but, in reality, a rich date-palm farm hun- 

 dreds of acres in area, whose ownership is shared by the resi- 

 dents of bordering villages. The water for irrigation flows from 



