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 



