24 



GEOLOGY, 



sage-brush hold the soil immediafcely about them, but between the 

 clumps of brush, where there is little other vegetation, the wind has 

 often blown away the soil to such an extent that each clump of brush 

 stands up several inches, or even a foot or two, above its surroundings 

 (Fig. 5), Such mounds are often partly due to the lodgment of dust 

 about the bushes. 



Fig. 5. — This figure shows the effect of sage-brush or other similar vegetation in hold- 

 ing sand or earth, or in causing its lodgment, in dry regions. 



Where the earthy matter is moist, the cohesion of the particles 

 is great, and the wind cannot pick them up. Furthermore, if the 

 surface is generally moist, it is likely to be covered with vegetation 

 which protects it against the wind. But even where vegetation is 



Fig. 6. — Diagram to illustrate the way in which the wind sometimes strips the soil 

 from the edge of a bluff. This phenomenon is not rare in the basin of the Columbia 

 River in Washington. 



prevalent the wind finds many a vulnerable point. Thus on the edges 

 of plains or plateaus facing abrupt valleys, the wind attacks the soil 

 from the side, and in such situations all earthy matter may be stripped 



