WEATHERING 



17 



nel is due mainly to the mechanical wear or friction of the material 

 carried by the water (Fig. '.)) Clear water abrades very slowly. 

 A rapidly flowing stream carrying large amounts of material abrades 

 its bed very rapidly. This may be illustrated in the valleys that 

 have been cut by streams that contain water only after very heavy 

 rains. Level plateaus have been dissected and changed into a rugged 

 country of hills and valleys by comparatively small wet weather 

 streams. The entire land surface has been greatly modified by this 

 process and the transported material used largely in soil formation. 





Fio. 10. Inner gorge of Grand Oaflon of tho Colorado Hivor, Arizona. (Walrott, I'. S. 



Cionl. Survey.) 



Captain C. E. Dutton 2 estimates that 10.000 IVct of rock strata 

 have been removed from an area of LS.OOO to 15,000 square miles 

 by the Colorado River (Fig. 10). 



When quart/ is ground up through the action of moving water 

 much sand is produced, and after these particles have been reduced 

 to a certain size the permanent water film protects them largelv from 

 further attrition. On the other hand, feldspars when subjected to 

 attrition form an impalpable mud or clay accompanied by consider- 

 able loss of bases such as potassium, sodium, or calcium, according 

 to the kind of feldspar. 

 2 



