21 the what and the why of desert country 



northern Arizona and southern Utah owe their unusual 

 forms to the sculpturing of wind-blown sand, or that sheer 

 cliffs often rise from a sloping cone of rocks and boulders 

 because the talus slopes can accumulate in just that way 

 only where there is not enough draining water to dis- 

 tribute them over the whole surrounding plain, as they 

 would be distributed in regions of heavier rainfall. But 

 the most striking example of all is the greatest single 

 scenic wonder of the region, the Grand Canyon itself. 

 This narrow gash, cut a mile deep through successive 

 strata until the river flows at last over some of the oldest 

 rock exposed anywhere on earth, could have been formed 

 only in a very dry climate. 



As recently as two hundred years ago the best informed 

 observer would have taken it for granted that the river 

 was running between those sheer walls at the bottom of 

 the gorge simply because it had found them out. Today 

 few visitors are not aware that the truth lies the other way 

 around, that the river cut its own course through the rock. 

 But most laymen do not ask the next questions: Why is 

 Grand Canyon unique, or why are such canyons, even on 

 a smaller scale, rare? And the answer to those questions 

 is that a set of very special conditions was necessary. 



First there must be a thick series of rock strata slowly 

 rising as a considerable river flows over it. Second, that 

 considerable river must carry an unusual amount of hard 

 sand or stone fragments in suspension so that it will be 

 able to cut downward at least as rapidly as the rock over 

 which it flows is rising. Third, that considerable river must 

 be flowing through very arid country. Otherwise rain, 

 washing over the edges of the cut, will widen it at the 

 top as the cut goes deeper. That is why broad valleys are 



