9 8 



PHYSICAL GEOLOGY 



The effect on 

 stream flow of for- 

 ested and deforested 

 (Fig. Jj) areas is well 

 illustrated near Bilt- 

 more, North Caro- 

 lina. The David- 

 son River has its 

 upper drainage basin 

 in the Pisgah for- 

 est; the Tuckasegee 

 River in a defor- 

 ested land that has 

 been logged, burned 

 over, pastured and 

 farmed. The two 

 areas drained are of 

 geologically the same 

 age and structure ; 

 the headwaters of the 

 streams are found 

 within the same 

 range of mountains; 



the rainfall of the two areas is the same; the steepness of slope of 



the two watersheds is about the same. Yet the Tuckasegee, though 



the larger river, shows greater fluctuation in discharge than the 



Davidson; and the Davidson is practically free from sediment, 



while the Tuckasegee 



bears gravel and 



sand which it often 



spreads over fertile 



lands. 



Growth of Valleys. 



— It is possible to 



study the growth of a 



valley in almost ;m\ 



region. Water docs 



not flow down a 



slope in sheers for |. [G ?s 



Fig. 77. — Rapid erosion of deforested land and one 

 method of preventing further erosion. (U. S. Geol. Surv.) 



loriK distances, but 



— A young valley, western Nebrask. 

 work of the strcaui h;is only begun. 



