156 PHYSICAL GEOLOGY 



Least favored ? At what stage is road building most difficult ? 

 (Compare Plates III, IV, and V.) 



6. At what stage in the life of a river is it most likely to furnish 

 water power ? Navigation ? 



7. What is the age, in terms of erosion, of the area shown in 

 Figure 148? 



8. Make a diagram showing (1) the wet-weather and dry- 

 weather positions of the ground-water surface, and (2) the cross 

 sections of valleys which (a) frequently, (6) seldom, and (c) never, 

 "go dry." 



9. Compare and contrast the relief of two topographically 

 mature regions of the same original altitude, one of which is near 

 the sea, and the other far inland. 



10. Assuming that the rainfall is equal everywhere and the char- 



acter of the rocks everywhere the 

 same, what change will occur in the 

 position of the divide now at a in Fig- 

 ure 149 ? When will the divide cease 



to shift ? How will the character of the 

 FIG. 149. Diagram of a shift- , i . .-, j- -j r. 



in di id slopes change after the divide becomes 



stationary ? What would be the effect 

 if the rocks to the left of a were harder than those to the right ? 



11. What would be the width of valleys if they were due only 

 to the down cutting of streams? What inferences may be made 

 from the fact that most valleys are several times as wide as they are 

 deep? 



12. At what stage in its life should a river be engaged chiefly in 

 deepening its valley ? In widening it ? 



13. What is the age, in terms of erosion, of the area shown in 

 profile in Figure 150 ? The evidence ? 



,5600' 



Jsooo' 



FIG. 150. Profile showing the character of the surface in a portion of 

 southeastern Colorado. Length of section nearly ten miles. Vertical 

 scale about six times the horizontal. (U.S. Geol. Sura.) 



1600 



1200 

 800 

 400 







FIG. 151. Profile of an area in West Virginia, near Charleston. Length 

 of section, Q 1 /* miles. Figures show elevation above sea level. 



14. In what stage of erosion is the region shown in profile in 

 Figure 151 ? How told ? What is the age of the valley at the right 

 end of the section ? Of the rest of the valleys ? 



