170 PHYSICAL GEOLOGY 



water gap, whose hard rocks continue to act as a base level 

 for the drainage crossing them. It is evident that the divide 

 between the beheaded stream and the pirate system will shift 

 at the expense of the former, for the slope in that direction is 

 very gentle, while that toward the latter is steep. The result 

 is shown in Figure 176, where the divide has migrated to a per- 

 manent position at the outcrop of the hard rock. The be- 

 headed river now rises to the left of the mountain ridge, 

 and the gap, abandoned by the stream which cut it, has 

 become a wind gap. Wind gaps are common in the Appa- 

 lachian Mountains. Through piracy, streams tend so far 

 as possible to get off the hard rocks and upon the soft 

 rocks. Thus they adjust their courses to the structure of 

 the beds. The result is appropriately called structural 

 adjustment. 



While stream capture is most common in regions of tilted 

 or folded strata, it is not confined to them. A river may 

 be able to capture the waters of neighboring streams because 

 favored by larger volume, less resistant rock, the character 

 or amount of its load, or a shorter course to the sea. 



Questions 



1. Why are steep slopes characteristic of arid regions? 



2. State the conditions necessary for the development of a sub- 

 sequent falls. 



3. Enumerate all the factors upon which the length of life of a 

 given waterfall will depend. 



4. What inference concerning the structure of the beds under- 

 lying a given region may be made : (1) From the fact that its eleva- 

 tions are ridges in parallel arrangement ? (2) From the fact that 

 the eastern slopes of its north-south ridges are relatively long 

 and gentle, while the west-facing slopes are short and steep? 

 (3) From the fact that its elevations are without systematic ar- 

 rangement ? 



5. How many cycles of erosion are recorded by Figure 162 ? 



6. (1) What is the age of the valley shown in Figure 177, and how 

 is it shown ? (2) What kinds of work is the river doing ? (3) How 

 will the river modify its valley in the future? (4) What was the 

 origin of the waterfall ? (5) What will be its future ? (6) What 



