70 



GEOLOGY. 



It is not to be inferred that the method of valley development which 

 has been sketched is the only one. The processes of valley develop- 

 ment are complex, and the history of some valleys has run a different 

 course; yet the processes outlined above are in operation in all cases, 

 and they were probably the most important ones in the development 

 of the first drainage system on any land surface. As will be seen in the 

 sequel the history of valleys is subject to serious accidents, and they 

 are often of such a nature as to mask the simplicity of the more normal 

 processes. 



Fig. 51.— Diagram illustrating the further development of Fig. 49. The land hero 

 has been reduced greatly, though not yet to base-level. 



The permanent stream. — From the foregoing discussion, it is seen that 

 a valley may be developed by the run-off of successive showers. If 

 supplied only from this source surface streams would cease to flow soon 

 after the rain ceased to fall, and a valley might attain considerable size 

 without possessing a permanent stream. How does the valley devel- 

 oped by the run-off of successive showers come to have a permanent 

 stream? The answer to this question involves a brief consideration of 

 that part of the rainfall which sinks beneath the surface. 



