154 



r.KOLOGY 



rapids are slight (a, Fig. Ill), but they become more considerable 

 (b) as time and erosion go on. When the bed of the rapids becomes 



Fig. 111. Diagram illustrating the development of a fall where the hard 

 layer dips up-stream. 



sufficiently steep, the rapids become falls 1 (cd). When the water 



falls rather than flows over the 

 rock surface below the hard 

 layer, erosion assumes a new 

 phase. The hard layer is then 

 undermined, and the under- 

 mining causes the falls to recede. 

 This phase of erosion is some- 

 times called sapping (Fig. 113). 

 If the hard layer which 

 occasions a fall dips up-stream 

 (Fig. Ill), its outcrop in the ! 

 stream's bed becomes lower as 

 the fall recedes (c). When it 

 has become so low that the 

 water passing over it no longer 

 reacts effectively against the 

 less resistant material beneath 

 (/), sapping ceases, and the fall 

 is then transformed again into | 

 rapids. The history of rapids 

 which succeed falls is tin- 

 verse of the history which lire- 

 ceded. Tne later rapids are 



steepest at the beginning of their history, the earlier at their end. 



Stated in other terms, rapids are steepest when nearest falls in 



1 The terms rapids, falls, and cataracts are rather loosely used. Many 

 moderate rapids are incorrectly called falls. The "Falls of the Ohio" is 

 an example. The term cataract is often applied to very steep rapids or falls. 



Fig. 112. The lower fa 

 lowstone. 



