EFFECTS OF UNEQUAL HARDNESS 



EFFECTS OF UNEQUAL HARDNESS 



Inequalities of hardness give rise to many peculiarities of topog- 

 raphy, and to many scenic features. To this category belong m;my 

 rapids, falls, narrows, terraces, and many striking hills and ridges. 



I-'i^. 74. Diagram illustrating the development of a fall where the hard layer 

 dips up-stream. 



Falls and rapids. Falls and rapids are most commonly de- 

 veloped where streams pass from more resistant to less resistant 

 rock. The greater wear of the 

 latter gives origin to rapids. 

 At first the rapids are slight (a, 

 Fig. 74), but they become more 

 considerable (b) as time and 

 erosion go on. When the bed 

 of the rapids becomes so steep 

 'that the water falls (as at cd) 

 rather than flows over the 

 rock surface below the hard 

 layer (ha), erosion assumes a 

 new phase. The hard layer is 

 then undermined (Fig. 76), and 

 the undermining causes the 

 falls to recede. This phase of 

 erosion is sometimes called 

 sapping. 



If the hard layer which 

 occasions a fall dips up-stream 

 (Fig. 74), its outcrop in the 

 stream's bed becomes lower as 

 the fall recedes. When it has 



become so low that the water 



Fig. 75. Lower fall of the \ ellowstone. 

 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 the reverse of the history 



