DENUDATION .-- \ : : ,.,^ r .ipl 



produce pot-holes or curving hollows and recesses (Fig. u). From 

 this we can pass to the relation of stream to valley, and show that 

 the inevitable effect of water running with sufficient velocity will 

 be to excavate and deepen the course along which it runs ; the 

 excavation being more rapid where the fall is most abrupt and 

 the speed of the running water greatest. Thus the work of a 

 stream is such that, even if there were no valley to start with, but 

 only a slope, a valley would certainly be excavated, and it is 

 therefore more likely that the stream has made its valley than 

 that it found one ready made and took possession of it. This is 

 one of the many cases where observations like those advised in 

 the last chapter can be utilised. 



Not all the work of a stream is carried out mechanically. A 

 good deal of it is performed by invisible chemical means. The 

 evaporation of filtered stream water will show that much dissolved 

 matter, chiefly consisting of salts of lime, magnesia, iron, and 

 alkalies, is carried by all rivers. 



It will be noted that while a stream is always carrying out a 

 certain amount of work, this is very much intensified during 

 storms and floods. At such times more erosive and trans- 

 porting work may be done than during years of steady flow. 

 This is an important principle which applies to nearly all forms 

 of denudation ; a slight increase above the normal power increases 

 the effect in a geometrical ratio. 



In limestone countries the absorption of much of the rainfall 

 into the ground and its travel along underground channels can be 

 demonstrated and its work observed or inferred, but it will be 

 hardly possible at this stage to explain the causes which bring 

 the water again to the surface. Underground water or that 

 issuing from springs might, however, be examined for suspended 

 and dissolved contents to prove that its denuding work is con- 

 tinued during its underground journey. The issuing of under- 

 ground waters at springs is one of the most important sources of 

 streams (Fig. 71). 



Neither will it be possible in this country to study directly 

 the transporting and erosive power of glaciers, which in high 

 altitudes and latitudes replace the streams with a flow of ice. 

 That is better deferred until deposition has been studied. 



