VIII RIVERS AND LAKES 317 



I cannot therefore resist the conclusion that 

 glaciers have taken an important part in the 

 formation of lakes. 



The question has sometimes been discussed 

 as if the point at issue were whether rivers or 

 glaciers were the most effective as excavators. 

 But this is not so. Those who believe that 

 lakes are in many cases due to glaciers might 

 yet admit that rivers have greater power of 

 erosion. There is, however, an essential dif- 

 ference in the mode of action. Rivers tend 

 to regularise their beds ; they drain, rather 

 than form lakes. Their tendency is to cut 

 through any projections so that finally their 

 course assumes some such curve as that 

 below, from the source (a) to its entrance into 

 the sea (b). 



Fig. 46. —Final Slope bf a River. 



Glaciers, however, have in addition a scoop- 

 ing power, so that if similarly a d b in Fig. 

 47 represent the course of a glacier, starting 

 at a and gradually thinning out to e, it may 



