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 (0). 
PS 
UR yie tay eee 
Fig. 46. — Final Slope of 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 
