826 THE GEOGRAPHICAL SIGNIFICANCE OF FLOODS, 



(ii.) Wide valleys jji-otected by fairly high hut Jlaring sides. 



The general shape and arrangement of the valley will determine 

 which side is specially selected for attack. Here again one must 

 look for local ice-currents as in the analogous case of eddies in 

 streams. One side of the valley will doubtless be protected 

 while active cutting is in progress on the other side. As in the 

 case of streams, one must not forget that although intense ice- 

 action is probable, it can have but local application. And the 

 wider the valleys, and the flatter the grades, all other things 

 being equal, the more limited the intense action. This arises 

 from the significance of the term base-level. Every water-pond 

 is a potential waterfall, its capacity for work depending on the 

 amount of head it may possess. Wide sluggish streams forced 

 into narrows, or into any position where gravity has freer action, 

 will accomplish wonders. The lower Amazon turned into the 

 Yosemite caiion would accomplish such work as to amaze even 

 experienced engineers. The tourist finds difficulty in recognising 

 the smooth, and almost greasily-surfaced above-the-fall water 

 as the same with that which pursues its mad career along the 

 torrent-track under the falls. Yet the sluggish upper mass and 

 and the lower " river gone mad, with boulders and mud for 

 water," are one and the same. Niagara and Zambesi are the 

 grand examples. It is but a simple application of the truth that, 

 for streams, the power of transportation varies as the sixth 

 power of the velocity. And here we perceive the reason for 

 limited evidence only of intense former ice-cutting in areas of 

 low relief. The points of intense action are those of great gravi- 

 tative thrust, such as great fall or marked convergence. In fiord 

 and alpine regions these conditions abound, hence magnificent 

 contours; but as one rarely finds a Niagara on the low country, 

 so one but rarely finds signs of magnificent rock-basining by ice 

 over country of gentle relief. 



Therefore the study of such glacial contours should properly 

 commence in alpine regions, and thence be carried back among; 

 areas of less marked cravitative thrust. 



