378 



EARTH FEATURES AND THEIR MEANING 



amazing rate for each increase of its grade. No such law controls 

 the processes of plucking and abrasion by which the glacier lowers 



its floor, for these processes 

 appear to depend for their 

 efficiency upon the depth of 

 the ice, and the supply of 

 cutting tools, quite as much 

 as upon the grade of the 

 bed. To apply a homely 

 illustration, the hollowing 

 of flagstones upon our walks 

 is dependent more upon the 

 number of persons that pass 

 over them, and upon their 

 size and the number of pro- 

 truding nails in their boot 

 heels, than upon the grades 

 upon which they are placed. 

 At all events we find that 

 the main glacier valleys are 

 cut deeper than the side 

 valleys, so that the latter 

 FIG. 403. Gorge of the Albuia Paver near become hanging v alley s - 



Berkum in the Engadine, cut through a rock they enter the main Valley, 



distance above it (Fig. 404) . 

 The U-shaped hanging valleys, like the main valley, are much 

 too large for the 

 streams which now fill 

 them, and these di- 

 minutive side streams 

 plunge over the steep 

 wall of the main valley 

 in ribbon-like falls so 

 thin that the wind 

 turns them aside and 

 disperses the water in T 



u FIG. 404. Idealistic sketch showing both glaciated 



the Spray OI a bridal and non-glaciated side valleys tributary to a glaci- 

 veil." Such falls are ated main valley (after Davis). 



