129 



itself furiously aofainst the outer bank of the curve, eatins: into the 

 clay of which it is composed, until it is undermined, allowing a great 

 mass of clay to slide down into the stream bed where it is eaten up 

 and carried away by the rushing water (Fig. 82). Farther on the 

 brook dashes down a steep, rocky incline, and if we listen and watch 

 we may hear the thud of boulders hurled along or even see a pebble 

 bound out of the muddy foaming water. These moving pebbles 

 strike against each other and grind along the bottom, wearing them- 

 selves out as well as the large unmovable boulders or the rocky bed 

 of the l^rook. Thus the larger stones are ground down, rounded at 

 lirst but in time reduced to sand, adding in this way to the moving 

 burden of the brook. By this slow process of cutting and grinding 

 the deep rock gorges of Xew York State, those at Watkins, Ithaca, 

 Au Sable Chasm, and even the mighty gorge of Niagara, have been 

 made. The Grand Canyon of the Colorado, over a mile in depth, is 

 one of the greatest examples of stream cutting to be found in the 

 world. 



Now the brook leads us into a dripping woodland and just ahead 

 we can lieai* the roar of a little waterfall, for at this point the cut- 

 ting stream has uncovered and flows upon the bed rock with its 

 alternating bands of hard and soft rock through which the busy 

 brook is cutting a miniature gorge. Here is a hard layer which the 

 stream has undermined until it stands out as a shelf and over which 

 the w^ater leaps and falls in one mass with a drop of nearly ten feet. 

 Watch how the water below boils and eddies ; think with what 

 force it is hammering its stony catting tools upon the rocky floor. 

 Surely here is a place w^here the brook is cutting fast. Notice that 

 swirling eddy where the water is whirling about with the speed of 

 a spinning top ; let us remember this eddy and when the water is 

 lower we will try to see what is happening at its bottom. 



On the other side of the woods our brook emerges into a broad 

 meadow ; let us follow it and see wdiat becomes of its load, whether 

 it is carried always on or whether the tired brook lays it down occa- 

 sionally to rest. Out of the woods the brook dashes down a steep 

 incline until the foaming tide comes to rest in a deep pool. What 

 becomes of the large pebbles wdiicli have been swept down ? Do they 



521 



