THE TWO GREAT SCULPTORS. 115 
Shall you not feel a fresh interest in all the little 
valleys, ravines, and gorges you meet with in the 
country, if you can picture them being formed in this 
way year by year? There are many curious differ- 
ences in them which you can study for yourselves. 
Some will be smooth, broad valleys, and here the rocks 
have been soft and easily worn, and water trickling 
down the sides of the first valley has cut other chan- 
nels so as to make smaller valleys running across it. 
In other places there will be narrow ravines, and here 
the rocks have been hard, so that they did not wear 
away gradually, but broke off and fell in blocks, leav- 
ing high cliffs on each side. In some places you will 
come to a beautiful waterfall, where the water has tum- 
bled over a steep cliff, and then eaten its way back, 
just like a saw cutting through a piece of wood. 
There are two things in particular to notice in a 
waterfall like this. First, how the water and spray 
dash against the bottom of the cliff down which it 
falls, and grind the small pebbles against the rock. 
In this way the bottom of the cliff is undermined, and 
so great pieces tumble down from time to time, and 
keep the fall upright instead of its being sloped away 
at the top, and becoming a mere stream. Secondly, 
you may often see curious cup-shaped holes, called 
" pot-holes," in the rocks on the sides of a waterfall, 
and these also are concerned in its formation. In 
these holes you will generally find two or three small 
pebbles, and you have here a beautiful example of 
how water uses stones to grind away the face of the 
earth. These holes are made entirely by the falling 
water eddying round and round in a small hollow of 
