166 ELEMENTARY GEOLOGY 
Fia. 81. 
So a_ broad 
and gently slop- 
ing river val- 
ley represents 
greater age or 
longer period of 
work than the 
narrow, angular 
A general view of the Horseshoe Falls of Niagara. Where 
gorge. 
weathering is slow, as in the arid regions of the West, 
even valleys of considerable age are angular, and 
rugged gorges or canons 
are common. Gorges are 
also numerous In moun- 
tains, because the torrent 
flowing in a steep valley 
deepens it faster than it 
is widened by weathering. 
The river is the chan- 
nel in which the surplus 
rainfall flows away. In 
its course it either picks 
up or otherwise receives 
sediment by the aid of 
which it deepens its val- 
ley, beg aided in this 
work by a certain slight 
Fic. 82. 
Niagara gorge below the falls. The hard 
limestone is seen at the top of the cliff, 
while the softer shales form a more 
gentle slope below. 
