xni Action of Water on the Land 251 



on a magnificent scale in the plateaux west of the Rocky 

 Mountains, where they have received the name of Canons. 

 The most wonderful example is the Grand Canon of the 

 Rio Colorado about 400 miles in length, in many parts from 

 4000 to 7000 feet beneath the level of the plateau, and 

 with very steep terraced sides that strike the eye as vertical 

 walls. 



328. River Work on Rainy Plateaux. A river flowing 

 over a rainy plateau cannot form a canon or V-shaped 

 gorge because of the number of small tributaries it receives, 

 each of which helps to reduce the slope of the valley walls. 

 The action of rain on the cliffs leads to occasional landslips, 

 forming a gently sloping talus which protects the lower 

 rocks from erosion and gives the valley a U-shaped section. 

 Only in places where the rocks are hard and vertically 

 jointed and the river strong can the talus be swept away as 

 it is being formed, and a steep-sided gorge result. The 

 valleys excavated across a plateau in rainy regions become 

 wider as they grow older ; and according as the rate of 

 denudation over the whole area is nearly equal to, quite equal 

 to, or more rapid than the deepening of the river-bed, the 

 apparent depth of the valley increases very slowly, remains 

 unchanged, or actually diminishes. 



329. Mountains of Circumdenudation. To a traveller 

 ascending the Colorado River the sides of the canon 

 appear like lofty and precipitous mountain ranges, and 

 where a tributary canon enters, the appearance of the two 

 meeting slopes is exactly that of a mountain. On the 

 summit instead of a peak there is a vast plateau stretching 

 out as a boundless plain, broken by massive buttes, the 

 remnants of more resisting rocks left as monuments of 

 denudation. In a rainy region the valleys of adjacent rivers 

 cut up the plateaux into rounded blocks of elevated land, the 

 exact form of which depends on the composition and 

 arrangement of their rocks. Most geologists believe that 

 the mountains of Scotland and of Norway have been carved 

 out in this way from a solid plateau of great height by the 

 agency of rain, streams, springs, and ice, guided by the 

 durability and structure of the rocks (contrast 295, 303). 



