RIVERS AND THEIR WORK 237 



257. Erosion. From the time rain falls upon the earth 

 until it reaches the sea, the water is doing the work of 

 erosion. Mantle rock and soft rock are carried away grain 

 by grain, leaving sometimes deep gullies, sometimes pyra- 



FIG. 125. THE COLORADO RIVER IN THE GRAND CANYON 



The river is flowing here in a gorge worn in granite rock which is as old 

 as any known. The highest elevations shown are not so high as the plain 

 in which the work of erosion began. 1. Describe the rock resting upon the 

 granite. 2. Give a reason why the valley is wider at the upper levels. 

 3. What may have become of the material removed from this valley? 



mids or irregular columns of material slightly harder than 

 the rest. Erosion continues to take place after the stream 

 is formed. The grains of sand carried by the current wear 

 upon the bed or banks of the river and scour them, as 

 sandpaper rubs off wood or soft stone. 



The Colorado River shows remarkable results of erosion 



