252 The Realm of Nature CHAP. 



330. Rivers and the Land Surface. When a river is 

 fairly established in its valley it is the most permanent 

 feature of a land surface. Upheaval, which acts very 

 slowly, may even elevate a range of mountains across its 

 course, while the river cutting its way downward remains 

 at the same absolute level. The Uintah mountains were 

 elevated in this way across the course of the Green River, 

 one of the tributaries of the Colorada ( 364). The 

 range in such a case rises divided, like a bar of soap 

 pressed upward against a horizontal wire. Where a 

 river crosses soft and regularly placed rocks its valley 

 is comparatively wide, the sides of gentle slope, and the 

 gradient of the stream uniform ; but where a strip of 

 hard rocks is encountered the valley narrows into a 

 steep-sided gorge, and the gradient of the river will be 

 suddenly changed. In such circumstances the hard rock 

 is cut through more slowly, and above it the gradient is 

 reduced to what is termed the base -level of erosion, where 

 no destructive action can take place but alluvial deposits 

 are formed. The softer rock farther down stream being 

 eroded more rapidly, a waterfall is formed over the hard 

 ledge, which is worn through in time, and a line of rapids 

 formed in the short portion of steep slope. Eventually the 

 gradient of the bed becomes uniform and the rapids also 

 disappear. The great waterfall of Niagara is caused by 

 thick beds of hard limestone (black in Fig. 53) resting on 

 soft shale. The river flowing over the cliff formed by the 



FIG. 53. Ideal Section of Falls of Niagara. 



edge of the limestone cuts away the soft shale from below 

 and so produces occasional slips of the overhanging rock, 

 causing the falls steadily to recede. The falls are now at 

 the head of a gorge 7 miles from the escarpment of the 

 limestone cliff, where the rock is being eroded much less 

 rapidly by weathering. From recent surveys it is stated 



