THE WORK OF STREAMS 



89 



the flood has subsided the channel thus deepened may be entirely 

 filled with sediment. This process is called scour and fill. The 

 Missouri River sometimes scours out its channel to depths of from 

 70 to 90 feet and later fills it again. It is evident that the deepening 

 of the beds of such rivers is largely confined to high water. A fail- 

 ure to understand scour and fill has led some observers to assign a 

 great age to stone implements found deeply buried in river gravels 

 (p. 680). 



Lateral Erosion. — When a young river is deepening its valley it 

 flows in a narrow channel between steep banks, but since its course 

 is seldom straight it tends in places to cut more on one bank than 

 on the other, with the result that as it cuts downward it also cuts 

 sidewise, thus widen- 

 ing its valley. By the 

 time grade is reached 

 the valley walls will 

 have flared open, but 

 will be steeper on the 

 outside of each curve. 



Unsymmetrical valleys are formed (1) in this way and also (2) by the 

 greater hardness of the rock on one side of the stream than on 

 the other (Fig. 63) (where the strike of the rock parallels the course 

 of the stream). 



When two neighboring streams have ceased to degrade their beds, 

 they will cut laterally and may in time wear away the divide which 

 separates them, thus causing one to flow into the other. 



Fig. 63. 



Unsymmetrical valley formed as a result 

 of the dip of the rock. 



Features Due to Stream Erosion 



Falls and Rapids. — Falls and rapids result from a number of causes. 

 (1) Regions in which a harder layer of rock overlies a softer one fur- 

 nish most favorable conditions for the formation of falls (Fig. 64). 

 When a stream, in deepening its valley, encounters a harder bed of rock 

 lying in the position shown in the diagram (Fig. 65), the less resistant 

 beds are worn more rapidly than the harder ones, and a rapid will 

 result first, which upon further erosion will become a fall. Falls 

 become lower and lower in the course of time, until the resistant beds 

 form mere ledges in the stream bed and the falls cease to exist (Fig. 

 65). Niagara Falls (Fig. 66) have gradually cut back until now they 

 are seven miles from their original position. The recession of these 



