LAND WATERS STREAMS 



191 



STREAM TERRACES 



Stream terraces 1 are bench-like flats or narrow plains along the 

 sides of valleys (Fig. 163-4) and above their bottoms. They are 

 usually narrow, but sometimes have great length in the direction 

 of the axis of the valley. 



The commoner river terraces are remnants of former flood- 

 plains, below which the streams which made them have cut 

 their channels. They originate in various ways. (1) Some are 

 due to inequalities of hardness (Fig. 117). (2) If an alluvial flood- 



I Fig. 163. Terraces on the Fraser River at Lilloet, B. C. (Photo, by Calvin.) 



plain has been built as the result of an excessive supply of sediment 

 (p. 183), the exhaustion or withdrawal of the excessive supply 

 would leave the stream again relatively clear, and free to erode 

 where it had been depositing. It would forthwith set to work to 

 carry away the material which it had temporarily unloaded on the 

 plain. The plains built up in many valleys in the northern part 



'For discussions of terraces see Gilbert's Henry Mountains, p. 126: 

 Davis River Terraces in New England, Bull, of the Mus. of Comp. Zool. 



rrt v1 es xx o1 ' V ' pp ' 282 ~ 346; and Dodge ' Proc - Boston Soc. of Nat. 



