754 N- M. FEN NEMAN 



it is quite the rule that the sides of the smaller valleys are upwardly 

 convex from top to bottom. 



In the course of its development the young valley whose cross- 

 section shows simple curves increasing in steepness as the axis is 

 approached, exchanges these for compound curves as described above. 

 The stage of development at which this exchange is made depends 

 partly on the absolute rate of down-cutting of the axis and partly on 

 the behavior of the wash. A full explanation of the latter would 

 involve discussion of materials composing the surface. Observa- 

 tion indicates that with a given rate of down-cutting a loess cover is 

 specially favorable to valleys, whose sides increase in steepness down 

 to the axis, that is, are simple curves and upwardly convex. 



