Introduction — Principles Involved. 7 



If conditions now change so that deposition again succeeds 

 erosion, the flat GH (Fig. 3) will be aggraded, as shown in Fig. 

 4, giving rise to formation 3. At the same time that the main 

 stream is aggrading its plain, its tributaries are of necessity 

 aggrading theirs, but with material of a different sort, if the 

 tributaries drain terranes different from those drained by the 

 main stream. 



Fig 4 



While formation 3 is being deposited, the accumulation of 

 waste on the gentle slopes between formations 3 and 2, and 

 between. 2 and 1 is in progress. At this stage then, the detritus 

 on the surface between formations 1 and 2 may be of any age 

 and all ages younger than 1, while that on the slope between 

 formations 2 and 3 may be of any age and of all ages younger 

 than 2. 



If subsequently erosion again succeeds deposition, new valleys 

 will be excavated in formation 3 of the main valley, and in the 

 corresponding formation in the tributary valleys. 



