DRAINAGE MODIFICATIONS 579 



movement, when A^ is depressed to TV', the stream M, N, O will 

 have been seriously retarded by the decreased gradient of that 

 portion of its upper course which is within the limits of the 

 crustal movement. In this first stage, as shown by Fig. 6, the 

 stream has lost almost all of its gradient, and corrasion along 

 the lower course of the stream has not been able to relieve the 

 sluggish portion of its upper course. It is incorrect to consider 

 the entire upper course as retarded ; the portion below the axis 

 of depression, or that portion which is tilted toward the head of 

 the stream, is rendered sluggish ; but that portion which is 

 above the axis is considerably accelerated by the downward 

 tilting and the stream will corrade its channel back into the 

 former divide. The stream Q, P, will suffer by the depres- 

 sion of its headwaters, and so will be deprived of the power to 

 hold its own against the opposing stream. As in the cases 

 already discussed this divide will migrate toward the weaker 

 stream, or away from the axis of depression. 



If the subsidence continues, the point TV will soon be lower 

 than the stream channel farther down, and consequently pond- 

 ing will ensue, until the ponding waters can find an outlet in 

 some other direction. This is supposed to have occurred in Fig. 

 7, and a transverse stream is located at the point where the axis 

 crosses the course of the former stream. A new divide is thus 

 formed between M and N, its position depending upon the 

 readiness with which the waters find an outlet at N. On the 

 other hand, the divide R will have migrated to R^. In the last 

 stage the divide 5 will have reached M and the divide R'^ 

 reached R-. 



In comparing the results obtained when the movement is 

 depression with those produced by elevation, it will be found 

 that the changes are of the same character whether the move- 

 ment is elevation or depression, or whether it is fast or slow. 

 In all cases the divides will tend to migrate up the slope of the 

 tilted surface, and they will so continue until the point is 

 reached where the surface is unaffected, or else is inclined in 

 another direction. 



