DRA IN A GE MODIFICA TIONS 5 7 5 



represented bv tlic arrow, it would take successively the 

 l)ositions Z?', Z)^ and D'^, and the divide C, if it remain stationary, 

 would occupy the positions C% C^ and C'^. If the movement 

 were sufficiently rapid, so that erosion produced no sensible 

 effect, it is obvious that when the point D reached D'^, it would 

 be at a greater altitude than C'^, and consequently the divide 

 would be shifted from C \.o D ; thus the stream flowing toward 

 E would be beheaded and the stream flowing toward A would be 

 increased by the beheaded portion. 



It is not at all probable that the great majority of crustal 

 movements are rapid enough to produce this effect. Let us 

 examine the problem and see if a slower rate of elevation would 

 affect the drainage. 



Suppose that the rate of elevation is but little more than the 

 rate of erosion. Under the supposition the condition of the 

 divide would be represented approximately by Figs. 2, 3, and 4, 

 Generally the first result of the uplift is the formation of a 

 barrier at the point D, but if the rate of movement is very slow 

 and the rocks soft, the stream may cut away this barrier as fast 

 as it is produced by the upward movement. The rising of the 

 land and the cutting of the stream continue until the elevation 

 reaches D''\ at that period of development the condition of the 

 streams and the divides is shown in Fig. 2. The original 

 surface is represented by the broken line, A, B, C^, D^, E. 

 According to our assumption, the streams were nicely balanced 

 against each other before the uplift occurred, hence the slightest 

 elevation at D would raise a barrier in the pathway of the stream 

 C^ D, E, and while this stream may be able to remove the barrier, 

 it involves a certain expenditure of time and energy which the 

 stream A, B, C, is not required to make. Thus the effect of such 

 a barrier is to retard the stream which it crosses, but in the case 

 under consideration the uplift not only retards one stream, but 

 it steepens the grade of the other and consequently accelerates 

 its corrasive power near headwaters. 



Under such favorable conditions, it would cut rapidly into 

 the divide at (7 while the other stream is expending its energy in 



