DRA INA GE MODIFICA TIONS 659 



course ; its constant tendency is toward courses at right angles 

 to, or parallel with the axis. If the movement is enough to 

 give the rocks an appreciable dip, the stream, in corrading its 

 channel, will tend to cut its banks on the lower side, and in doingr 

 so will migrate down the slope of the beds, but in all such cases 

 the stream will move as a whole, still retaining its parallelism 

 with the axial line. If the stream flows, in general, parallel with 

 the axis, but in a broadly meandering course, the tilting will give 

 to the stream a tendency to cut off its ox-bows and so straighten 

 its course and at the same time migrate away from the axial line. 

 This change is produced by the retardation of the current in 

 that portion of the bend in which the stream flows toward the 

 axis, and an acceleration in that portion in which the stream flows 

 away from the axis. Figure 9 represents such a stream on a 



Fig. 9. 



surface tilted in the direction of the arrow. In the course a b 

 the grade is lessened by the tilting and consequently but little 

 corrasion is accomplished but in the course c d\\\Q, grade is steep- 

 ened and corrasion is greatly stimulated. As a result of this 

 change in the grade of the stream, the channel at d is very much 

 lower than at a, hence a small stream may easily work back 

 across the neck of the bend and capture the main stream at the 

 point a. This process tends to straighten the course of the 

 stream and at the same time causes it to migrate away from the 

 axial line. 



Again the original course of a stream may be neither parallel 

 with, nor perpendicular to the axial line, but may pursue a diago- 

 nal course indicated by ^4 ^ in Fig. 10. If such a stream has a 

 branch {A C^ which flows parallel with the axis, under certain 

 conditions a small branch (c? b^ of the lower stream ma}' cut 

 through the divide separating the two streams and rob the diag- 

 onal stream of its upper portion. Ordinarily such a transfer 



