100 IOWA STUDIES IN NATURAL HISTORY 



reversed stream, that is, in a line roughly parallel with the 



dip of the strata and with the original slope of the surface. 



Finally when old age has been reached and the main 



Fig. 29. Block diagram showing the drainage conditions in Stage II of stream ad- 

 justment. 



Fig. 30. Block diagram illustrating Stage III of stream adjustment, in case the 

 south-flowing stream has the advantage of the north-flowing one. 



streams have been reduced to grade throughout their 

 courses, and not before that stream which has the shortest 

 route to the sea would capture all the drainage and would 

 adopt a course, which for the first time would be indepen- 

 dent of structure and original slope and dependent upon 

 the slopes of the graded plain. Some of the tributary 

 streams, of which there would be few, might still be depen- 

 dent upon structure (Fig. 31). 



