22 



IOWA STUDIES IN NATURAL HISTORY 



have been reduced to grade, those streams which flow the 

 shortest distance to the sea will have higher gradients than 

 others and will therefore gradually absorb these other 

 streams. The result is a drainage system in which the 

 main streams flow the shortest distances to the sea irres- 



Fig. 7. Diag-ram to illustrate the change from Stage I to Stage II of stream ad- 

 justment in regions of folded strata. (After Davis). 



pective of rock structure or hardness, and even the tribu- 

 taries flow into the mains by the shortest routes (Stage III, 

 Fig 8). Near the divides where the streams are not at 

 grade, tributaries may still be flowing parallel with the 

 strike, controlled by the structure. In this third and final 

 stage of adjustment the courses of the streams are again 

 controlled by the topography, but the topography is not the 

 same as it was in the first stage. Roughly, Stage I would 

 correspond with youth of the cycle of erosion. Stage II with 

 maturity, and Stage III with old age. 



If a region in which the streams have gone through the 

 three stages of adjustment, be uplifted relative to the sea 

 and the streams hold their courses during and after uplift, 

 the streams in the second cycle would be antecedent accord- 

 ing to the more recent use of the term. In a region of fold- 



