108 IOWA STUDIES IN NATURAL HISTORY 



ent ways, that the writer believes that these intrenched 

 curving streams, considered alone without reference to re- 

 lations between mains and tributaries, might have acquired 

 these curves without having experienced more than one 

 cycle. On the other hand it is believed that these intrenched 

 curves bear as much evidence in favor of the plural cycle 

 theory as intrenched meanders ever afford. 



Associated Sets of Crooked and Straight Streams 



But if the crooked courses of these streams within the 

 Driftless Area have been developed in a single cycle their 

 tributaries must have had the same histories as the mains 

 and should have courses as crooked as the courses of the 

 mains. Neither in the case of the above-mentioned streams 

 in the Driftless Area of Wisconsin and Illinois nor of those 

 in the slightly glaciated portion of Iowa do the mains and 

 the tributaries have comparable natures and degrees of 

 crookedness. 



The fact that the tributaries are less crooked than their 

 mains in Iowa is not significant, for it is possible that the 

 main streams developed their course in the drift and that 

 the tributaries were developed under different conditions 

 after superimposition had been accomplished. 



In the cases of those streams in areas which were not even 

 slightly glaciated, however, and in which the tributaries 

 are straight and the mains are notably curved, a suggestion 

 is offered that the histories of the main streams and of the 

 tributaries have not all been worked out in a single cycle. 

 It would seem likely that the mains developed meanders 

 when they were at grade in maturity or old age of a previ- 

 ous cycle of valley development and that either the tributar- 

 ies had not reached grade and therebore did not meander be- 

 fore the first cycle was interrupted, or the tributaries did 

 not exist at the close of the first cycle and have developed 

 their straight courses entirely under conditions of higher 

 gradient in the present cycle. 



There is a marked difference in degree of crookedness be- 

 tween tributaries and mains in practically all of the drain- 

 age systems intermediate in size between the largest and 



