26 IOWA STUDIES IN NATURAL HISTORY 



the strike and other portions parallel with the dip. This 

 stage is reached before the streams have reached their depth 

 limits, and piracy may occur. Indeed, the change from 

 Stage I to Stage II is not accomplished without piracy. In 

 such cases streams parallel with the strike have the ad- 

 vantage of streams flowing across the strike and windgaps 

 are formed which might be indistinguishable from those 

 due to piracy in the second cycle. 



Windgaps are not conspicuous in regions of horizontal 

 strata nor in regions of massive rock, a fact which prac- 

 tically limits the application of windgaps to regions of fold- 

 ed or tilted strata. 



Another limitation in the use of windgaps as criteria for 

 more than one cycle of erosion has recently been emphasized 

 by Miller^ who explains that windgaps may be formed by 

 two streams working headward from opposite sides of the 

 same divide, developing a permanent divide between their 

 heads and forming a col. Such a col could hardly be dis- 

 tinguished from gaps which had once been occupied by 

 streams and then abandoned. A study of the relative sizes 

 and gradients of the streams on either side of the divide 

 might aid in determining the histories of such gaps. 



So difficult is it to distinguish wind gaps resulting from 

 piracy in the second cycle from those developed during or- 

 dinary adjustment or in the establishment of permanent 

 divides in the first cycle that it is doubtful if they would 

 ever, even under the most favorable circumstances aff^ord 

 important evidence of more than one cycle, taken alone. 



Even-crested Summit Areas 

 Perhaps the fact that the uppermost surfaces of some 

 regions approximate planeness and that the summit divides 

 all come up to a nearly uniform level, has been more gen- 

 erally used as a criterion of more than one cycle of erosion 

 than has any other evidence. But the principle has been 

 abused. Various terms have been used in connection with 

 this point, such as "even-crested hogbacks," "even-crested 

 ridges," "upland plains," "accordant summit levels," "even- 



1. Miller, A. M., "Windsaps," Science, Vol. 42 (1915), pp. 571-573. 



