12 IOWA STUDIES IN NATURAL HISTORY 



occurs, and it is in youth of the second cycle at the time of 

 observation, it would be almost impossible to determine that 

 it had entered upon a second cycle. It would be still more 

 difficult to interpret the history of a region in which a cycle 

 of erosion had gone so nearly to completeness that all evi- 

 dences of a former cycle had been obliterated. Indeed, 

 satisfactory determination seems to be almost limited to 

 cases where each cycle of erosion is less nearly complete 

 than those which preceded. 



Evidences of More Than One Cycle of Erosion 

 Several different physiographic features have been used 

 as criteria for more than one cycle of erosion in the history 

 of land surfaces. If their relative values are to be fixed, 

 it is necessary that each of these features be analyzed. 



Interrupted Profile 

 Streams which have reached grade, normally have con- 

 cave profiles with progressively decreasing gradients from 

 source to debouchure, (abc Fig. 1). If such a profile be 



Fig. 1. A diagram in explanation of interiupted profile due to rejuvenation of a 

 sti-eam. abc is the profile of the stream in old age of the first cycle when sea level 

 was cd. abe is the profile after sea level has taken the position ef and the rejuve- 

 nated stream has worked headward to be. 



developed in old age of a first cycle of erosion and if the 

 region then be uplifted so that a steep bordering slope is 

 formed, rejuvenating the stream, degradation will be re- 

 newed first near the mouth of the stream, where a new 

 valley and a new profile will be developed. As this new 

 valley, with its profile, is extended headward, there will 

 come a time when there is a double or interrupted profile 

 (abe Fig. 1) in which the upper portion was made in the 



