194 THOMAS C. CHAM BERLIN AND ROLLIN T. CHAM BERLIN 



So, too, among those who believe in the efficiency of glacial 

 erosion, there has long been some doubt as to the truth, or at least 

 as to the adequacy, of some of the processes to which the erosion 

 has been attributed. 



It seems worth while, therefore, to add to the growing mass 

 of matter some notes suggested by phenomena recently seen by 

 us, without presuming that much is new either in the observations 

 or in the suggestions. 



I. THE CRITICAL STAGE FROM WHICH CERTAIN EROSION TYPES 



START 



It has seemed to us advantageous to study the initial stages 

 of erosion to see, if possible, precisely what action gives the start 

 to the type of erosion which thereafter controls the configuration, 

 for it is the initial turn that most delicately measures the balance 

 between the opposing tendencies. 



The contours that spring from ordinary wear and weathering 

 are well known and may be restored from remnants when the 

 greater part has been lost. Even when there has been no change 

 in the agent and only a slight change in its mode of action, the 

 old configuration can be distinguished from the new; as, for 

 familiar example, the remnants of a peneplain are commonly made 

 out with confidence after most of the plain has been cut away by 

 the rejuvenation of the very drainage system that formed it. 

 Much more clearly can remnants of contours be rebuilt into their 

 originals when some new agency intervenes, especially a new 

 agency whose habit of sculpture is distinctively at variance with 

 that of the previous agency. 



As surface configurations are traced from regions dominated 

 wholly by ordinary wear and weathering into regions that have 

 been affected by local glaciation, it is usual to find the lower slopes 

 of the unglaciated region and, in the main, the brows and tops of 

 its hills and higher elevations, up to a certain limit, "marked by 

 contours of the familiar wear-and-weather type whose interpreta- 

 tion is clear and whose restoration, when mutilated, may be made 

 with great confidence. As such contours are traced into higher 

 latitudes or higher altitudes where local glaciation has entered 



