EROSION AND THE SUMMIT LEVEL OF THE ALPS 27 



tion of the upheaval by which they are introduced, is a helpful 

 advance in systematic physiography, but the advance is not so 

 much in the way of a correction of previous discussions as it is in 

 their extension. Penck impHes, however, on a number of his pages 

 that his present views are corrections of earher views. He states, 

 for example (p. 263), that the sharp-crested preglacial Alpine ridges 

 have been developed from pre-existent rounded ridges (dass manche 

 Schneiden aus runden Formen hervorgegangen sind), and that such 

 a sequence stands in opposition to a previously published scheme 

 of a typical cycle of mountain erosion, in which a reversed sequence 

 of forms is presented, the rounded or subdued ridges of late maturity 

 being explained in that scheme as developed from the higher and 

 sharper ridges of early maturity. But no real opposition occurs 

 here, for in the earlier pubHshed statement the tj^ical cycle of 

 mountain erosion was supposed to begin after a previous cycle of 

 erosion had reduced a region of deformed structure to a peneplain 

 which, when upheaved, is dissected in such a manner that the sharp 

 ridges of full maturity naturally enough precede the rounded and 

 subdued ridges of later maturity in the same cycle. On the other 

 hand, in the special case of the Alps, the sharp ridges of the pre- 

 glacial stage of the present cycle were developed, as Penck clearly 

 explains, out of the rounded ridges of an earlier cycle, which was 

 interrupted by upheaval before peneplanation ensued. In other 

 words, the rounded ridges were introduced, ready made, from an 

 earher cycle, and in that cycle they had presumably had sharp 

 crests before they were rounded; it was in the following cycle that 

 they were again sharpened. In this special case it is just as natural 

 for the sharp preglacial ridges of the present Alpine cycle to have 

 been developed out of the rounded crests of the earher cycle, as it 

 is for the rounded, late-mature forms of a single typical cycle which 

 begins with an uphfted peneplain to develop out of the sharp forms 

 of early maturity. Furthermore, had the sharp preglacial ridges 

 of the Alps not been made still sharper by glacial erosion but had a 

 normal climate continued instead, the ridges would have probably 

 been somewhat rounded today in preparation for a more complete 

 rounding in the future. Indeed, the development of rounded ridges 

 out of sharp ridges during the progress of a typical cycle is a well 



