EROSION AND THE SUMMIT LEVEL OF THE ALPS 29 



Directly after his irrelevant statement that the actual sequence 

 of forms in the Alps — rounded ridges converted into sharp-crested 

 ridges — stands in opposition to the sequence that I have given as 

 typical of an erosion cycle, he goes on to say that the scheme of 

 the cycle should be treated, not as involving the action of erosion 

 on an already upheaved mass, as Davis has done, but as involving 

 the action of erosion during upheaval as well as afterward (pp. 263, 

 264). It is perfectly true that I have frequently presented the 

 scheme of the cycle as if introduced by upheaval and continued by 

 erosion; the reason for such presentation being that it is the simplest 

 way of placing the general idea before beginners; but it is also 

 true that Penck has presented the scheme in the same simple way. 

 One of the first articles, if not the first, in which he recognized the 

 scheme of the "Erosionszyklus," subdivides it into five stages. 

 The first stage is the emergence of a sea bottom in the form of a. 

 gently inclined plain; in the second stage, streams incise valleys 

 and subdivide the inchned plain into flats; in the third, the incision 

 of the valleys is continued, their side slopes are washed down, and 

 the flats are thereby narrowed and converted into divides, which 

 under certain conditions may be sharp; in the fourth, valley 

 deepening ceases and the valley floors are widened at the expense 

 of the dividing ridges between them; and in the fifth, the ridges 

 are worn down so low that neighboring valley floors become con- 

 fluent and a plain represents the final result of the metamorphosis.^ 

 Not a word is said here about upheaval after the first stage, and no 

 mention is made of valley erosion during that stage. Nine years 

 later Penck again made a brief analysis of the erosion cycle, in 

 which the incision of a valley is as before said to take place upon a 

 slope (of upheaval), but not during the upheaval of the slope; and 

 the strongest erosion is said to be instituted where the greatest 

 surface unevenesses exist, but not during the production of the 

 unevennesses.^ 



Interaction of upheaval and erosion. — We have therefore both 

 presented the scheme in a simple, elementary fashion. But besides 



^"Die Geomorphologie als genetische Wissenschaft," Ber. 6tcn Internal. Geogr. 

 Kongr. (189s), pp. 735-47; see p. 736. 



^ "Die Physiographie als Physiogeographie," Geogr. Zeitschr., Vol. XI (1905); see 

 pp. 9, 18. 



