EROSION AND TEE SUMMIT LEVEL OF THE ALPS 5 



valley sides are graded up to the ridge crests, as they must be if 

 crest height is to be dependent on valley spacing according to the 

 principle above cited, then the streams also must be graded, even 

 though, on account of persistent upheaval, they are constantly 

 degrading their graded courses and are therefore not yet permitted 

 to widen their valley floors in flood plains: but this point is not 

 mentioned in Penck's analysis. If the point be well taken, the 

 postulated rate of upheaval should be conceived as rather moderate, 

 even though it is specified as strong [stark]. It would further seem 

 as if a very special though accidental relation must exist between 

 rate of upheaval, climate, drainage area, and rock resistance, in 

 order that the good-sized streams here considered should maintain a 

 graded flow, and that their valleys should maintain graded slopes, 

 even while upheaval is in progress. For it must be remembered 

 that as the streams are not yet deepening their valleys as fast as 

 the mountain mass is rising, their fall must be increasing; and 

 with increasing fafl, their graded condition might be lost. Besides, 

 with increase of mountain height, there must be increase of rainfall 

 and of stream volume; and increase of stream volume would still 

 further promote a return to a youthful, non-graded condition 

 rather than a persistence in a mature or graded condition. On the 

 other hand, the increase in valley depth and the accompanying 

 increase in the area of wasting vaUey sides — ^part of this second 

 increase being due to the development of many side ravines — will 

 cause an increase in the detrital load that is to be swept away by 

 the streams; and this may permit them to develop graded courses 

 in spite of their increased fall; but none of these details are men- 

 tioned in Penck's deductive analysis. In order to avoid misunder- 

 standing, let it be explicitly stated that the introductory clause of 

 this parenthical note does not apply to the small headwater streams 

 of an upHfted peneplain; they deepen the valley heads so slowly 

 that their side slopes are kept graded while the deepening is in 

 progress.^ It is only the larger streams that grade their beds 

 before the valley sides are graded.) 



Balance of upheaval and erosion.— Next comes a stage when the 

 larger streams, which are supposed to be continually invigorated 



' See my Erkldrende Beschreibung der Landformen (Leipzig, 191 2), p. 259. 



