EROSION AND THE SUMMIT LEVEL OF THE ALPS 3 



been fairly sharp, its sides would have had fairly uniform slopes of 

 too steep an inclination for the retention of more than a thin sheet 

 of creeping detritus, and its height over the adjacent valley bottoms 

 would have been about half the horizontal distance from them, 

 or a quarter of the distance between them. In other words, the 

 ridge slopes must then have been essentially graded with respect 

 to the principal streams. Under such conditions the degradation 

 of the ridges would have still been in fairly rapid progress, and their 

 altitudes would have been as nearly alike as the valleys between 

 which they rose were equidistant. (This modifies and presumably 

 improves a conclusion that Penck had reached eleven years earlier, 

 when he thought that erosion under the leadership of the ancient 

 glaciers had carved the present very sharp Alpine crests out of 

 maturely rounded preglacial ridges.^) It is the origin of the sharp 

 preglacial ridges of similar altitude and rapid degradation that is now 

 sought for, and in this search the deductive analysis of several ideal 

 cycles of mountain erosion is undertaken with the object of discover- 

 ing at what stage of their progress and under what relations of 

 upheaval and degradation the inferred uniform altitudes of the 

 preglacial Alps can be best accounted for. Although the essay is 

 the work of a geographer, the study as a whole is more largely 

 concerned with conditions and processes of the past than with the 

 forms of the present and hence its character is dominantly geological; 

 and in this respect it is characteristic of German physiography in 

 general. 



First ideal cycle: Erosion during prolonged upheaval. — Three 

 ideal cycles are examined. All begin with a mass of deformed 

 structures, previously worn down to a lowland. Structural com- 

 plications and differences of rock resistance are left out of considera- 

 tion, presumably to simplify the problem. A broad upheaval 



^ . . . . "aus den reifen Firstformen jahe Grate herausschnitt." This statement 

 is to be found on p. 154 of an admirable simmiary of the physiography of the lands 

 as resulting from the interaction of deformation and erosion, entitled "Die Erdober- 

 flache," which makes the third chapter of Scobel's Geographisches Handbuch, 5th ed.; 

 Leipzig, 1908. It will be referred to below as Sc, with page numbers. An interesting 

 measure of the progress of rational geography may be had by comparing this chapter 

 of 1908 with the corresponding chapter which Penck contributed to an earlier edition 

 of the same work in 1895. 



