EROSION AND THE SUMMIT LEVEL OF THE ALPS 21 



in the fine-textured forms of bad lands, the divides are delicately 

 rounded, instead of being acutely sharp, as Gilbert was led to expect 

 they should be in his study of land sculpture by running water 

 and the law of divides.^ The rounding of such crests appears to 

 be due to soil-creep, as he later showed.^ But in torrid lands of 

 rapid weathering and heavy rainfall, the ridges that rise between 

 broadly opened valleys are so extraordinarily sharp that, in propor- 

 tion to their breadth, they reahze the knife-edge acuteness which 

 Gilbert beheved ought to result from degradation by running water; 

 and the meaning of their sharpness appears to be that, under the 

 extra-heavy rainfall they receive, they really are shaped chiefly 

 by running water rather than by soil-creep, in spite of the rapidity 

 with which soil is there produced. These broad and sharply 

 divided valleys resemble in a surprising degree certain broadly 

 opened and sharply separated troughs of deglaciated mountains, 

 except that their cross-profiles have no ''shoulder" between the 

 higher weathered slopes and the lower wall of glacial scouring. 



As to the marine cycle, Gulliver's general account of "Shore-line 

 Topography"^ has been greatly extended in Johnson's Shore 

 Processes and Shore Line Development (New York, 1919). An item 

 that characterizes the mature and later stages of this cycle to which 

 attention has seldom been called is the then frequent occurrence 

 of cliffs of decreasing height ; that is, cliffs which have been cut back 

 so far that they now stand behind the former summit of a hill or 

 former crest of a ridge, so that the more they are abraded the 

 lower they become, until the valley beyond them is reached. In 

 regard to the cycle of glacial erosion, valiant efforts have been made 

 by various observers in the Alps to detect the effects of successive 

 epochs of decreasing glaciation, not only in form of trough- valleys, 

 but also in the pattern of valley-head cirques. If these efforts 

 prove successful they will give an increasing delicacy to the descrip- 

 tion of mountain forms. The solution cycle, not formulated until 

 after the normal cycle had become familiar, has greatly facihtated 

 the description of Karst lands. 



^ Geology of the Henry Mountains, Washington (1877), p. 122. 



2 "The Convexity of Hilltops," Jour. GeoL, Vol. XVII (1909) pp. 344-50. 



3 Proc. Amer. Acad. Acts and Sci., Vol. XXXIV (1899), pp. 149-258. 



