SUMMIT LEVELS AMONG ALPINE MOUNTAINS 119 



responsible for most of the peaked and serrated topography of the 

 Swiss mountains. He supposes that in pre-Pleistocene times the 

 range had the comparatively smooth, flowing profiles of well-graded 

 mountains ; that the present ruggedness is mostly due to the recession 

 of head- walls in cirque-making. 1 The view may be extreme, but it 

 illustrates the importance which the distinguished European physiog- 

 rapher attaches to the work of local glaciers. Their gnawing action 

 is just as manifest about the countless glacier-beds among the highest 

 peaks and sierras of the Rockies, Selkirks, and Coast Range of 

 British Columbia, of the Washington Cascades, of the mighty ranges 

 of Alaska. In all these fields the highest peaks and ridges long 

 suffered specially powerful attack, as they alone stood high enough 

 to wear the fatal belts of bergschrund. During the ice period, they 

 were nunataks and lost substance like nunataks ; the loftiest peaks 

 losing most, the lower ones with less linear extent of bergschrund, 

 losing proportionately less. Peaks and ridges not penetrating the 

 general surface of the Cordilleran glacier lost nothing by special 

 bergschrund attack. 2 



It is certain that this differential erosion was long continued 

 during the Pleistocene period in each of the ranges where accordance 

 of summit levels has been discussed. There is every reason to sup- 

 pose that like conditions and like results would characterize still 

 earlier glaciations. 



For the present purpose it is not necessary to inquire as to the 

 deepening or other modification of main valleys in the range. Im- 

 portant as may be such valley changes to the future scenery of the 

 range, they cannot have anything like the same control over summit 

 altitudes as the direct trimming down of the summits by glacier heads. 

 Moreover, head-wall recession among the higher summits continues 

 throughout the whole epoch of glaciation; the excavation of the 

 main valleys occurs only during maximum glaciation. 



In summary, then, it may be said that partial explanation for 



1 Petermann's Mittheilungen, Erganzungsheft No. 132, 1900. 



2 Compare the views of W. D. Johnson and G. K. Gilbert, as announced in the 

 Journal of Geology, December, 1904. The special glacial attack on the highest summit 

 of the Big Horn Range (Cloud Peak) is excellently illustrated in the well-known aper 

 by Matthes, Twenty-first Annual Report, U. S. Geological Survey, Part II, 1899-1900, 

 Plate XXIII. 



