FEATURES OF ICE-CAP EROSION 727 



than that postulated by the Glacial epoch. The chief point of interest is 

 the first step by which a ronghly -undulating upland surface may be 

 produced resembling an uplifted and modified peneplain. Other factors, 

 as isostatic adjustment, with simultaneous, differential degradation of 

 the rising blocks, erosion above and below timber limit, highland glacia- 

 tion, accordance of levels of upper surfaces of intrusive batholiths, with 

 subsequent removal of overlying less resistant intruded rocks, etcetera, 

 which have also been considered possible agents that might assist in 

 bringing about the frequently observed tendency toward accordance in 

 summit levels of mountainous areas, have been disregarded altogether in 

 the above discussion in order to simplify the presentation. Such factors 

 are obviously important, and must be and have been regarded in a con- 

 sideration of the whole subject,^ which, however, would lead too far in 

 the present paper. 



THE FIELD PROBLEM 



The practical field problem, to determine precisely the part ice-sheet 

 beveling has played in the formation of the present upland surface, is 

 difficult, but the fact that this surface coincides with the upper limit of 

 glaciation and follows this limit roughly down to the coastal margin of 

 the ice-sheet is in itself a strong argument in favor of fairly intense ice- 

 cap beveliDg. In mountainous regions, where these conditions might 

 prevail, the slopes and gradient are usually such that all loose deposits, 

 as original soil, which might serve to indicate peneplanation, or even 

 morainic materials, are rapidly carried off and obliterated. Planation by 

 glacial action should be most rapid in areas of soft, friable rocks, while 

 remnants of an original peneplain should disappear most rapidly in such 

 an area. 



Many factors enter into erosion problems of this nature, and criteria 

 have not been developed which might be used to determine definitely the 

 influence and importance of the factor of ice-cap beveling. The attempt, 

 only, has been made to show that under special conditions such ice-cap 

 planation is possible to a certain extent, and, if so, to indicate that it 

 was probably a factor in the development of the present baseleveled char- 

 acter of the upland areas in certain parts of Iceland and Alaska. It 

 does not preclude the existence of a former peneplain, now warped, dis- 



^ In a paper, "Accordance of summit levels among Alpine mountains : the fact and 

 its significance," Journal of Geology, vol. xiii, 1905, pp. 105-125, Mr. R. A. Daly has 

 considered these factors in detail and with special reference to their bearing on the 

 present problem. 



