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726 F. E. WRIGHT EFFECTS OF GLACIAL ACTION IN ICELAND 



however, which stretches over, and is supported solely by, the present 

 pinnacles is a much warped surface, so irregular in fact as to render its 

 interpretation as an original peneplain a mere guess rather than a work- 

 ing hypothesis which can be properly tested by observed facts. 



It seems possible, however, that, in Alaska as in Iceland, ice-sheet 

 beveling may have been an important factor in the observed planation of 

 the present upland surface. This idea first occurred to the writer in 

 1904 while engaged in field work in southeastern Alaska, and although 

 it is still only a working hypothesis it emphasizes a factor which seems 

 not to have been regarded sufficiently heretofore. 



At the time of the ice-flood the land area in the Coast Eange of 

 Alaska was covered with ice which reached all but the highest peaks. 

 The upper limit of this ice-cap averaged about 2,000 meters elevation 

 and is still plainly marked on the mountains. Below that limit the 

 mountain slopes are rounded and abound in large, smooth curves, while 

 above it sharp peaks and serrate ridges are the rule. That the ice-sheet 

 remained for a long period of time at about the same elevation is evident 

 from the distinctness with which its upper limit is chiseled in the peaks 

 themselves. As in Greenland at the present time, the surface of the ice- 

 sheet sloped gradually from the center of the range toward the sea. Such 

 an ice surface is comparatively smooth and may well have served as the 

 datum plane toward which the upland areas tended to be reduced. 



CYCLE OF ICE EROSION 



Undoubtedly, if this kind of erosion were sufficiently permanent in 

 character, it would be possible to speak of a complete cycle of ice erosion. 

 The deepening of the glacial valleys can not and does not continue in- 

 definitely, as is evident from the present fiords and valleys, which are 

 often over-deepened, where ice erosion was greatest, with consequent 

 reversal of valley gradient for short distances and the formation of large 

 hollows or basins in the valley floor. If the ice-cap were to remain long 

 enough, the projecting mountain peaks would be practically obliterated, 

 while at the same time some erosion would be effected along the top of 

 the baseleveled portion as well as at the valley sides and heads, since the 

 outward flow of the ice would still be kept up. A general lowering of 

 the whole surface would thus result and the tendency toward baselevel 

 of ice erosion would be still further accentuated, as with the lowering of 

 the uplands a spreading out of the ice-cap might be expected. This last 

 step of the process would require an exceedingly long time, much longer 



