FEATURES OF ICE-CAP EROSION 725 



the present surface owes to inheritance and how much to ice-sheet 

 beveling. 



TEE UPLANDS OF THE COAST RANGE, ALASKA 



This particular difi&culty is not encountered in southeastern Alaska, 

 where glaciation was unusually severe during the ice deluge, and where a 

 similar surface sweeps over the uplands of the Coast Eange. There the 

 backbone of the range is not flat lying basalt, but granite, in huge batho- 

 liths, extending from Alaska into British Columbia and covering an area 

 1,000 miles long and 100 miles wide. Its mountain tops show a decided 

 tendency toward uniformity in elevation, and are, moreover, usually 

 broad and only slightly arched; if the intervening precipitous valleys 

 and canyons were filled to their apparently original profiles, an undulat- 

 ing, warped surface, sloping seawards from the center of the range, 

 would result. This has been interpreted as proof of an original pene- 

 plain, uplifted and deeply incised, and general opinion among observers 

 seems to be that the evidence warrants this conclusion.^ The attempt to 

 correlate this peneplain with others in the interior of Alaska has, how- 

 ever, met with serious difi&culties, and the problem can not yet be regarded 

 as completely solved. These difficulties are due in part to lack of de- 

 tailed knowledge of the regions involved, but, until they are removed, 

 the present peneplain hypotheses can not be considered entirely satis- 

 factory. 



That there is an accordance of summit levels among the mountains of 

 certain parts of the Coast Eange is a matter of observation; it is, also, a 

 fact of observation that the surface passing over the tops of the upland 

 areas is gently warped and rises from the coast toward the center of the 

 range, the average gradient being about 1 :150. Here and there above 

 this surface sharp pinnacles and mountain spires pr«oject, which, on the 

 peneplain hypothesis, would be considered monadnocks rising above the 

 baselevel. 



Another interpretation has been advanced to account for these rela- 

 tions, namely, that the original peneplain passed just above the serrated, 

 non-glaciated peaks, in which case the undulating surface below has re- 

 sulted from ice sculpture and postulates great ice erosion. This surface, 



5 A. C. Spencer : Bulletin of the Geological Society of America, vol. 14, 1902, pp. 

 117-132. 



A. H. Brooks : Professional Paper no. 45, U. S. Geological Survey. 



G. K. Gilbert : Harriman Alaska Expedition, vol. iii, 1904, pp. 122-139. 



