LimU of an,cient Glaciation. 159 



as that. The point to which glaciation extends in the White 

 river basin has already been indicated, with the evidence on 

 which the conclusion is based. The extension of the line Avest of 

 White river is less satisfactorily fixed than its eastern portion, 

 depending on a statement of Lieutenant Allen that he saAV no 

 drift north of the Alaskan mountains, as he called the Tananah- 

 Copper river divide. The ice sheet, a part of whose northern 

 limit is thus approximately outlined, had its principal center of 

 dispersion in the high plateau of British Columbia, between the 

 Coast and Rocky mountains. From this center two subordinate 

 lines of dispersion diverged toward the north and northwest, 

 following the axes respectively of the Rocky mountains and the 

 St Elias range, while the non-glaciated area formed a deep em- 

 bayment in the Yukon basin between these divergent lines. 

 The northern limit of glaciation is shown approximately on 

 plate 18. 



It is probable, however, that many lobes from the main 

 glacier extended down the valleys beyond the limit above in- 

 dicated, while the confluent ice sheet was not sufficiently thick 

 toward its northern border to override the greater inequalities of 

 the surface. Thus the White River valley, at least, must have 

 been occupied by ice well north of the general glacier front even 

 after a considerable amount of recession had taken place. The 

 altitude of the valley at the moutli of the Nisling is about 2,400 

 feet ; so that it must have formed an estuary during the period 

 of subsidence marked by the white silt deposits, and the forma- 

 tion of lake Wellesley is probably analogous to that of lake 

 Ahklen. 



