V . Age of the St. Elias Range. 175 



westward extends beyond the end of the northeast fault. The 

 pomt of union is at the pass between Mount St. Elias and Mount 

 Newton. The upturned block, bounded on the southwest by a 

 great fault, projects beyond the junction with the northeasterly 

 fault. It is this projecting end of a roof-like block that forms 

 Mount St. Elias. That this is the case may be clearly seen when 

 viewing the mountain from the glacier near the base of Mount 

 Owen. Such a view is shown on plate 20. The crest-line of St. 

 Elias extends with a decreasing grade northwestward from the 

 culminating peak, and the northern slojje of the ridge is the sur- 

 face of the tilted block. 



From what has been stated already, it will be seen that the St. 

 Elias range is young. Its upheaval, as indicated by our present 

 knowledge, was since the close of the Tertiary. The breaking of 

 the rocks and their upheaval is an event of such recent date that 

 erosion has scarcel}^ modified the forms which the mountains 

 had at their birth. The formation of glaciers followed the ele- 

 vation of the region so quickly, that there was no opportunity 

 for streams to act. The ice drainage is consequent upon the 

 geological structure, and has made but slight changes in the 

 topography due to that structure. 



About Mount Cook, and in the elevated plateau east of Yakutat 

 bay, there has been deeper erosion than about Mount St. Elias. 

 The glaciers in this region occupy deep valleys radiating from 

 the higher peaks ; but whether these are really valleys of erosion 

 is not definitely known. In some instances, changes of dijj on 

 opposite sides of the valleys indicate that they may in part be 

 due to faulting ; but, owing principally to the fact that every 

 basin has its glacier, it has not been practicable, up to the present 

 time, to determine how they were formed. 



The crests of the mountains are always sharp and angular, by 

 reason of the rapid weathering of their exposed summits, but 

 while disintegratioii is raj)id, no evidences of pronounced decay 

 are noticeable. The peaks on the summits of the St. Elias range 

 are either pyramids or roof-like crests with triangular gables. 

 These forms have resulted from the weathering of schist in which 

 the planes of bedding are crossed by lines of jointing. 



