1 I ^ i. i . RUSSEL1 L'RFAOE GEOLOGY OF ALASKA. 



il explanations of the origin of the lake may be suggested, bul each 



dditional Reld observations in order to prove or disprove it. 



The first and least probable of these hypotheses is thai the drainage of the 



Yukon was obstructed and dammed by the large lava flow al the mouth of 



the Pellv. alone the border of which the river has excavated a recenl channel. 



ither possible explanation is that moraines were deposited aboul the 



northern border of the Cordilleran glacier, obstructing the drainage and 



in:: origin to the lake when the glacial ice was melted. This supposition 



finds some support in the approximate coincidence of the northern limit of 



glaciation with the northern extension of the old lake. 



Still another hypothesis is thai the weight of ice forming the great Cor- 

 dilleran glacier was sufficient to depress the earth's crust in the manner 

 suggested by Btudents of glaciation in other regions. As the ice retreated, 

 the depression thus originated was occupied by a lake, which was slowly 

 drained ;i< the channel of discharge was deepened or as the land regained its 

 former elevation. The observed increase in the elevation of the terraces of 

 the Yukon from mouth to source seems to be a direct and important con- 

 firmation of this hypothesis. 



Km-i in'. Glacii R8. 



Observations at Chilkoot Pass and about Limn Canal. — In describing the 

 nonglaciated condition of the Yukon region, it was mentioned that all 

 mountains in that region are now bare of snow in Bummer, and hence 

 arc without glaciers. Snov was absenl from all mountains -ecu during 

 my journey up the Yukon and Lewes until reaching Lake Bennett, when the 

 1 as! Range of southern Alaska came in sight. 



In crossing Chilkoot pass I sav five or six small glaciers on the north 

 Blope of the range. Some of them were in cirques; others on the Bides of the 

 more lofty mountain -pin- along the crest of the range. 'Their lower limit 

 appeared to be about 3,000 feel above the sea. About Crater lake, snow- 

 banks and -one ice on the Bteej tuntain side extended down to the very 



margin of the water. These accumulations, however, did uot have the 



characteristics of true glaciers, ('rater lak scupies the bottom of an 



immense amphitheatre, which was the source of a large glacier during quite 

 • nt times. Tl ds of ice action are to be seen every v here about the 



■ and in the wild valley leading from it toward* Lake Lindemann. It 

 evident that a slight change of climatic condition-, favorable to the 

 umulatiou of -now, would reproduce the counterpart of the ancient glacier 

 which once tlo no the amphitheatre of < rater hike. 



The weather was thick ami extremely unfavorable for observation during 

 my journey from Lake Lindemann to Lynn canal The only observations 



