ABSTEACTS OF PAPEES 87 



eral important lines of evidence — some of them new — will be presented which 

 show that there are two distinct drifts west of the mapped Wisconsin moraine, 

 the outer one being clearly Illinoisan in age, the inner probably early Wis- 

 consin. While the precise boundary is yet to be drawn, yet it is quite plain 

 that the Illinoisan drift extends in some places east of Rock River, and hence 

 the new boundary must be radically different from the former Illinoisan-Iowan 

 boundary. 



Read from manuscript. 



Discussed by Prof. R. D. Salisbury. 



ORIGIN AND HISTORY OF LAKE CHELAN 

 BY J. J. RUNNER^ 



{Abstract) 



The Chelan Basin, in north-central Washington, exhibits many striking 

 effects of the work of glacial ice. Its linear form, abrupt, smooth slopes, and 

 great depth have for some time been attributed to the erosive action of a large 

 valley glacier. At its maximum development, the Chelan Glacier and its trib- 

 utaries had a surface area probably exceeding 400 square miles. Its length 

 was nearly 80 miles and its thickness throughout a considerable portion of its 

 length doubtless exceeded 4,000 feet. For nearly 15 miles in its mid-section 

 the bottom of the lake lies near to or below sealevel and its depth exceeds 

 1,000 feet. Data are now at hand to show that in this section several tribu- 

 tary hanging valleys that doubtless once entered the main valley at accordant 

 levels now enter at elevations over 2,000 feet above the bottom of the latter. 

 Locally, then, the amount of glacial erosion exceeded 2,000 feet, for these trib- 

 utary valleys were glaciated and their floors lowered by a considerable but 

 unknown amount. The deepest section of the lake occurs in the stretch where 

 the valley is narrowest, which in turn coincides with the portion in which the 

 structure of the rock is most favorable for down-cutting. Above the narrow, 

 deep section the bedrock is dominantly granite and gneiss ; below it is largely 

 a complex of various fine-grained, igneous intrusives, while within this section 

 the valley runs parallel to the strike of a steeply dipping series of schists and 

 slates. Along the valley walls at the eastern end of the lake occur minor ter- 

 races marking the levels of small lakes that lay between valley spurs at the 

 sides and the glacier in front. 



Following the Chelan glaciation, after an unknown interval, came an inva- 

 sion of the Cordilleran ice-sheet into the lower valley from the east. This 

 blocked the natural drainage channel eastward to the Columbia River and 

 caused the lake to rise first to an elevation of 1,800 feet and flow out through 

 Navarre Coulee southward. Then, as the ice-tongue receded, the lake found a 

 lower outlet through Knapp Coulee, at an elevation of 1,430 feet, and finally 

 through the lower Chelan Valley, over the drift-dam, at 1,120 feet above tide. 

 Since that time the level of the lake has been lowered by erosion of the outlet 

 to 1,080 feet. The preglacial channel of Chelan River may be clearly seen one 



' Introduced by George F. Kay. 



