386 J. T. PARDEE 



It appears then that escape of the water through the more northerly- 

 passes or by way of the lower Kootenai was effectually blocked by 

 ice-barriers of much more than the necessary height. 



One of the very largest of these lobes flowed south through the 

 Purcell trench and dammed the only remaining avenue of escape, 

 the Clark Fork Valley. From 7,300 feet elevation at the 49th 

 parallel the glacier's surface sloped down to about 4,200 feet here, 

 where it forced the lake to seek its outlet. 



While the vagueness and close spacing of most of the lower 

 beaches seem to depend on the inconstant level of an ice-dam that 

 formed one wall at least of the outlet, the more prominent ones, 

 indicating constancy of level for a relatively long period, suggest that 

 the outlet may at times have been a col in some opposing spur from 

 the Coeur d'Alene Mountains. Until the ice, by rising, should invade 

 such an outlet, or by sinking below it should capture its flow, the water 

 level would be comparatively stable. 



I 

 1 



