362 THE JOURNAL OF GEOLOGY. 



flow of subglacial water. Now, loss of effectiveness through 

 absolute and relative increase of weight must eventually become 

 potent in retarding direct excavation of the depression ; also, 

 whenever the depression becomes so considerable as to possess 

 reverse slope toward its distal extremity, gravity will no longer 

 enhance, but instead oppose, direct transportation of detritus ; 

 again, with increased depth of depression will go increased cross- 

 section and concomitant and material diminution of velocity and 

 eroding capacity in the ice-stream ; and finally, the longitudinal 

 perimeter of the depression must continually increase until the 

 fricton along it approaches and ultimately equals the shearing 

 strength of the ice along its chord, whence the movement of the 

 basal segment .must concurrently diminish and gradually cease. 

 In like manner, when the normal slope becomes reversed, gravity 

 will oppose and not enhance transportation by subglacial water ; 

 also, as the reverse slope increases, the flow of such water will 

 become sluggish and its capacity diminished ; and finally, when 

 the depth of depression below its distal rim reaches 0.92 of the 

 maximum depth of ice (or when b-c equals 0.92 a-c, fig. 4), the 

 subglacial water will assume static equilibrium, the incumbent ice 

 will suffer flotation, and both corrasion and transportation will 

 practically cease. Thus the excavation of depressions by direct 

 ice- action has a definite though indeterminate limit, and can prob- 

 ably never exceed a moderate fraction of the depth of the ice ; 

 and thus also indirect glacial erosion in depressions through the 

 cooperation of subglacial water alike in corrasion and transporta- 

 tion will remain effective until the depth of excavation approaches 

 the thickness of the incumbent ice ; whence, in the general case, 

 the measure of maximum excavation of rock-basins is a large 

 fraction of the depth of the glacier. 



(Evidently embouchures of valleys, zones of abrupt diminu- 

 tion in declivity, points at which for any reason glaciers termi- 

 nate for considerable periods, broad cross -valleys beneath 

 continuous ice-sheets, and all localities where the surface slope 

 of the ice materially exceeds the slope of its base, will form as 

 definite loci of active excavation as do the ordinary planes of 



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