Page:The Journal of geology (IA journalofgeology21894univers).pdf/378

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 friction 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 probably never exceed a moderate fraction of the depth of the ice; and thus also indirect glacial erosion in depressions through the coöperation of subglacial water alike in corrasion and transportation 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 diminution in declivity, points at which for any reason glaciers terminate 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