Page:The Journal of geology (IA journalofgeology11893univers).pdf/97
ice, or whether they be supposed to continue to rise (more and more slowly) till they meet the descending plane of ablation.
If currents rise by reason of differential movements to certain heights, but not beyond them, notwithstanding the extension of the differential movements all the way up to the surface, a very distinct statement of this limitation and of the dynamics involved, qualitative and quantitative, would be appropriate. Perhaps such an explanation is intended in the following quotation from Mr. Upham, which I introduce to give ampler expression to his views, though I dissent from his interpretations of the crevasses of the alpine glaciers and of the esker, Bird's Hill, as well as from his fundamental proposition.
"The conditions of the flowing ice which seem to me to have been efficacious to carry drift upward into it from tracts of plane or only moderately undulating contour, were the more rapid onflow of the ice-sheet in its upper and central parts and even in the portion near the ground but not in contact with it, than upon the bed of the ice-sheet where its movement was much retarded by friction. A very good analogy with the slowly rising currents which I believe to have existed in many portions of the base of the ice-sheet is afforded by the edges of alpine glaciers, where the crevasses extending diagonally up stream into the glacier testify that the movement of its friction-hindered border is from the side of the valley into the ice mass. But the arched surface of the glacier and the great supply of its central current prevent the drift so worn off and borne away from being carried into the axial portion of the ice stream. Similarly the steady accession to the mass of the ice-sheet over any place by onflow from its thicker central part and by the accumulating snowfall forbade the drift of the upwardly moving basal current from being carried far into the ice in comparison with its total thickness. The evidence of the esker called Bird's Hill, near Winnipeg, Manitoba, shows that much englacial drift had there been uplifted from a nearly level country to a height of more than 500