RELATIONS OF ENGLACIAL DRIFT TO SUBGLACIAL TILL. 88 



nearly stationary during several years the vigorous outflow of the ice to 

 its steep frontal slope brought much drift which had been englacial and 

 on account of the ablation became superglacial, dumping it, as we may 

 sa}'', in the irregular moraine embankments. As these marginal accumu- 

 lations of drift record the position of the terminal line of the ice-sheet 

 when they were formed, the name terminal moraines has been usually 

 applied to them, but they may also very properly be called retreatal or 

 recessional moraines. Along all the pathway of retreat of the ice, that 

 is, over all the land which it had covered, as in front of the places of the 

 moraines, beneath the tracts where they were soon to be accumulated, 

 and farther northward, I think the deposition of the subglacial till to 

 have been somewhat rapidl}" in progress upon a belt probably 20 to 50 

 miles in width next backward from the receding ice boundary, this belt 

 being in gradual retreat from south to north. 



tilBCial rccess/on. 



y5r5^^S;5T\YTT^\TmTr 



Figure \.— Section of the Ice-sheet in the Mississippi Basin along a Distance of 300 Miles north from its 



most southern Boundary. 



The manner in which the ice gathered drift into its basal portion from 

 any plain tract seems to me explainable by a consideration of the cur- 

 rents of outflow toward its border. Even in the central area of the ice- 

 sheet the currents of its upper and lower portions probably moved out- 

 ward with quite unequal rates, the upper movement being faster than at 

 the base. This contrast was increased, however, upon a belt extending 

 many miles back from the margin, where the slope of the ice surface had 

 more descent, for there the upper currents of the ice, unsupported on the 

 outer side, would move very much faster than its lower currents which 

 were impeded by friction on tlie land. Within this belt, accordingly, 

 there would be a strong tendency of the ice to flow outward with some- 

 what curved currents (shown by dotted lines with arrows in the figure), 

 tending first to carry the outwardly moving drift graduall}^ upward into 

 the ice-sheet, and later to bear it downward and deposit it partly beneath 

 the edge of the ice as subglacial till, and partly along the ice boundary 

 as moraines when any halt permitted such accumulation. 



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 afl'orded 

 by tlie edges of alpine glaciers, where the crevasses extending diagonally 

 upstream into the glacier testify that the movement of its friction-hin- 

 dered border is from the side of the valley into the ice. But the arched 



