Pleistocene Ice-sheet in JSf. America. 325 



overridden by tlie readvance of an earlier sheet, the height attained 

 by the englacial drift depending then upon the thickness of the 

 overridden sheet. 



Belations of Englacial Drift to Modified Drift. 



The modified drift, or washed and stratified gravels, sands, and 

 clays, of glacial origin, forming deltas, terraces, overwash or apron 

 plains, eskers, kames, etc., occur almost wholly in valleys or on 

 lowlands. This drift is due in part (1) to the washing and assorting 

 of the till or ground-moraine by glacial streams during and following 

 the waning and disappearance of the ice-sheet ; (2) to the englacial 

 drift through the agency of subglacial streams ; and (3) to the 

 melting of the ice and liberation of the englacial drift on its surface 

 where it was washed and assorted by superglacial streams. 



The Transportation Argument. 



While a small part of the drift of the Pleistocene ice-sheet is 

 far-travelled, the great bulk is of local origin. 



Much of the preglacial residuary soil was probably swept away 

 by aqueous erosion during the elevation of the continent and before 

 the formation of the ice-sheet. What was left of it probably became 

 incorporated with the ice-sheet in its earliest stage, and it has 

 ultimately been carried by various agencies, glacial, lacustrine, and 

 fluvial, in large part beyond the limits of the glaciated area. There 

 is little indication of the presence of highly oxidized residuary clay 

 in the till : its matrix is chiefly the finely comminuted and unoxidized 

 detritus. 



In its earliest stages the ice-sheet wore away and absorbed 

 a considerable thickness of rotten rock underlying the soil, and 

 during its maximum stage the hard rocks suffered glacial abrasion. 

 On the decline of the ice-sheet, as before mentioned, there was 

 a rending of ledges and detachment of fragments and boulders. 

 These must have been, in general, the last material to be absorbed 

 by the ice-sheet and the first to be deposited by basal melting. 

 Under favourable conditions of flexing and shearing a small part 

 of this material attained a high level in the ice and enjoyed a long 

 glacial transport. 



Although the total forward movement of the ice, as indicated by 

 far-travelled erratics, appears to have been as much as five or six 

 hundred miles, and even in some parts of the glaciated area perhaps 

 a thousand miles, a basal slipping of one-twentieth of that distance 

 or less would probably be regarded as sufficient to account for the 

 erosion of the bed-rock surface and the normal distribution of 

 the identifiable fragments. Possibly the total movement of the 

 ice has been overestimated, the more distant erratics having been, 

 perhaps, transported in part by water, and not wholly by the 

 ice-sheet, each marked recession of which provided a series of 

 glacial lakes and rivers along its margin. 



It is evident that the entire volume of the drift was englacial 

 during the active erosion of the bed-rock, and that an efficient 



