Pleistocene Ice-sheet in JSf. America. 323 



Relatively slight obstructions to the flow of an ice-sheet, whether 

 of solid rock or uncompacted drift, are sufficient to originate sharp 

 overthrust flexures and oblique shear-planes, marked by bands of 

 debris. Thus drift is transferred from a subglacial to an englacial 

 position. It may be concluded that from the summit or crest of 

 nearly every elevation with an abrupt lee-slope, a stream of detritus 

 flowed onward and upward into the ice-sheet during its progress 

 over the land. On the stoss-slope glacial erosion reaches its 

 maximum intensity, as both vertical and flowage pressure co-operate, 

 and much basal deti'itus is carried forward and from the summits 

 of elevations into the body of the ice, and not down the 

 lee-slopes. 



During the period of maximum glaciation a large proportion of 

 drifts may have been dragged down the lee-slopes in the bottom 

 of the ice, and thus striation and polishing of those slopes were 

 produced. Later, when the vertical pressure was less and the 

 velocity of flow greater, the ice hugged the lee-slopes less closely, 

 and the conditions became favourable for the detachment of blocks 

 of rock. Thus ice penetrated the joint-cracks of the rocks and 

 severed blocks, and these were eventually carried away by the 

 moving mass of ice to which they became attached. Indeed, we 

 often find a surface train of angular blocks leading away from the 

 lee-slopes. 



During the later stages of the decline of the ice-sheet, basal 

 melting set free considerable volumes of the hitherto englacial 

 drift to form the ground- moraine ; and just as the frontal or 

 terminal moraine, also composed of material set free by the melting 

 of the ice, records the cessation of the forward movement or invasion 

 of the ice-sheet, so the basal or ground-moraine records the gradual 

 cessation of the glacial abrasion of the bed-rocks. Generally the 

 ground-moraine rests upon striated surfaces of the rocks, and as 

 it accumulated it was pressed down by the ice to form the typical 

 " hard pan." 



Thus, in the earlier and maximum stages of the ice- sheet the 

 basal temperature was below freezing, and the freezing of sub- 

 glacial waters made and kept the detritus a part of the ice-sheet; 

 in the later stages of the ice-sheet, the basal temperature rose above 

 freezing, and the ice relaxed its hold on the detritus and flowed 

 over it. 



Transportation of drift by simple drag is relatively unimportant, 

 if not impossible ; the transportation is almost wholly englacial, as 

 insisted by Upham (Bull. Geol. Soc. Amer., vi, 348), but highly 

 differential, being extremely slow in the basal layers and more and 

 more rapid at higher levels. The detritus reaching the highest 

 levels in the ice is carried farthest, not only because of the higher 

 velocity, but also because it remains for a longer time in the ice. 



The efficiency of glacial erosion is clearly proved by the fact that 

 over practically the entire glaciated area north of the terminal 

 moraine all the preglacial soil and partially decayed rock have been 

 worn away, and there has been an extensive erosion of the hard 



