362 W. Upham — Diversity of the Glacial Drift. 



wardly moving air currents, abundantly laden with moisture 

 from that region, were chilled in their farther progress over 

 the ice-sheet north and east of the driftless area, and there 

 gave exceptionally heavy snowfalls, permitting that part of 

 the ice-sheet to grow thick and high, with only slight reces- 

 sion. At last, when there came a temporary general reversal 

 of the warm climate under which the ice-sheet had been mainly 

 retreating, its halt or re-advance producing the first of the 

 prominent moraines carried the ice-front in Wisconsin forward 

 upon a part of the area which previously had no drift. 



During the great recession of the ice in the Mississippi basin, 

 it probably withdrew much less, perhaps mostly from ten to 

 thirty or forty miles, in Ohio, Pennsylvania, northern New 

 Jersey, and Long Island, and south of Rhode Island and 

 eastern Massachusetts. Meanwhile, as in eastern Wisconsin, it 

 had grown thicker than during its time of maximum area, 

 and the sudden and short climatic changes leading to the 

 formation of the moraines allowed the ice in the eastern states 

 to flow out again almost to its earlier limit, and in some places 

 even beyond it. Considering how nearly coincident the earlier 

 and later drift boundaries are for this long distance from the 

 Scioto River in Ohio eastward, we naturally feel much reluc- 

 tance against referring them to distinct epochs of glaciation 

 separated by a long interglacial time, as some have supposed, 

 when the ice-sheet made a long retreat to the north or was 

 wholly melted as now from this continent. It seems to me 

 more reasonable to appeal, as Prof. James D. Dana has re- 

 cently done,* to meteorological differences between the Mis- 

 sissippi basin and the eastern states, whereby comparatively 

 long glacial retreats and re-advances could take place at the 

 west while in the east the ice-border more steadily remained 

 near the drift boundary. 



What shall be said, consistent with this view, concerning the 

 extra-morainic drift in New Jersey, some of which occurs 

 more markedly in isolated patches than any of the early drift 

 before noted in the Mississippi basin ? Prof. Salisbury esti- 

 mates that a very long time of ordinary subaerial erosion inter- 

 vened between the times of deposition of the earlier and later 

 drift in New Jersey, so that the denudation of the land had 

 removed the greater part of the earlier drift, leaving only its 

 present patches on the extreme boundary, before the late 

 moraine-producing ice advance. If this is a needful explana- 

 tion, it goes far toward establishing a longer and probably 

 more complex history of the Ice age than the view taken in 

 this essay. It seems to me, however, that the manner of trans- 

 portation and deposition of that early drift may explain its 



*This Journal, III, vol. xlvi, pp. 327-330, Nov., 1893. 



