MARGINAL MORAINES OF NORTH AMERICA. 23 



tered, and the rare occurrence of lone drumlins, yet of large size and 

 typical form. With much englacial drift gathered in layers and patches 

 in the lower part of the ice-sheet, the inequalities of ablation and super- 

 glacial drainage, when extended at certain times over a somewhat broad 

 belt of the ice border, ma}^ have produced convergent currents of the 

 lower ice sufficient for amassing these hills in all their variety of group- 

 ing and occasional solitary occurrence.^ 



North American marginal Moraines. 



In the subdivision of the Glacial period by Geikie, Chamberlin and 

 others, the time of principal accumulation of marginal moraines is re- 

 garded as an epoch distinct from the previous portions of the Ice age ; 

 and Chamberlin has named the earlier divisions of this period, when the 

 North American ice-sheet reached its culmination, the Kansan and lowan 

 stages, while the later moraine-forming time is called the Wisconsin stage, 

 from the magnificent development of the moraines in eastern Wisconsin. f 

 Between these glacial stages, which appear well recognizable and syn- 

 chronous in North America and Europe, these authors suppose that there 

 were prolonged intergiacial ep6chs when the ice-sheets were in large part 

 or wholly melted away. Instead of this view, the Ice age seems to me 

 to have been essentially a single glacial epoch, with moderate fluctuations 

 of the ice borders during both the growth and the wane of the ice-sheet. 

 The marginal moraines I consider to have been very rapidly formed 

 while the ice was retreating from its lowan stage, with no important 

 general re-advance dividing the lowan from the Wisconsin or moraine- 

 forming stage. 



From my studies of the glacial lake Agassiz, whose duration was prob- 

 ably only about 1,000 years, the whole Champlain epoch of land de- 

 pression, the departure of the ice-sheet because of the warm climate so 

 restored, the accumulation of the great marginal moraines during pauses 

 of the glacial recession, and most of the reelevation of the unburdened 

 lands, appear to have required only a few (perhaps four or five) thousand 

 years, ending about 5,000 years ago. This closing part of the Glacial 

 period, when the moraines were being amassed, was apparently far shorter 

 than its earlier stages of oncoming and culmination. 



Three or four of the most prominent moraines of the country on each 

 side of lake Agassiz were formed contemporaneousl}^ with the highest 

 beach of the glacial lake, but the formation of that beach could not have 



* More detailed statements of this theory are given in the Am. Geologist, vol. x, pp. 389-362, Dec, 

 1892, and in Proc. Bost. Soc. Nat. Hist., vol. xxvi, pp. 2-17, for Nov., 1892, with ensuing discussion 

 by Professor W. M. Davis and Mr George H. Barton, pp. 17-25. A later note which I published in 

 the Am. Geologist, vol. xv, pp. 194, 195, March, 1895, I should now wish to change so far as it sug- 

 gested departure from the earlier papers here cited. 



tThe Great Ice Age, third edition, 1894. Journal of Geology, vol. iii, pp. 241-277, April-May, 1895. 



