ESTIMATED TIME RATIOS . 513 



mapped in Minnesota were accumulated, marking pauses or slight re- 

 advances in the wavering general recession of the glacial boundary. The 

 seventh to the twelfth of these moraines were formed contemporaneous] \ 

 with the Glacial Lake Agassiz, besides probably numerous other's "farther 

 north, beyond the twelfth, or Vermilion, moraine. But the entire dura- 

 tion of Lake Agassiz, estimated from the ratio between the erosion and 

 beach deposits of its shores and those of the Great Lakes tributary to the 

 Saint Lawrence, can not comprise more than two thousand years, the 

 ratio being certainly no greater than as one to four or five. Therefore it 

 may be inferred that the Wisconsin stage of glaciation, from its earliest 

 Altamont moraine to the end of the existence of Lake Agassiz, did not 

 exceed four or five thousand years, having begun about ten thousand years 

 ago. The principal uplift of the continent from its subsidence beneath 

 the weight of the ice-sheet took place while Lake Agassiz yet existed, the 

 Champlain submergence being thus mainly terminated at the same time 

 with the melting of the latest remnants of the vast continental icefields. 



Recurring to Gilbert's estimate of the ratio for the high stages of Lake 

 Bonneville, we may refer the Kansan and later stages of the ice-sheet, 

 with the Yarmouth, Sangamon, and Peorian recessions and readvances 

 of the glacial boundary, to a relatively late part of the Ice Age, during 

 the second high rise of that lake, measured probably by about 40,000 years 

 and ending nearly 5,000 years ago. Under this view the maximum area 

 of the Kansan icefield was perhaps attained, as thought by Leverett, twice 

 as long ago as the farthest advance of the Illinoian icefield, which over- 

 lapped the edge of the Kansan drift. The successive farthest limits of 

 the ice-border in these stages, noted approximately, may have been about 

 40,000, 20,000, 12,000, and 10,000 years ago, respectively, for the Kansan, 

 Illinoian, Iowan, and Wisconsin glaciation. 



The long Aftonian interval, bringing far retreat of the outer part of 

 the ice-sheet, was probably about 50,000 years ago. Earlier the very pro- 

 longed Nebraskan glaciation, correlated with the first great rise of Lake 

 Bonneville, appears to have extended through fully 150,000 years. Strict 

 acceptance, however, of the ratio from the lacustrine record, as in the 

 proportion of five to one, would give even 200,000 years, on this pro- 

 visional time scale, for the formation of the Nebraskan drift sheet, ac- 

 counting very amply for its observed thickness beneath the Aftonian beds 

 and the later glacial drift in Iowa and southern Minnesota. 



Within the first half or third of the Nebraskan stage very thick ice 

 accumulation southwest of Hudson and James bays, upon the present 

 area of Patricia, a district of Canada, caused currents of glaciation to 



