AFTONIAN INTERGLACIAL STAGE 499 



west and west eight miles to Mountain Lake, in Cottonwood County, its 

 whole extent being 37 miles. Its successive portions from south to north 

 are tributary to the East Fork of Des Moines River, to Center and Elm 

 creeks, and to the South Fork of Watonwan River, This West chain has 

 about 25 lakes, extending through a region of moderately undulating till, 

 deposited by the ice-sheet, with no noteworthy areas nor unusually thick 

 included layers of water-deposited gravel and sand, which, indeed, is true 

 of all this county. 



An explanation of these series of lakes, which appears most probable, 

 is that they mark avenues of drainage and occupy portions of valleys that 

 were excavated in the till during the Aftonian interglacial stage, after 

 ice had long covered this region and had deposited the greater part of the 

 drift, but before other glacial stages again enveloped this area beneath 

 the ice-sheet, partially refilling these valleys and leaving along their 

 courses the present chains of lakes. Detailed description has therefore 

 seemed needful, that we may compare the great amount of early glacial 

 erosion and drift transportation and deposition, which preceded the 

 Aftonian stage, with the relatively small amount that can be attributed 

 bore to later glaciation. 



Why a widespread and long amelioration of the cold climate melted 

 back the ice-sheet in Aftonian time so far from the former Nebraskan 

 border, with which the later Kansan and Illinoian border nearly coin- 

 cided, it is hardly possible to conjecture, unless there was a subsidence of 

 the continent during the later part of the Nebraskan stage, followed after 

 the Aftonian stage by a second high continental uplift. 



Kansas Glaciation 



It is reliably proven that the Ice Age was much diversified and even 

 very complex. Yet I believe it more reasonable to ascribe all our North 

 American drift formations to one prolonged and continuous Glacial 

 period, with great fluctuations of the ice-border, especially in the interior 

 of the continent, than to regard our Ice Age as twofold or threefold, in 

 the sense of having had its vast ice-sheet wholly melted away, or even 

 nearly so, with ensuing renewal of snow and icefields. 



Geologically very rare, an ice age would scarcely be duplicated with so 

 nearly the same limits of ice extension upon half of our continent. The 

 same general conclusion is also, as I think, applicable to the European 

 glaciation. Almost inconceivable geologic duration divided the Permian 

 and Pleistocene ice ages. In this most recent ice age, which j continued 

 to the threshold of the historic period, I can not think that the stupen- 



