THE CAUSE OF THE GLACIAL PERIOD. 525 



the different view that probably the earlier interglacial lake 

 in this valley may have even cut its channel of southern out- 

 let, at the site of Brown's Valley, to a lower level than the 

 well in Barnesville, or that the attitude of the land then 

 was unlike what it now has, having then such an ascent from 

 south to north that the Barnesville locality in that inter- 

 glacial time was above the general surface of the region at 

 Brown's Valley, into which the River Warren, outflowing 

 from Lake Agassiz, cut its deep continuous valley. So I 

 now think that we have there, in the section of this well, 

 only evidence of an ice retreat (that is, a departure of the 

 outer part of the ice-sheet) so far north as to that region, 

 about halfway between the south and north boundaries of 

 this state.* My numerous papers show how, as I think, 

 good forests and other luxuriant vegetation may have 

 flourished near the border of the ice-sheet, accompanying 

 the recession of that border. 



The questions are sure to be asked: Why is the boundary 

 of the glaciated region in the United States so irregular? 

 What was the cause of its withdrawing so far north in western 

 New York, and of its sudden bend to the south in eastern 

 Ohio, and of its lobe-like projections in southeastern Indiana 

 and southern Illinois ? And what was the cause, at a later 

 stage, of the lobate contour of the moraines west of Lake 

 Michigan in Wisconsin, Iowa, Minnesota, and Dakota ? These 

 questions we can only answer by saying that the distance to 

 which the great American ice-sheet penetrated the southern 

 latitudes was evidently not dependent, to any great extent, 

 upon the elevation of the land over which it was compelled to 

 move. The ice did not uniformly move farther south where 

 there was a depression of the land, and the boundary does 

 not ordinarily retreat to the nortli on account of the higher 

 elevations opposing its progress. South of New England the 



* Geology of Minnesota, vol. ii, pp. 661-2, and 668; with the map 

 facing p. 656. 



