182 T. G. Chamberlin — Diversity of the Glacial Period. 



thicker than that of to-day. Pine, oak, elm, sumac, walnut, 

 willow, ash, hickory, and tamarac recognized. 



3. An upper till, ranging in thickness from a veneering to 

 probably luO feet, averaging perhaps 20 feet. Erratics gener- 

 ally granitic. Grades horizontally at the south into loam, and 

 vertically either grades upward into loess, or is overlain with 

 a certain unconformity by loess. Only rarely and locally do 

 terminal moraines mark the margin. 



4. A loess mantle, which in part grades into the upper till, 

 and in part overlies it with a definite dividing plane between. 

 Regarded as a deposit contemporaneous and continuous with 

 the upper till. 



These formations are older than the moraine-bordered 

 tongue of till which occupies the north central part of the 

 State and they pass under it. They are separated from it by 

 great general erosion, oxidation, leaching, etc. 



Back of this lie the complicated moraine-bordered deposits 

 of Minnesota. We have here, therefore, a series comparable 

 to that on the east side of the Mississippi. The upper till of 

 Mr. McGee was deposited at a time of slack drainage. The 

 moraine-bordered tongue of Central Iowa was deposited at a 

 higher gradient, as shown by the freer drainage from its edge. 



It thus appears that the latest studies of these two experi- 

 enced investigators, the fullness and detail of whose work in 

 this region is unsurpassed, bring into clearer and sharper defi- 

 nition than ever before, the distinction between these drift 

 sheets, and magnify the importance of the intervals. 



While the question of one or more epochs is not necessarily 

 dependent upon that of the origin of glaciation, there is so 

 intimate a connection between them, that whatever bears upon 

 the one, is not without application to the other. If the great 

 intervals of deglaciation and the great advances of the ice were 

 due to astronomical causes of the type advocated by Croll, 

 every great change, being due to a reversal of the action of 

 the controlling agency, is entitled to distinct recognition. If 

 the chief cause of glaciation is northern elevation, any evi- 

 dence which shows a depression far to the northward carries 

 with it as a logical conclusion the assumption of general deglaci- 

 ation. Prolonged glacial action at low altitudes puts the theory 

 to a severe strain. Halts and advances at low altitudes impose 

 still greater difficulties, the more severe the greater the interval. 

 Even if it is granted that just after the ice reached its max- 

 imum extension, a depression took place because of the load- 

 ing of the ice and that low gradient deposits attended the 

 retreat of the ice, it is evident that there could be no consider- 

 able readvance without a restoration of high altitude. For 

 instance, the ice could not rationally be presumed to retreat to 



