294 G. F. WRIGHT POSTGLACIAL EROSION AND OXIDATION 



their methods of accumulation. The upper till is for the most part 

 material which was held in the upper strata of the ice, and which gradu- 

 ally accumulated in thickness toward the south as the ice melted and 

 deposited its earthy material upon the lower surfaces which were exposed. 

 During this long exposure this material of the upper till imderwent a 

 great amount of oxidation, while the lower till was protected from oxi- 

 dizing agencies. Moreover, the material which had been oxidized in 

 pre-Glacial time was that which was first picked up by the continental 

 glacier and incorporated in its mass and carried onward far toward the 

 southern limit. When the movement had proceeded as far as to the 

 time of the Wisconsin episode, the surface rocks of Canada had been 

 denuded of this oxidized pregiacial material, which readily accounts for 

 the comparatively small oxidation of the Wisconsin till. 



One other point should be kept in mind. As the surface of the ice 

 moved faster than the bottom, much of the material which was carried 

 high up in the glacier would be thrown forward upon the margin and 

 become mingled with the material which was underneath the glacier; 

 so, in periods of advance, after a time of recession this would become a 

 part of the ground moraine and be overridden by the ice, thus furnishing 

 a lower stratum of highly oxidized material covered, as the ice advanced, 

 by a later stratum of less oxidized material. Failure to keep these con- 

 siderations in mind will lead to erroneous inferences concerning the age 

 of deposits of highly oxidized material lying underneath deposits of less 

 oxidized material, for on a little thought it is evident that the two de- 

 posits need not be far separated in age, for the earlier oxidized material 

 is constantly being thrown forward, to be overrun by later movements of 

 the ice in close succession. Every pause in the movement of the front, 

 or temporary recession, would be marked by an accumulation of highly 

 oxidized material, to be covered by deposits of less oxidized material that 

 were continually advancing. The complete oxidation of such a basal 

 deposit would be no evidence of great age. 



Conclusion 



While this evidence does not give us any very definite conclusions as to 

 the age of the drift upon the attenuated border of the glaciated region, 

 it is sufficient to throw doubt on the extreme figures made to represent 

 the length of inter-Glacial and post-Glacial time. The physical condi- 

 tions of the Glacial' period were so abnormal that we are not permitted 

 to apply to that period measures which are drawn from present earth 

 movements and rates of erosion and deposition. It is pretty certain that 



