SUBDIVISIONS OF THE ICE PERIOD. 181 



the ice period into two epochs may not be final. If this classification 

 is to undergo modification, it is believed that the change will be in the 

 direction of greater complexity. Data have heen accumulating for sonic 

 time past which would seem to be best explained on the basis of three ice 

 epochs instead of two. Tins suggestion is less of an innovation than it 

 may at first seem to be. President Chamberlin long since recognized two 

 distinct episodes in the first glacial epoch, as classified by him, the two 

 being separated by an interval of milder climate and ice retreat. The 

 suggestion here made would simply emphasize this division already 

 recognized. While President Chamberlin has hitherto regarded this in- 

 terval of mild climate as marking a subordinate interruption of glaciation 

 determining the division of the earlier ice epoch into episodes, Mr. McGee 

 has regarded it as marking the greatest interruption of glaciation during 

 the glacial period, determining the division of the ice period into two 

 epochs. Mr. McGee's first glacial epoch would therefore correspond to 

 the first glacial epoch of the classification here suggested, while his sec- 

 ond glacial epoch would embrace the second and third as here proposed. 

 On the other hand. President Chamberlin's first epoch embraces the first 

 two, and his second the third epoch, if the ice period he divided into 

 three epochs.* 



Briefly characterized, the drift representing the ice advance of the first 

 epoch has no marginal accumulation of the nature of frontal moraines. 

 Its margin is attenuated. The drift representing the ice advance of the 

 second epoch, according to the suggestion here made, is limited by mo- 

 rainal ridges, which are bordered and often covered by loess, loess-loam 

 and silt deposits, which indicate slack drainage; Avhile the drift of the 

 third epoch is limited by stronger terminal moraines of more pronounced 

 topography, in which valley trains and overwash plains of gravel take 

 their origin. These valley trains of gravel often extend many miles 

 down the valleys from the moraines, and demonstrate that the attitude 

 of the Land was such as to determine vigorous drainage. The degree of 

 erosion, oxidation and disintegration of the drift of the several epochs is 

 progressively less, from oldest to youngest. The significance of the silt 

 and loess bordered moraines, as distinct from those bordered by gravel 

 plains and trains in indicating continental attitudes, was long since 

 pointed out by President Chamberlin, as was also the significance of the 

 varying degrees of erosion, decomposition and disintegration of the drift. 

 In briefly indicating, therefore, the broad divisions of the drift, corre- 

 sponding to the three epochs suggested, the features noted are in noway 



Because oi the importance attaching to hi- opinion <>n this question, I am glad t<> say thai 

 Presidenl Chamberlin i- very hospitable n> Lhe suggestion here made of :i tripartite division 

 of tin' glacial period, 



\ \ l\ Bom Hoi \m.. \ ni , ::, 1891. 



