578 THE IGE AGE IN NORTH AMERICA. 



not have belonged to the original forests which covered the 

 country in front of the first sheet of advancing ice. These 

 logs may have been picked up like the bowlders, and trans- 

 ferred to the south a long time after their original deposi- 

 tion. Thus, it may be that the " forest-beds " near the mar- 

 gin of the glaciated area are of more recent origin than those 

 some distance back, since the ice in its final retreat may 

 have proceeded with few and slight oscillations. As Presi- 

 dent Chamberlin suggests, also, " certain subaqueous deposits 

 so closely resembled true till that they have been mistaken 

 for it, and there is perhaps no case of superposition of beds 

 supposed to represent two glacial periods that is not still 

 open to these doubts."* 



President Chamberlin, whose knowledge of the facts 

 bearing on this subject is wider than that of any one else, 

 therefore does not rely so much upon the existence of in- 

 closed forest-beds and a supposed superposition of distinct 

 beds of glacial debris, in proof of distinct glacial epochs, as 

 upon certain other considerations of a more general nature, 

 such as the following : 



The earlier drift is characterized, in the interior basin, 

 by a wide but relatively uniform distribution, manifesting 

 only occasional and feeble tendencies to aggregation in mo- 

 rainic ridges. It is not bordered, except in rare instances, by 

 a definite terminal moraine, but ends in an attenuated border. 

 It is not characterized by the prevalence of prominent drum- 

 lins or other similarly ridged aggregations. The phenomena of 

 glacial erosion connected with it are generally feeble. Glacial 

 striae are indeed present, even in the peripheral portions, but 

 the surface of the rock is not usually extensively planed. The 

 whole aspect of the deposit indicates an agency which spread 

 the drift over the surface smoothly, and relatively gently, with 

 little forceful action. The drainage phenomena are also of 

 the gentle order. We have yet failed to find evidence of very 

 vigorous drainage connected with the older drift of the in- 



* See " Geology of Wisconsin," vol. i, p. 272. 



