848 W. UPHAM — PREGLACIAL AND POSTGLACIAL VALLEYS. 



period, that is, to a time during the Wisconsin or moraine-forming stage, 

 rather than to the distinct ghicial epochs which Coleman infers from 

 them. The evidence of two closely consecutive glacial recessions and re- 

 advances in Scarboro' and the thinning out of the thick deposits of till 

 so formed within a few miles westward imply that the ice border during 

 that whole time was near, but seem quite inexplicable on the hypothesis 

 that these till formations record great re-advances of the ice, as either to 

 the lowan stage or to the AVisconsin moraines. Furthermore, the thick 

 Scarboro' stratified beds were evidently amassed as deltas, and the origin 

 of so large a suppl}^ of sediments seems referable only to their derivation 

 chiefly from englacial drift exposed b}^ ablation on the margin of ice- 

 fields within the drainage area of the delta-forming streams. In this 

 tract of confluence between the great eastern and central lol)es of the 

 Laurentide ice-sheet, represented b}" the angle of the drift boundary at 

 Salamanca in southwestern New York, there undoubtedly was brought 

 an exceptional volume of the englacial drift by the confluent glacial 

 currents. 



The Ice age is found divisible into two parts or epochs, the first or 

 Glacial epoch being marked by high elevation of the drift-bearing areas 

 and their enveloi)ment by vast ice-sheets, and the second or Champlain 

 epoch being distinguished by tlie subsidence of these areas and the de- 

 parture of the ice, with abundant deposition of both glacial and modified 

 drift. Epeirogenic movements, first of great uplift and later of depression, 

 are thus regarded as the basis of the two chief time divisions of this 

 period. Each of these epochs is further divided in stages, marked in the 

 Glacial epoch by fluctuations of the predominant ice accumulation, and 

 in the Champlain epoch l)y successively diminishing limits of the waning 

 ice-sheet, which, however, sometimes temporarily re-advanced, inclosing 

 stratified and fossiliferous beds between the unstratified glacial deposits. 



