DISTLXCT GLACIAL EPOCHS. TJ 



was placed upon it. The argument is stronger the farther from 

 the margin of the newer drift such erosion on the surface of the 

 underlying older drift can be proved to have taken place. In 

 other words, if, in addition to the greater surface erosion of the 

 older drift sheet as now exposed outside the limit of the newer 

 drift, we find a notable unconformity between the newer and the 

 older drift, and especially if this unconformity lie far enough 

 north of the margin of the newer drift, the argument becomes 

 conclusive. 



When differential erosion and drift unconformities are not in 

 themselves conclusive, they may have great corroborative value 

 in conjunction with differential weathering, forest beds, or other 

 indications of separate ice epochs. 



The absence of observable unconformity between sheets of 

 drift would be no proof that there were not distinct and widely 

 separated ice epochs, since the later ice invasion might have so 

 far modified the surface which it transgressed, as to destroy all 

 patent evidences of unconformity. It would have been antici- 

 pated that distinct unconformities in the drift would be rare, 

 even if there were distinct ice epochs, for the same reason that 

 weathered zones and forest beds would be rare. But if the drift 

 which lies outside a line supposed to mark the limit of a sheet of 

 drift belonging to a later ice epoch, be not more eroded than 

 that which lies within such line, the absence of greater erosion in 

 the outer drift is positive evidence against the reference of the 

 drift of the two areas to distinct ice epochs, if conditions for 

 erosion in the two areas are equally favorable. 



( 8 ) Valleys Excavated Betiveen Successive Depositions of Drift. 

 A closely related, but not identical, point may be found in the 

 extent of the valley excavations which can be proved to have 

 taken place between the deposition of the earlier and later drift. 

 We do not refer to valleys excavated in the drift especially, but 

 to those excavated in other formations as well. If it can be 

 shown, for example, that after the deposition of an earlier drift 

 sheet, and before the deposition of a later, valleys were exca- 

 vated which extended not merclv into the drift itself, but far 



