DISTINCT GLACIAL EPOCHS. 7 1 



events would certainly be exceptional. In order to constitute 

 the inter -stratified gravel and sand evidence of separate ice- 

 epochs, its continuity for great distances between beds of till, 

 and in the direction of ice movement, would need to be demon- 

 strated. In themselves, these beds, und^r the conditions 

 indicated, would simply be a minimum measure of the amount 

 of ice recession between the deposition of the underlying and 

 overlying bodies of till. It is hardly likely, though possible, 

 that the continuity of inter -bedded gravel and sand could be 

 proved for a sufficient distance north of the southern limit of 

 the less extensive bed of ground moraine, to alone constitute 

 evidence of a recession of ice great enough to make it necessary 

 to refer its re-advance to a new epoch. Beds of silt in like 

 relations, deposited by waters beyond the edge of the ice, would 

 have a like significance so far as the question here under consid- 

 eration is concerned. Such beds of stratified drift might some- 

 times have corroborative value when their testimon}-, taken by 

 itself, is inconclusive. If, for example, their surfaces are marked 

 by forest beds, and especially by forest beds whose plants denote 

 a warm climate, the association becomes most significant. 



In view of what has been said, it is evident that the absence 

 of beds of subaerially stratified silt, sand, and gravel, between 

 beds of till can not be brought in evidence against separate ice 

 epochs. It would rarely be true that topographic and hydro- 

 graphic conditions would make possible the construction of 

 plains of sufficient extent to serve as criteria for the purpose 

 here indicated, and few of those formed would escape such a 

 degree of destruction as to leave them demonstrably continuous. 

 There is also the further possibility that such beds exist, even 

 though their continuity be not known. To prove the contin- 

 uity of a buried bed of stratified and incoherent drift, even if it 

 existed, would be a most difficult task. 



(6) Differential Weathering. If, after covering a given 

 region, the ice retreated, the drift which it left in the area which 

 it previously covered would be subject to oxidation, leaching 

 and disintegration. The depth to which this oxidation, leaching 



