DISTINCT GLACIAL EPOCHS. 79 



be made in the case of drift sheets along the margins of con- 

 fluent or proximate ice lobes. In such cases, if the one lobe 

 temporarily secured the advantage ' of the other, drift beds 

 formed by movements from opposite directions might be found 

 in vertical succession, without being evidence^ of separate ice 

 epochs. 



It is no part of the purpose of this essay to point out the 

 difficulties which might arise in the application of this criterion 

 of diverse directions of ice movements. It is possible that 

 gradual changes in the direction of movement might leave records 

 which would seem to indicate abrupt changes instead. This possi- 

 bility makes care necessary in the application of the criterion, 

 but does not destroy its value. When not itself conclusive, this 

 criterion may be so associated with differential weathering, dif- 

 ferential erosion, forest beds, etc., that their combined testimony 

 makes but- one conclusion possible. 



The absence of evidence of radically diverse directions of 

 movement during the time of deposition of the various sheets of 

 drift, would be no proof that there were not distinct epochs. In 

 the first place, the movements of different epochs might be har- 

 monious — a condition of things more probable than any other if 

 the more common views of the causes of glaciation be correct. 

 In the second place, if the movements were diverse, the deposits 

 might still be so similar that their differentiation, when the one 

 is buried, might not be easily made. In the third place, the 

 later ice might have so far incorporated the older drift material 

 with that which belonged more properly to it, as to have 

 destroyed all definition between them. 



(10) The Superposition of Beds of Till of Different Physical 

 Constitution. After the retreat of an ice-sheet, the surface of the 

 countrv thus discovered would be largely mantled with drift. 

 This drift would serve to protect the underlying rock from dis- 

 integration. But where there was little or no drift, the rock sur- 

 face would be subject to all the disrupting agencies which affect 

 surface rocks. The same would be true of all rock surfaces 

 bared by subaerial erosion after the disappearance of the ice. 



