DISTINCT GLACIAL EPOCHS. 8 1 



the deposits made by the glacial streams would be correspond- 

 ingly coarser. In these deposits, if they persist to the present 

 day, we should find conclusive evidente of the swiftness of the 

 streams. If it can be shown that during the deposition of one 

 sheet of drift drainage was sluggish, and that during the deposi- 

 tion of a later body of drift the drainage was vigorous, these 

 facts are evidence of an interval between the two times of drift 

 deposition, sufficiently long to accomplish the corresponding 

 changes in elevation or attitude. Since such changes of altitude 

 and attitude are generally believed to have been accomplished 

 slowly, the interval must be believed to have been of considera- 

 ble duration. 



It is true that continental altitudes and attitudes might 

 change during a single epoch of glaciation. If the change thus 

 brought about resulted in increased slope, the more sluggish 

 drainage of the earlier part of the epoch would be gradually 

 transformed into the more vigorous drainage of the later part. 

 In this case, if the evidence of both the earlier sluggish drainage 

 and of the later vigorous drainage remain, there should also 

 remain the evidence of the intermediate stages. If the deposits 

 representing the intermediate condition of drainage do not exist, 

 while those representing both extremes do, there would be the 

 best of reason for believing that the intermediate phases of drain- 

 age did not exist during a glacial epoch, but during an interglacial 

 epoch, when streams were not handling glacial debris, and when 

 they were eroding rather than depositing. The deposits of the 

 slow and of the swift drainage might occur in such relations as 

 to prove, beyond peradventure, that intermediate stages oi glacial 

 drainage never existed. 



If the sluggish drainage accompanied the maximum ice inva- 

 sion, while the vigorous accompanied a lesser, the evidence of 

 the swift streams might be found far north of the southern limit 

 of the earlier drift. The farther north of the outer border of 

 the older drift the gravel representing the vigorous drainage of 

 the later and minor ice-sheet occurs, the further the ice must 

 have retreated before the change from the one type of drainage 



