T. C. Chamberlin — Diversity of the Glacial Period. 197 



the necessary support of a recognizable outlet, the hypothetical 

 lake may well be dismissed from the literature of the subject. 

 The whole phenomena fall into perfect consonance with the 

 phenomena of the Delaware and Susquehanna. 



As one of the strong, and as it seems to me, unanswerable 

 arguments in favor of a considerable interval between an 

 earlier and a later drift formation, I appeal to the cutting of 

 the Delaware gorge 200 feet or more, which was demonstrably 

 later than the high terraces bearing glacial silts and gravels 

 and demonstrably earlier than the Belvidere moraine from 

 which a gravel stream was poured down into the gorge cut 

 during the intergiacial interval. I make a like appeal to a sim- 

 ilar class of phenomena on the Susquehanna, and I repeat the 

 appeal in respect to the precisely analogous phenomena of the 

 Allegheny and other tributaries of the Upper Ohio, and the 

 upper part of the river itself. I make a similar appeal to the 

 erosion of the lower Mississippi valley and several of its 

 branches, the erosion here however sustaining a different rela- 

 tion to the old erosion plane. Doubtless the appeal could be 

 made to all branches if the import of the phenomena were 

 equally clear in all cases, or had been equally studied. The 

 form which the valley erosion took and the material eroded 

 varied with the antecedent and concurrent conditions which 

 were not the same in all valleys, nor the same in all parts of 

 the same valley, but a correspondent erosion occurs on all 

 branches that have been carefully studied so far as I am in- 

 formed. All this class of phenomena repeating itself over and 

 over from the Atlantic to the western plains, carries a force 

 from which I think there will be found no escape when the 

 phenomena are critically and judicially investigated. 



But the case does not rest upon the intergiacial cutting of 

 thesa river channels alone. It is supported by concurrent 

 erosion over the whole surface of the old drift. If there were 

 any error in the interpretation of the river channels it would 

 be shown in the facies of the old drift. But its topography 

 shows an amount and kind of erosion that indicates similar 

 antiquity. The phenomena are so wide spread that special 

 enumeration is wholly impracticable. In a paper on the Drift- 

 less Area, Prof. Salisbury and myself attempted a careful 

 description of a part of these erosion features and drew a com- 

 parison between them and those of the new drift. There, ic is 

 to be noted, the two borders were subject to the same condi- 

 tions save that of age. 



One of the best fields for a comparison between the fresh 

 little-eroded topography of the later drift and the much-eroded 

 topography of the earlier drift, is found on the borders of the 

 tongue of the later drift which terminates at the capital of 



