THE TERTIARY PERIOD 



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New York and much of New England. Hence we assert, with 

 emphasis, that all the principal topographic features of this region 

 as we see them today date from the uplift of the Cretaceous 

 peneplain, because they have been produced by the dissection of 

 that upraised surface. This dissec- 

 tion was largely the work of erosion, 

 though more locally (e.g. the eastern 

 Adirondack Mountains) faulting has 

 produced notable effects. All the 

 valleys, great and small, such as the 

 Champlain, Connecticut, Mohawk, 

 Hudson, the Great Lakes valleys, 

 and the valleys of the Appalachians, 

 have been produced since the uplift 

 of the peneplain. 



The uplift greatly revived the 

 activity of the streams, so that they 

 became very active agents of ero- 

 sion, first cutting channels through 

 the alluvial deposits, and then into 

 the underlying bed rock. Thus 

 these large original streams had 

 their courses determined in the over- 

 lying deposits, and when the un- 

 derlying rocks were reached the 

 same courses had to be pursued 

 entirely without reference to the 

 underlying rock character and struc- 

 ture. Fine examples of such (super- 

 imposed) streams, which are now 

 entirely out of harmony with the 

 structure of the regions through 

 which they flow are the Susque- 

 hanna, Delaware, and Hudson. 

 Thus the Susquehanna cuts across 

 a whole succession of Appalachian 

 ridges, while, in accordance with the same explanation, the 

 Delaware cuts through the Kittatinny ridge at the famous Dela- 

 ware Water Gap. The lowei Hudson pursues a course no less out 

 of harmony with the country through which it passes. It flows at 



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