70 NEW YORK STATE MUSEUM 



o feet to perhaps 2000 feet within the borders of the area (con- 

 tinental tiking or warping j. 



Late Tertiary reelevation. Traces of such an intermediate and 

 incomplete peneplain are to be seen in the compound nature of the 

 large valleys of the present day. ^lost of them are essentially broad 

 valleys into the bottoms of which narrower valleys and gorges are 

 cut. The tops of the minor hills and ridges of the broad valleys 

 represent the intemiediate Tertiary peneplain that was interrupted 

 in its development before completion (interrupted erosion cycle). 

 The inner narrow valleys indicate that for the second time a re- 

 gional elevation rejuvenated the streams and they began their 

 work of cutting to a new grade. They have made a good begin- 

 ning at this task, and as a consequence have carved some reliei 

 in the old valley bottoms. These new streams have not yet reached 

 a graded condition. 



When the glacial ice began to invade this region all of the surface 

 features had had such a history. Leaving out of account minor 

 fluctuations of elevation and depression, of which there may have 

 been several of too transient character to make a lasting impres- 

 sion on the topography, the stages become comparatively few and 

 the general tendencies are easily understood. 



The measurable dillerences of elevation between the Cretaceous 

 and Tertiary peneplains give some reasonable conception of the 

 amount of the first continental or regional elevation. Concerning 

 the altitude reached in subsequent regional elevation there is less 

 certainty. Xcne of the streams, not even the master streams such 

 as the Hudson, reached grade, for it exhibits strictly a gorge t}^e 

 not only within the present land borders, but it is now known to 

 show gorge development far beyond the present coast line. Judg- 

 ing from the Hudson, therefore, it seems necessary to conclude 

 that this continental region stood at a much greater elevation in 

 some portions of the later period than had formerly prevailed. 

 Probably the maximum elevation immediately preceded the glacial 

 invasion. 



Conservative estimates as to the amount of elevation of that 

 time in excess of the present would place it at not less than 2000 

 feet. ^luch more than that is believed to be indicated, possibly 

 5000 feet or more. 



In the meantime, the master stream, the Hudson and several 

 of the tributaries cut into their valley bottoms to such extent as 

 to make typical gorges so deep that their beds now, since the sub- 



