TERTIARY UPLIFT OF THE CRETACEOUS PENEPLAIN. 565 



altitude above sea-level attained by its several parts ; second, in tlie valleys 

 and lowlands that have been etched into it, as well as in, third, the stripping 

 off of a good part of the Cretaceous cover. 



From this it appears that both in the Cretaceous submergence and in tlie 

 post-Cretaceous elevation the movement concerned was the same, in so far 

 as it involved a tilting on a fulcrum or axis; but between these two dates 

 the axis ^vas shifted seaward, and thus a different warping resulted in the 

 two cases, particularly at points situated between the tw^o positions of the 



_.^---^^V3-- 







Figure 5— Section illustrating the Development of middle Atlantic Slope Topography. 



fulcrum. In the later movement the fulcrum (F,, figure 6) lay further to 

 seaward than in the earlier (F^, figure 5) ; and hence the previously depressed 

 portion of the Cretaceous peneplain was then elevated : from having been 

 below sea-level, it rose above sea-level. 



The general location of the sea-level line on this strip of disputed territory 

 after the Tertiary uplift may be briefly indicated, although the determi- 

 nation of its position is much complicated by late Tertiary and Quaternary 

 oscillations of level of some magnitude and persistence. In New England, 

 siuce the uplift and the cutting of the valleys, there has been a distinct de- 

 pression, by W'hich the lower valleys were submerged ; the shore-line during 



C;^ / \ 1 -^ • I ' Csf 



Figure G— Section illustrating tlie Development of middle Atlantic Slope Topography. 



the greater lapse of time since the elevation of the old peneplain has there- 

 fore presumably stood somewhat to the seaward of the present shore-line. 

 Going southward, signs of considerable Tertiary oscillations, a late Tertiary 

 elevation, and a recent and brief submergence complicate matters; but in a 

 very general way it may be said that the line marking the position of the 

 sea-shore assumed by the old Cretaceous peneplain as it rose with the Cre- 

 taceous cover on its back, lay two-thirds or more of the way from the present 

 inner margin of the Cretaceous formation to the present sea-shore. 



Altitude of the interior Portion of the Peneplain. — The elevation given to 

 the peneplain as we advance inland can be measured by the altitude of its 



LX XX 11 1— Bull. Geol. Soc. Am., Vol. 2, 1890. 



