TERTIARY EROSION INTERVALS NORTH CAROLINA AND VIRGINIA 673 



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EROSION INTERVALS IN THE TERTIARY OF NORTH CAROLINA AND VIRGINIA 1 



BY BENJAMIN L. MILLER 



Contents Page 



Introduction . 673 



Eocene formations 674 



Post-Eocene erosion interval 675 



Miocene formations 676 



Pliocene formation 677 



Relations of the North Carolina Tertiary formations 677 



"Hatteras axis" 678 



Introduction 



In the Atlantic Coastal plain the recognition of unconformities is not an 

 easy matter. This is due to several causes. In the first place, the formations 

 in general represent littoral deposition, and the character of the materials 

 changes very rapidly, both horizontally and vertically. For this reason it is 

 difficult to definitely recognize the same formation by its lithologic character- 

 istics over wide areas except where we have continuous exposures. Second, 

 the covering of Pleistocene sands, gravels, and loams has concealed the under- 

 lying deposits over the stream divides to such an extent that outcrops can only 

 be found at occasional intervals in the stream valleys. With practically all of 

 the streams that are of sufficient size to have cut through this surface covering, 

 flowing in the same general direction — to the southeast — there are few con- 

 tinuous exposures from one major drainage basin to the other, and since the 

 lithologic characteristics may be dissimilar in two adjacent valleys, it becomes 

 impossible to determine definitely whether the strata in the two places repre- 

 sent the same beds or not. Further, the fossiliferous strata occur in the form 

 of lenses of variable extent separated by non-fossiliferous strata. Thus the 

 absence of a fossil layer in one valley and its presence in an adjacent one does 

 not necessarily mean that erosion has removed it from the former place. 

 Again, the dip of the strata composing the Coastal plain is slight, especially in 

 the case of the Tertiary strata, seldom exceeding 15 to 20 feet per mile. This 

 varies somewhat in the different formations, though it is very unusual to have 

 two formations in contact that exhibit different inclinations of sufficient mag- 

 nitude to prove the presence of an erosional unconformity between them. 



During the Tertiary period there is evidence to prove the submergence and 

 elevation of the Coastal plain as a whole a number of different times, although 

 seldom was the region elevated sufficiently to permit streams to carve valleys 

 of any considerable depth before a succeeding depression took place. For the 

 reasons given above, throughout the Coastal plain generally the various forma- 

 tions recognized are apparently conformable in that few irregular lines of con- 

 tact can be discovered between them. The unconformities determined are 

 mainly those of overlap. These overlap unconformities are exhibited in the 

 northern part of the Atlantic Coastal plain in New Jersey, Delaware, and 

 Maryland particularly, and have been described in the published literature. 

 Similar unconformities are recognized in Virginia, and no doubt they have 



i Manuscript received by the Secretary October 15, 1909. 



