ATLANTIC COASTAL PLAIN AND ADJACENT OCEAN BASIN 



137 



greatest amount, because most all the formations thicken seaward, and 

 each successively higher sedimentary surface dips somewhat less than the 

 "basement" floor. From a number of deep wells that have penetrated the 

 crystallines, the ancient surface can be contoured as shown in Fig. 10.6. 

 Its gentlest slope is in North Carolina, where a dip of 10 to 14 feet per 

 mile exists from the inner margin ( fall line ) to the coast in the southeast- 

 ern part of the state. 



It then breaks seaward into a steeper slope of 122 to 124 feet per mile 

 (Berry, 1948). Two deep wells in northern Maryland demonstrate an off- 

 shore dip there of about 100 feet per mile (Balsley et al., 1946), and a 

 uniform slope is indicated. The two slopes in North Carolina are taken 

 to mean two peneplains by Berry ( 1948 ) , but their local development is 

 puzzling if this theory is true. 



Unconformities. The great unconformity at the base of the Cretaceous 

 has already been implied in the discussion of the slope of the surface of 

 the crystallines. This ancient erosion surface, buried by the Cretaceous 

 sediments, has been called the fall zone peneplain. See block diagrams 

 2 and 3 of Fig. 10.8. Since an outer and sharper slope has recently been 

 defined, the ancient surfaces appear more complicated. It will be dis- 

 cussed further when the continental shelf is considered. 



The Lower Cretaceous beds do not crop out anywhere along the At- 

 lantic Coastal Plain; they form a subsurface wedge between the crystalline 

 floor and the Upper Cretaceous. The dashed lines of Fig. 10.6 show the 

 extent and thickness variations of the Lower Cretaceous. It will be seen 

 that the wedge corresponds in position approximately to the outer steeper 

 slope of the crystalline floor. The isopachs should be related to those of 

 Plate 11 which depicts the distribution of Lower Cretaceous strata in the 

 Gulf Coastal Plain and the Caribbean regions. Not enough is known of the 

 Lower Cretaceous and Upper Cretaceous contact to decipher the rela- 

 tions. The Lower Cretaceous Potomac formation is regarded as nonma- 

 rine, and the overlying Tuscaloosa as marine (Richards, 1945). 



According to Richards' ( 1945 ) correlations the Eocene bevels the Up- 

 per Cretaceous beds near Asbury, New Jersey ( section 32, Fig. 10.2 ) and 

 rests on the Lower Cretaceous in parts of Virginia ( section 33, Fig. 10.3 ) . 



Fig. 10.2. Cross sections of the Atlantic Coastal Plain, after Richards, 1945. Section 32, from 

 Allentown, N. J., to Asbury Park, N. J. Section 34, from Goldsboro, N. C, to Cape lookout, 

 N. C See index map, Fig. 7.1, for location of sections. 



