MARYLAND GEOLOGICAL SURVEY 179 



mantle of Lafayette gravel it was once more elevated above the level 

 of the ocean and remained in this position for a considerable period. 

 It is probable that the drainage which established itself on the La- 

 fayette surface, worked its way downward through the body of that 

 formation and finally became fixed in the underlying crystalline rocks, 

 and as explained above, the origin of the curious river system of the 

 Piedmont Plateau in Cecil county is perhaps to be ascribed to the 

 erosion which followed the uplift of the Lafayette formation. 



SEDIMENTARY RECORD OF THE COLUMBIA GROUP. 



At the close of the post-Lafayette period of erosion, one of the 

 most interesting chapters of the geologic history of Cecil county was 

 commenced. The unsteadiness of the coast-line, which had manifested 

 itself repeatedly during Potomac, Upper Cretaceous, Aquia and La- 

 fayette time, now became intensified, and during Columbia time, a 

 remarkable series of oscillations have been recorded in the deposits 

 of that group. These oscillations are known to have affected the 

 North Atlantic Coastal Plain, and many and possibly all of them will 

 ultimately be recognized in the Coastal Plain further south. As the 

 southeastern two-thirds of Cecil county falls within the province of 

 the North Atlantic Coastal Plain, it shared in all these movements, 

 and each oscillation which is recorded in other portions of the Coastal 

 Plain is here also represented by its distinct formation. 



These formations, to which the names Sunderland, "Wicomico and 

 Talbot have been applied, are developed in terraces lying one above 

 the other in a vertical range from tide to an altitude of about 180 

 feet. Beneath these three terraces, there is forming to-day a fourth 

 which extends from high-tide downwards beneath the waves to deeper 

 water. 



The key to the interpretation of these terraces is secured by study- 

 ing the manner in which this recent terrace is forming. At the 

 present time the waves of the Atlantic Ocean and Chesapeake Bay 

 are engaged in tearing away the land along their shores and in de- 

 positing the detritus on a submarine platform or terrace. This ter- 



