24 The Lower Cretaceous Deposits of Maryland 



The Coastal Plain is the name applied to the low and partially sub- 

 merged surface of varying width extending from Cape Cod southward 

 through Florida^ and confined between the Piedmont Plateau on the 

 west and the margin of the continental shelf on the east. The line of 

 demarkation between the Coastal Plain and the Piedmont Plateau is 

 sinuous and ill-defined, for the one passes over into the other oftentimes 

 with insensible topographic gradations, although the origin of the two 

 districts is quite different. A convenient, although somewhat arbitrary 

 boundary between the two regions in the Maryland area is furnished 

 by the Baltimore and Ohio Eailroad in its extension from Wilmington 

 southwestward through Baltimore to Washington. The eastern limit of 

 the Coastal Plain is at the edge of the continental shelf. This is located 

 about 100 miles off shore at a depth of 100 fathoms beneath the surface 

 of the Atlantic Ocean. It is in reality the submerged border of the 

 North American continent, which extends seaward with a gently sloping 

 surface to the 100-fathom line. At this point there is a rapid descent 

 to a depth of 3000 fathoms, where the continental rise gives place to the 

 oceanic abyss. 



The Coastal Plain, therefore, falls naturally into two divisions, a sub- 

 merged or submaHne division and an emerged or subaerial division. The 

 seashore is the boundary line which separates them. This line of de- 

 markation, although apparently fixed, is in reality very changeable, for 

 during the past geologic ages it has migrated back and forth across the 

 Coastal Plain, at one time occupying a position well over on the Pied- 

 mont Plateau, and at another far out at sea. At the present time there 

 is reason to believe that the sea is encroaching on the land by the slow 

 subsidence of the latter, but a few generations of men is too short a 

 period in which to measure this change. 



The subaerial division is itself separable in Maryland into the Eastern 

 Shore and the Western Shore. These terms, although first introduced 

 to designate the land masses on either side of Chesapeake Bay, are in 

 reality expressive of a fundamental contrast in the topography of the 

 Coastal Plain. This difference gives rise to an Eastern Shore and a 

 Western Shore type of topography. Chesapeake Bay and Elk Eiver sepa- 



