24 The Upper Cretaceous Deposits of Maryland 



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 often ill-defined, for the one frequently passes over into the 

 other with insensible topographic gradations, although the origin of 

 the two districts is quite different. A convenient, although somewhat arbi- 

 trary, 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 approximately 100 fathoms 

 beneath the surface of the Atlantic Ocean. It is in realitj' the submerged 

 border of the North American continent, which extends seaward with a 

 gently sloping surface to the 100-fathom line. From this point there is a 

 more 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 submarine division and an emerged or subaerial division. The 

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

 cation, although apparently stationary within narrow limits, 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 Piedmont 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 River sepa- 

 rate the two. Areas showing the Eastern Shore type are found along 



