22 THE GEOLOGICAIi STRUCTURE, ETC. 



these beds dip south-eastward at a low angle, which will carry 

 them below the level of Chesapeake Bay. 



In Virginia, few exposures of the impermeable Upper 

 Cretaceous beds have been noted, which is partly due to thfr 

 extensive area over which the surface of the State is occupied by 

 Tertiary beds, which surface erosion has not entirely denuded. 



At Fort Monroe, the water-carrying sandy strata (the 

 Potomac Group) were met with in a well at a depth from 835ft. to 

 907ft. The overlying impervious Cretaceous beds were appar- 

 ently absent. The total thickness of the Potomac Group has 

 not been ascertained, but the United States Geological Survey 

 estimates it at from 500ft. to 600ft. 



In the States of North and South Carolina, the sandy 

 porous beds at the base of the Cretaceous, are seldom exposed. 

 Those which are known are few and unimportant, but such as 

 they are they afiford evidence of the continuity of the formation 

 throughout the whole of the Atlantic Border region. 



In a deep well at Aiken, South Carolina, the full thickness 

 of the Potomac Group is exhibited. The formation consists 

 successively of sand (55ft.), fine white sand and kaolin, locally 

 called chalk (30ft.), and sand and soft sandstone, with some clay 

 (335ft.), reaching a total thickness of 420ft. The well ended in 

 granite. The few exposures of the Cretaceous strata in North 

 and South Carolina, demonstrate the dip of the beds to be 

 seaward, and the overlying Tertiary strata have a similar 

 inclination. 



With reference to the Cretaceous beds of the Atlantic 

 region, the northern New Jersey section shows that the western 

 margin of the formation reaches an altitude of 300ft. above sea 

 level, while the eastern margin is 900ft. below high water 

 mark ; the North Carolina section shows a similar state of 

 affairs, but the depth of the eastern margin beneath the sea 

 level is not nearly so great. 



From the Atlantic Border, the Cretaceous and overlying 

 beds are continuous into what is known as the Gulf Border 

 Region, which includes a portion of the States of Georgia, 

 Alabama, Mississippi, Louisiana, Tennessee, Kentucky, and the 

 whole of Florida. The maximum elevation of this region is 

 scarcely over 600ft. 



