MESOZOIC 



CRETACEOUS. 



The Cretaceous deposits of Virginia constitute part of the basal sedi- 

 ments of the Coastal Plain and overlie the crystalline rocks of the Pied- 

 mont Plateau along its eastern margin. Well borings near the eastern 

 border of the Coastal Plain have never penetrated deposits of earlier age 

 overlying the floor of crystalline rocks although they may exist still farther 

 to the eastward beneath the submarine portion of the Coastal Plain. The 

 Virginia deposits of this age are confined entirely to the Lower Cretaceous 

 along the line of outcrop, although Upper Cretaceous deposits have been 

 recognized in the deeper well borings. Where exposed at the surface these 

 Lower Cretaceous sediments have been transgressed by the Tertiary forma- 

 tions, either Eocene or Miocene deposits overlying them uncomformably, 

 as the case may be, although in the absence of the latter the Quaternary 

 deposits often rest immediately on the Cretaceous beds. 



LOWER CRETACEOUS. 



BY 

 EDVS7AKD W. BERRY. 



THE POTOMAC GROUP. 



Along the eastern border of the Piedmont Plateau, lying for the most 

 part directly on the flanks of its ancient crystallines and constituting the 

 basal element of the Atlantic Coastal Plain, is a series of mostly uncon- 

 solidated, arenaceous, arkosic, argillaceous and often ferruginous or lignitic 

 sediments of highly varied character. Their outcrop constitutes a relatively 

 narrow belt, extending from Pennsylvania to Alabama and ranges from a 

 few to 20 miles in width, its landward boundary lying somewhat westward 

 of the "fall-line". The thickness of the beds at the point where they pass 

 beneath tide, ranges from 100 to 1,000 feet, dependent largely upon whether 

 the full sequence of formations is present or not, although the formations 

 themselves are very variable in thickness. 



