GEOLOGY OF THE VIRGINIA COASTAL PLAIN 



INTRODUCTORY. 



The great body of deposits forming the Coastal Plain of Virginia has 

 been laid down along the border of the Piedmont Plateau on the floor of 

 crystalline rocks of which that district is composed. These deposits are 

 very thin in the vicinity of the "fall-line," but the accumulation of materials 

 gradually increases to the eastward imtil the thickness reaches several 

 thousand feet. At Fortress Monroe a government well penetrates the entire 

 thickness of the Coastal Plain, reaching crystalline rocks at a depth of 

 2246 feet. At first estuarine in character, these deposits were later of 

 marine origin and continued to be chiefly such until the later geological 

 epochs when the marginal deposits of the Pleistocene were laid down in 

 the inclosed bays and estuaries of the dissected Coastal Plain. 



Although composed of a succession of formations which represent nearly 

 every period from the Cretaceous to the Eecent, the Coastal Plain deposits 

 do not succeed each other in a conformable series, nor do they possess the 

 same strikes and dips. Ditferential movements took place that materially 

 affected the attitude of the beds by which, in certain sections, the landward 

 exposures of whole formations that appear in adjacent regions have been 

 eliminated. Thus the Upper Cretaceous deposits, so well exhibited in 

 Maryland, are gradually transgressed by the Eocene southward, shutting 

 out the former throughout the entire area of outcrop in Virginia, althougli 

 recognized in the deep-well borings at Fairport and Fortress Monroe. 



In general the beds strike from north to south although some variation 

 occurs. The strata have a general easterly dip, which changes from 50 feet 

 in the mile in the lowest formations, about the slope of the crystalline floor 

 on which the deposits rest to less than 5 feet in the mile in the highest de- 

 posits. With this relatively low dip the beds generally appear horizontal 

 in any particular section and may be actually so locally, so that measure- 

 ments must be extended over a wide area before the average dip of any 

 particular formation can be determined. 



