48 PHYSIOGRAPHY AXD GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



the mouths of these inlets and bnilt up largely of material carried out from 

 the lagoons are very changeable so that the buoys marking the channel ap- 

 proaches must be frequently shifted. 



Tlie lagoons are only temporary features as they are gradually being 

 filled by the accumulation of vegetable material, by the debris carried from 

 the mainland, and to a certain extent by the sand carried by the wind from 

 the sandy beaches. If the land remains at its present level with respect to 

 the ocean, these lagoons will ultimately disappear by this infilling and the 

 sandy islands will be connected with the mainland as they are along many 

 parts of the Atlantic coast at tlie present time. 



SUBAERIAL DIVISION. 



GENERAL CHARACTERISTICS. 



The emerged portion of the Coastal Plain declines with gradually lessen- 

 ing slope from the Piedmont Plateau l)order to the shore-line. The highest 

 portions of the Virginia Coastal Plain are found in Fairfax County near 

 Washington. Southward the Western margin gradually declines in altitude. 

 The outlier in Fairfax County already referred to covers a flat-topped divirl-^ 

 almost 500 feet above sea level, while in Spottsylvania County similar 

 deposits are found abont 400 feet above sea level. In the southern portion 

 of the State the greatest elevation of Coastal Plain deposits is probably 

 little more than 200 feet. These differences in elevation are responsible for 

 the much more diversified topography in the northern than in the southern 

 portion of the State. The easterly slope of the Coastal Plain from the 

 "fall-line" which near the western margin may reach 10 or 15 feet to the 

 mile, or even more, gradually declines to less than a foot to the mile in the 

 extreme eastern part of the State. On either side of Chesapeake Bay there 

 are wide areas of monotonously flat lands only a few feet above the water 

 level. The average elevation of the Virginia Coastal Plain is slightly more 

 than 100 feet above tide, 



TERRACES. 



The topography of the Virginia Coastal Plain, although of low relief. 

 is in reality more complex than at first appears, this complexity being due 

 to a series of terraces which stretch across the region, each in succession 

 wrapping about the preceding in concentric arrangement. These terraces 

 are five in number and have been designated by the names of the formations 

 whose deposits cover them. They are in order, from the highest to the 

 lowest, the Lafayette, the Sunderland, the Wicomico, the Talbot, and the 

 Eecent. 



