INTRODUCTIOX. 17 



Coastal Plain deposits resting upon an uneven eastward-sloping floor of 

 Piedmont Plateau crystalline rocks. In the depressions of this floor much 

 thicker deposits accumulated than in other places. Everywhere also the 

 deposits thin out toward the west and, since the deposition of the beds, 

 erosion has in many places destroyed the continuity of the mantle of un- 

 consolidated materials. Thus outliers of Coastal Plain sediments, separated 

 from the continuous cover by miles of intervening crystalline rocks, are 

 common. 



The deposits of the Lafayette formation particularly are frequently 

 found overlying the Piedmont crystallines as isolated patches many miles be- 

 yond the limits of the great mass of Coastal Plain sediments. A few miles 

 to the west of Falls Church in Fairfax County is a striking instance of such 

 an outlier of Coastal Plain deposits, covering several square ' miles, and 

 separated from the continuous mantle farther east by a distance of quite five 

 miles. A similar outlier of considerable extent occurs near the small village 

 of Midlothian in the northwestern part of Chesterfield County. Several 

 miles of crystalline rocks intervene between it and the main body of the 

 Coastal Plain deposits. Many such Coastal Plain outliers of large and 

 small extent occur throughout the eastern part of the Piedmont Plateau. 

 They furnish evidence of a former much more extensive Coastal Plain 

 whose marginal deposits have been largely removed by erosion, leaving only 

 small remnants over the divides or in places where, on account of depres- 

 sions, the deposits were unusually thick. The present streams which have 

 cut through the thin marginal strata have exposed the underlying crystalline 

 rocks at times many miles to the east of the points where they are still 

 concealed from view on the uplands of the stream divides. Along the 

 larger streams these crystalline rocks disappear near the level of tide. 

 Even disregarding the outliers mentioned above, the boundary line between 

 the Piedmont Plateau and the Coastal Plain in Virginia is a very irregular 

 line running in a general north and south direction but bending eastward 

 in the valleys of all the eastward-flowing streams and westward over the 

 intervening divides. 



Along the Potomac Eiver the crystalline rocks finally disappear at 

 Washington ; along the Eappahannock they are not seen to the east of Fred- 

 ericksburg; along the James Eiver they disappear at Eichmond, but 

 because of great irregularity in the crystalline floor or on account of post- 

 Tertiary disturbance, they again appear in one place a short distance above 

 Dutch Gap canal; while along the Meherrin Eiver the crystalline rocks 

 disappear below water level a short distance below Emporia. Over the 



