GEOLOGICAL HISTORY OF THE VIRGINIA COASTAL PLAIN. 215 



beaches great accumulations of fragmental shell deposits Avere formed. 

 These constitute the shell beds so well exposed in the cliffs at Yorktown. 

 All during the Miocene there had been a gradual change in marine life, 

 and in this latest period the forms were more closely related to the present 

 fauna than those that lived earlier. 



Pliocene history. — During the earlier Tertiary periods the adjacent land 

 of the Piedmont Plateau had been subjected to erosion. The low character 

 of the country prevented the weathered products of the Piedmont from being 

 carried off by the sluggish streams. In late Pliocene time a subsidence 

 occurred which brought the entire Coastal Plain and the margin of the 

 Piedmont Plateau beneath the water. This was the greatest submergence 

 that had occurred since the close of the Lower Cretaceous and perhaps was 

 even more extensive than that during the Patuxent epoch. Coincident 

 with the subsidence there seemsi to have been a slight elevation and tilting 

 of the region west of the shore-line. The heads of the streams were given 

 renewed force, enabling them to carry down and deposit over this region 

 large quantities of gravel and sand derived from the Piedmont and Appa- 

 lachian rocks to the AvestAvard. The evidence for the source of this material 

 is found in the many different pebbles which, by their lithological character 

 or the fossils Avhich they contain, can be traced to their origin. In the 

 vicinitj" of Washington and Eichmond gravels are found containing fossils 

 of Devonian and Carboniferous age, brought from beyond the Blue Eidge. 

 These show that the Potomac and James rivers had extended their drainage 

 basins far to the Avest. During the submergence of the region beneath the 

 Lafayette sea conditions Avere not uniform over the entire area, as we have 

 gravel deposits forming in some places at the same time that other beds were 

 being deposited in adjoining regions, yet on the whole, sedimentation was 

 remarkably uniform over the entire area, considering the circumstances 

 under which it took place. Over the former land surface a persistent cap 

 of graA'el was deposited. ToAvard the close of Lafayette submergence the 

 land slowly rose, and the velocity of the streams was so changed that gravel 

 could no longer be carried doAvn except in occasional freshets. Fine sand 

 and loams Avere laid down over the gravel which had been previously 

 deposited. This loam, Avhich is so extensively developed over a large part of 

 the region occupied by the Lafayette deposits, apparently marks the end of 

 Pliocene sedimentation. It marks also the last time that the entire Coastal 

 Plain of Virginia has been beneath the ocean waters. 



Pleistocene history. — At the close of the Pliocene epoch the region was 

 raised again and extensively eroded. It appears that after the close of the 



