GEOLOGICAL HISTORY OF THE VIRGINIA COASTAL PLAIN". 317 



terrace was deposited as a flat bench at the base of this escarpment. Ice-borne 

 boulders are also extremely common in the Talbot formation, showing that 

 blocks of ice charged with detritus from the land, were carried down by the 

 major streams and deposited their load over the bottom of the Talbot sea as 

 the ice melted, in precisely the same manner as during the Wicomico and 

 Sunderland epochs. 



Imbedded in the Talbot formation in several places are deposits of 

 dark-colored clay filled with plant remains. The most important one occurs 

 on the Eappahannock River a short distance above Tappahannock. The 

 stratigraphic relation of this and similar lenses of clay occurring elsewhere 

 in the Coastal Plain shows them invariably unconformable on the under- 

 lying beds and apparently so with the overlying sands and loams, although 

 all form a part of the Talbot formation. Although the clay lenses seem to 

 represent erosion unconformities, they do not, however, represent an appre- 

 ciable lapse of time. The clays carrying plant remains are regarded as 

 the lagoon deposits made in ponded areas and gradually buried beneath 

 the advancing beach of the Talbot sea. In these ponds or marshes cypress 

 trees and many marsh plants grew in abundance and their remains now 

 form the vegetable debris found in the clay lenses. As the Talbot sea con- 

 tinued to advance, these marshes were finally covered with deeper water, and 

 deposits of sand and loam continuous with the strata elsewhere were laid 

 down, the line of separation appearing as a line of unconformity. 



Recent history. — At the present time the waves of the Atlantic Ocean, 

 of Chesapeake Bay, and of the other large estuaries are at work tearing away 

 the land along their margins and depositing it on a subaqueous platform or 

 terrace. This terrace is everywhere present in a more or less perfect state 

 of development and closely resembles the earlier terraces. The materials 

 which compose it are variable, depending both on the detritus directly 

 carried down from the land to the sea and on the currents which sweep 

 along the shore. On an unbroken coast the material has a local character 

 while in the vicinity of river mouths the terrace is composed of debris 

 contributed by the entire river basin. 



Beside building a terrace the waves of the Atlantic Ocean and of Ches- 

 apeake Bay and its tributaries are cutting sea cliffs more or less pronounced 

 along their coasts. The height of these cliffs depends not so much on the 

 force of the breakers as on the relief of the land against which the waves beat. 

 A low coast line yields a low sea cliff and a high coast line a high one. The 

 one passes into the other as often and as suddenly as the topography changes 

 so that as one sails along the shore of the Bay or estuaries high cliffs and 



