180 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIN PROVINCE. 



Physiographic-ally the Columbia group is readily seen to consist of more 

 than a single unit. Each formation occupies a well-defined wave-built 

 terrace or plain separated by a wave-cut escarpment from the terraces above 

 and below it. At the base of each of the escarpments the underlying 

 Cretaceous and Tertiary formations are at times exposed. The highest 

 terrace is composed of the oldest deposits, the Sunderland, the next lower 

 by the younger, the Wicomico, while the lowest terrace is covered with the 

 youngest or Talbot materials. 



In most places Avhere good sections of Pleistocene beds are exposed the 

 materials were evidently deposited continuously from base to summit. In 

 some places, however, certain layers or beds are sharply separated from 

 underlying or overlying beds by uneven lines. These beds usually dis- 

 appear in short distances and seem to have no relation to similar lines in 

 adjacent regions, showing clearly that they are only local phenomena within 

 the same formation, produced by the contemporaneous erosion of shifting 

 shallow water currents. Since the Pleistocene formations occupy a position 

 so nearly horizontal it should be possible even with the relatively few 

 sections exposed to connect these lines if they represent subaerial erosional 

 unconformities. In the absence of any definite evidence therefore show- 

 ing these lines to be stratigraphic breaks between two different formations, 

 they have been disregarded. Yet it is not improbable that in some places 

 the waves of the advancing sea may not have entirely removed the beds of 

 the preceding period of deposition over the area covered by its transgres- 

 sion. Especially would deposits laid down in depressions be likely to per- 

 sist as isolated remnants which later would be covered by the next mantle of 

 Pleistocene materials. If this is the case, each formation from the 

 Lafayette to the Wicomico is probably represented by fragmentary deposits 

 beneath the later Pleistocene formations. Thus in certain sections the 

 lower portions may represent an earlier period of deposition than the 

 upper. 



In those regions where older materials are not exposed in the base of the 

 escarpment, each Pleistocene formation near its inner margin probably 

 rests upon the attenuated edges of the next younger formation. Since litho- 

 logic differences furnish insufficient criteria for separation of these late 

 deposits and the sections do not afford the data necessary lo distinguish 

 between local inter-forraational unconformities and widespread strati- 

 graphic unconformities resulting from an erosion interval, the whole mantle 

 of Pleistocene materials occurring at any one level is referred to the same 

 formation. The Sunderland is described as includino- all the surface 



