the lafayette foemation, 205 



Pliocene (?). 



The Lafayette Formation. 



The Lafayette formation is similar both in its materials and physiogra- 

 phic relations to deposits which have received the same name throughout the 

 Atlantic and Gulf borders. The formation occupies a relatively narrow 

 belt along the western margin of the Coastal Plain and at times occurs in 

 isolated areas on the Piedmont Plateau. It forms the oldest and highest of 

 the series of the late Tertiary and Quaternary terraces that characterize the 

 Coastal Plain. In the absence of fossils this physiographic feature becomes 

 one of the most important criteria for the correlation of the formation. The 

 structure and composition of the strata already described are also highly 

 characteristic and aid materially in the recognition of this formation where- 

 ever found. 



The age of the Lafayette formation is in much doubt. Prom its known 

 stratigraphic relations it is evidently younger than the latest Miocene beds 

 and probably younger than the marine Pliocene of the Atlantic border. 

 These earlier beds were apparently subjected to a long period of erosion 

 before the deposition of the Lafayette began. Again, after the close of the 

 Lafayette it is probable that an extensive epoch of erosion occurred before 

 the Columbia deposits of Pleistocene age wore laid down. This would assign 

 to the Lafayette formation a position between the marine beds of Pliocene 

 age and the oldest deposits of recognizable Pleistocene age. The Lafayette 

 formation is therefore either of late Pliocene or early Pleistocene age. The 

 consensus of opinion hitherto has been in favor of the former interpretation 

 but in the absence of fossils it is practially impossible to reach a positive con- 

 clusion. It is evident that more adequate evidence must be secured before 

 the question can be regarded as settled, especially, as Berry has shown, as 

 has previously been mentioned, that the type sections in Lafayette County, 

 Miss., are of Eocene age, thus necessitating a new name for the later 

 Atlantic coast deposits. 



QUATERNARY. 



The Quaternary formations of A^irginia comprise both Pleistocene and 

 Eecent deposits. Since they represent the most recent phase of deposition 

 and still preserve in a marked degree their original form, physiographic 

 criteria are of much importance in interpreting and correlating the deposits. 



