ESTUARINE DEPRESSIONS OF THE COASTAL PLAIN. 447 



inclination. There is also a series of similar transverse pre-Appomattox ter- 

 races and slopes along the great transverse drainage depressions, which add 

 complexity to the contour of the basal surface, but at the same time throw 

 much light on the geologic history of the region. 



In the first place, as all these shallow terraced basins appear to be pre- 

 Appomattox in age, their existence records the interesting fact that the 

 transverse depressions of the coastal plain region were first excavated by 

 the retreating waters which carved the lougitudiual terraces during the 

 interval between the deposition of the Chesapeake and Appomattox forma- 

 tions ; for the pre-Appomattox formations bear no records of the presence of 

 transverse drainage. In the second place, it is found that these terraces 

 present evidence of a post-Appomattox deformation in their extension to 

 progressively lower minimum levels from north to south from Maryland to 

 North Carolina. In the Roanoke basin the Appomattox is brought down 

 to tide-level at some points by the terraces, but northward the minimum 

 elevation of its base gradually increases finally to an altitude of 250 feet at 

 the head of Chesapeake bay. As it is altogether improbable that there was 

 longitudinal incliuation to the floor ou which the Appomattox formation 

 w^as deposited, this gradual northward slope indicates an uplifl approximating 

 250 feet in amount distributed through the interval of about 250 miles. 



The origin of the oblique southward deflection of the rivers across the 

 western part of the coastal plain does not appear to be related to this longi- 

 tudinal uplift. It is probably due either to a shallow pre-Appomattox flex-^ 

 ing along the Piedmont shore or to shallow channels just off the mouths of 

 the pre-Appomattox streams. In either case the result would be a south- 

 ward deflection of the drainage into these lines when emergence took place. 



As I have not yet studied the seaward extension of the Appomattox for- 

 mation, I have not observed its overlap by the Columbia formation ; but the 

 two formations are separated by a great uplift and erosion interval. This 

 epoch differed from its base-leveling predecessors by greater relative emer- 

 gence and consequent stream action which developed the greater part of 

 the present physiography of the region. This erosion deepened and greatly 

 widened the transverse drainage depressions, trenched the side drainage 

 depressions, and cut into the edges of the terraces to an extent gradually 

 increasing northward from North Carolina, and in northern Maryland result- 

 ing in the removal of wide areas of the coastal plain formations, especially 

 the Chesapeake and Appomattox. 



Taxonomy. — No fossils have, yet been discovered in the Appomattox for- 

 mation in the Virginia-Maryland region, but the structural evidence above 

 presented definitely places its stratigraphic position between the Chesapeake 

 and Columbia formations, and widely separated from both by long erosion 

 intervals. Its precise age is unknown. 



LXV— Bull. Geol. Soc. Adi., Vol. 2, 1890. 



