432 X. H. DARTON — LATER FORMATIONS OF VIRGINIA AND MARYLAND. 



Pago. 



Appomattox Formation 445 



Distribution and Characteristics .»-. 445 



Stratigraphic Relations 446 



Taxonomy 447 



Columbia Formation 448 



The Displacement 448 



Course and Relations 448 



Date 449 



Geologic History 450 



Introductory. 



The crystalliae rocks of the Piedmont region in Virginia and Maryland 

 are flanked eastward by an overlapping series of deposits of later Mesozoic 

 and Cenozoic age, which extend thence to the Atlantic ocean, a distance 

 averaging one hundred miles. This district has been designated the " coastal 

 plain," and is in general terms the continuation of the Piedmont plains 

 which, with gradually decreasing elevation, finally pass beneath sea-level. 



The eastward inclination of the crystalline rock-surface on which the 

 coastal plain deposits lie is slight, and the width of the zone of overlap from 

 the feather edges and outliers of the formations to the final disappearance of 

 this floor below tide-level usually averages about ten miles. 



The irregular western terminations of the various formations usually do not 

 give rise to notable topographic features, and in the larger drainage de- 

 pressions the crystalline rocks finally disappear below the elastics, generally 

 at a considerable distance below the head of tide-water. 



Excepting the most recent member, w^hich often forms low terraces, the 

 formations of the coastal plain series constitute a succession of thin sheets, 

 inclined gently seaward and gradually thickening in that direction. Nor- 

 mally their outcrops are from below upward, from west to east; but there is 

 considerable overlapping westward, and the sequence and extent of their 

 relations vary greatly in different parts of the region. 



The formations constituting the coastal plain series are as follows : The 

 Potomac of McGee ; the southern extension and termination of a portion of 

 the New Jersey Cretaceous greensand series, which I shall designate Severn, 

 from typical exposures, described by Clark, on the Severn river near 

 Annapolis ; a representative of the Eocene, restricted to Maryland and 

 Virginia, for which the term Pamunkey is appropriate on account of the 

 typical nature and extent of the exposures on the Pamunkey river, as 

 described by Kogers ; the Miocene formations, which I shall comprise under 

 the group name Chesapeake from Chesapeake, bay, adjacent to which the 

 formation attains its greatest development; and the Appomattox and the 

 Columbia of McGee. 



