434 N. H. DARTON LATER FORMATIONS OF VIRGINIA AND MARYLAND. 



Representative. ^9^- 



Columbia formation. Pliestocene (^early). 



Erosion interval. . 



Appomattox formation. Pliocene (?). 



Erosion interval. . 



Chesapeake formation. Miocene. 



Erosion interval. . 



Pamunkey formation. Eocene. 



Erosion interval. . 



Severn formation. Cretaceous. 



Erosion interval. Cretaceous. 



Potomac formation. Cretaceous (?). 



Erosion interval. Jurassic (?). 



Newark formation. Jura-Trias. 



Erosion interval. Early Mesozoic. 



Crystallines. (?). 



GENERAL DISTRIBUTION AND STRUCTURE. 



The accompanying map (plate 16) is a generalized reduction, in the main, 

 of large scale sheets mapped in detail during the past year. On account of 

 the thinness of the formations, especially in their feather edges, and the in- 

 tricacy of the boundary lines, great difficulty has been experienced in pro- 

 ducing a small scale map in black and white, and the result is not altogether 

 effective. Careful examination should, however, afford all data of general 

 interest concerning the distribution of the formations in the explored belt. 



The cross-sections in figure 1 illustrate, at intervals, the structural rela- 

 tions and the general configuration of the mass of each formation above tide- 

 level. 



The Columbia formation is omitted from both the map and the sections to 

 avoid the greatly increased complexity which its representation would intro- 

 , duce. In the western part of the region this formation is confined to the 

 lower terraces along the great transverse drainage depressions and for a short 

 distance up some of the side drainage lines. In the low coasts eastward it 

 fringes the shores for a considerable distance, and in the aggregate covers 

 wide areas. 



The Potomac formation lies directly on an irregular surface of the old 

 crystalline rocks excepting in a small area north of Richmond, where an out- 

 lying mass of the Newark formation intervenes. Southward from Fredericks- 



