Maryland Geological Survey 81 



the agency of iron oxide, we may suppose that the subaerial Newark 

 sandstones of that date, if consolidated at all, were considerably less 

 resistant than, for example, during the early Pleistocene, in the deposits 

 of which the Newark materials are abundantly represented. 



The basal deposits of the Potomac Group, produced by the initial warp- 

 ing of the continental border and described as the Patuxent formation 

 indicate in their arkosic character their proximity to the source of supply, 

 which was the extensively disintegrated Piedmont crystallines. It is 

 assumed that the Weverton peneplain, upon which the oldest known 

 Coastal Plain sediments were deposited, extended as a land surface to 

 the eastward of the present coast line. The epeirogenic movement, which 

 stimulated erosion and inaugurated the Potomac cycle of deposition was 

 undoubtedly a differential warping, with the focus near the present 

 Fall-line. This may have resulted in the formation of a broad, shallow 

 basin near or below sea level, from which the waters of the Atlantic 

 Ocean were largely excluded. McGee has compared the Potomac deposi- 

 tion with that of the present Gulf of California, although the lack of 

 any evidence of an invertebrate fauna in the Patuxent formation renders 

 such an interpretation unlikely. A gradual tilting of the coastal border 

 of the Weverton peneplain would seem more nearly to explain the facts, 

 since it is well known that sea coasts with an almost imperceptible 

 gradient like that of the present west coast of the Floridian peninsula, 

 show characters identical with those coasts which are separated from the 

 open ocean by barriers in the form of reefs or sand bars. It seems 

 probable that the inner marginal Patuxent beds, which alone are avail- 

 able for study, were largely continental deposits made up of an ever- 

 varying and complex combination of fluviatile, seolian, and lacustrine 

 sediments which merged in passing to the eastward with estuarine or 

 littoral sediments. The well-rounded and rarely flattened pebbles are 

 characteristic of fluviatile action, as is the presence of cobbles, often of 

 large size, which are so prominent in some of the Virginia outcrops. 

 The cross-bedding of so much of the arenaceous materials which pass 

 horizontally into clay lenses and which contain rolled clay balls is also 

 especially characteristic of fluviatile forces, and seolian forces may like- 



