THE PATAPSCO FORMATION. 67 



area of their most extensive development, while in the Virginia area the 

 source of the Potomac sediments wore the granitic and gneissoid rocks 

 which there constitute the eastern Piedmont. 



The Arundel is typically developed in the area between Baltimore . and 

 Washington in Anne Arundel and Prince George's counties, Maryland, It 

 is also present in Baltimore City and in Baltimore and Harford counties 

 but has not been recognized farther to the northeast in Cecil County, Mary- 

 land, or to the southwest in the valley of the Potomac Eiver. 



The Patapsco Formation. 



Name. — The Patapsco formation receives its name from the Patapsco 

 River, Maryland, in the valley of which excellent exposures of these beds 

 are found. 



Stratigraphic relations. — In Maryland the deposits of the Patapsco for- 

 mation rest unconformably upon the Arundel where it is present and in 

 other places upon the Patuxent formation or the crystallines of the Pied- 

 mont Plateau. Throughout their area of outcrop in A^irginia the Patapsco 

 deposits rest with marked unconformity upon the Patuxent. In the stream 

 valleys the deposits frequently have a covering of Pleistocene materials 

 while Lafayette gravels, sands, and loam often cap it when it rises to the 

 height of the stream divides. It disappears from view along the eastern 

 margin of its outcrop beneath the Aquia formation of the Eocene, which is 

 also found capping it in the higher hills and may be seen in several places 

 in Stafford County about Aquia, Accokeok, and Potomac creeks. 



The Patapsco Avas greatly eroded in pre-Eocene times, its upper surface 

 being extremely uneven, this undulating contact, emphasized by the marine 

 beds of the overlying Aquia formation, being one of the sharpest lines in the 

 whole Coastal Plain. 



From the record of the deep wells near the mouth of the James River, 

 notably at Fortress Monroe, it is believed that the Upper Cretaceous 

 deposits so extensively developed in Maryland and New Jersey intervene 

 between the Patapsco and Aquia formations. 



Litliologic character. — The Patapsco, like the Patuxent formation con- 

 sists of very variable materials — clays, sands, gravels, and conglomerates. 

 The deposits, however, are more uniform and finer than those of the 

 Patuxent, consisting chiefly of highly colored and variegated clays, which 

 grade over into lighter colored sandy clays, while sandy bands of coarser 

 materials are at times interstratified. The sands frequently contain much 

 decomposed feldspar and like those of the Patuxent are often cross-bedded, 



