JIAKVr.AM) CKOLOGICAL SUUVKY 45 



Dcroi'iiiatioii has also aH'cc-U'd the region lo a limited oxleMt, the 

 strata being slightly warped, so that the plane of bedding does not main- 

 tain a nniforni strike and dip. This is particularly marked along the 

 western border of the area. Tiiere have also been slight displacements 

 in various localities. ]\rcGee ' has described one of these, while others 

 have been observed by the junior aiitlioi' of Ihis report. 



It seems highly probable that every geological period from the Creta- 

 ceous (possibly Upper Jurassic) to the Pleistocene is represented, al- 

 though in one or two instances the lack of characteristic fossils renders 

 the taxonomic position of certain formations difficult of absolute deter- 

 mination. 



CKETACEOUS. 



The Cretaceous (in part possibly Upper Jurassic) is extensively repre- 

 sented in the Middle Atlantic Slope. The deposits of this period con- 

 sist of a series of basal formations that has been designated the Potomac 

 group, comprising the Patuxent. Arundel, Patapsco and Karitan forma- 

 tions, none of which was deposited under marine conditions, overlain in 

 succession by the Matawan, Monmouth, and Eancocas formations, which 

 are distinctly marine in origin. All but the Potomac formations gradu- 

 ally disappear southward, that group alone of the Cretaceous deposits 

 being recognized in Virginia. Unconformities characterize the several 

 members of the Potomac group while the marine deposits are also un- 

 conformable to the older strata. 



The Potomac group consists chiefly of sands and cla3'S. the former 

 frequently arkosic, with gravel at certain points where the shore accu- 

 mulations are still preserved. The deposits of the Patuxent formation 

 are highly arkosic, the sands and clays showing both a vertical and a 

 horizontal gradation into one another. The sand layers are seldom 

 widely extended, being generally lenticular masses, which rapidly dimin- 

 ish in thickness from their centers. Dark colored clays abound in the 

 Arundel formation and have yielded large amoimts of nodular carbonate 

 of iron. Highly colored and variegated clays largely make up the 

 Patapsco formation. Thick-bedded and widely extended white sands 



1 Tth Ann. Kept., U. S. Geol. Survey, pp. 616-633. 



