MARYLAND GEOLOGICAL SURVEY XXVll 



that successively later members of the series are encountered in passing 

 from the interior of the country toward the coast. 



From the beginning of deposition in the coastal region until tlie 

 present time sedimentation has apparently been constantly in progress 

 over some portions of the area. Differential movements of the sea 

 floor^ with its accumulated sediments, took place, however, from time 

 to time so that the formations present much complexity along their 

 western margins. It is not uncommon to find certain members of the 

 series lacking, as renewed deposition carried a later formation beyond 

 its predecessors. In the absence of distinctive fossils the discrimina- 

 tion of the different horizons at such points is often attended with great 

 uncertainty. 



Deformation has also affected the region to a certain extent, the strata 

 in places being slightly warped, so that they do not maintain a uniform 

 strike and dip. This is particularly marked along the western border 

 of the area where there have also been slight displacements in various 

 localities. 



Every geological period from the Cretaceous (possibly Jurassic) to 

 the Pleistocene is represented, although in one or two instances the 

 lack of characteristic fossils renders the taxonomic position of certain 

 formations difficult of absolute determination. 



Cretaceous 



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

 resented in the Middle Atlantic Slope. The deposits of this period 

 consist of a series of basal formations that has been designated the 

 Potomac group, comprising the Patuxent, Arundel, Patapsco and Eari- 

 tan formations, no one of which was deposited under marine condi- 

 tions. They are overlain in succession by the Matawan, Monmoutji and 

 Rancocas formations, which are distinctively marine in origin. All of 

 these formations gradually disappear southward, the lower formations 

 of the Potomac 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 unconformable to 

 the older strata. 



The Potomac group consists chiefly of sands and clays, the former 



