208 CLARK AND BIBBINS — GEOLOGY OF THE POTOMAC GROUP 



the last, considerabty nearer the marine is indicated by the much more 

 common occurrence and more active operations of Teredo. 



Raritan sedimentation was closed by an uplift more strongly marked 

 than before to the landward. The westward portions of the terrane were 

 extensively eroded, and resubsidence inaugurated Matawan sedimenta- 

 tion. This depression of the continental border was distinguished from 

 the others by its extent, which was such as to inaugurate the deposition 

 of more or less well defined marine sediments, including greensands, at 

 points where only estuarine materials were laid down during preceding 

 epochs; hence the term "Marine Cretaceous," which has been often 

 used by geologists to distinguish the later Cretaceous deposits from those 

 of the earlier estuarine beds of the Potomac group. 



The distinctly estuarine character of the Potomac sediments points to 

 the existence for a long period of an extensive area of more or less 

 brackish water along the eastern border of the North American continent. 

 It must have reached at least from cape Cod to the Gulf. That it was 

 either a sound, a lagoon, an embayment or an estuary, or a series of these, 

 on a vastly greater scale than any along the Atlantic coast today, is 

 probable. McGee's recent studies of the gulf of California suggested the 

 possibilit}^ of an Atlantic barrier comparable in scale to the peninsula 

 of Lower California. The evidence available, however, to establish the 

 actual existence of any such type of barrier in early Cretaceous time 

 appears to be scant. The comparatively sudden appearance of marine 

 sediments which marked the beginning of the Upper Cretaceous points, 

 to be sure, to the disappearance of some form of barrier, but what may 

 have been its character or extent seems impossible of determination 

 with the facts at hand. In the succeeding chapters the surface configu- 

 ration both of the crystalline floor and of the Potomac group is discussed, 

 and some possible interpretations advanced. 



The greater thickness of the formations of the Potomac group along a 

 belt somewhat to the eastward of the Fall line may have emphasized the 

 downward movements in this portion of the Coastal plain during Potomac 

 time. On the other hand, the gradual removal of the weight over the Pied- 

 mont region by the removal of its residuals has occasioned an upward 

 movement of that area as well as immediately adjacent Coastal Plain re- 

 gions. The accumulating results of these tendencies, particularly the first 

 mentioned, from the beginning of Potomac time until the present, have 

 been the weakening of the crystalline floor near the landward border of 

 the Coastal plain accompanied by monoclinal folding and even faulting 

 on a limited scale. McGee's studies of the upper Chesapeake area, and 

 others to the northward and southward, fully convinced that author 

 some years since that displacement had actually occurred, though no 



