MARYLAND GEOLOGICAL SURVEY 133 



sidence occurred which again brought the region under water. 

 Coincident with the subsidence there seems to have been a slight ele- 

 vation and tilting of the region west of the shore line. The heads of 

 the streams were given renewed force, enabling them to carry down 

 and spread over this region large quantities of gravel and sand, de- 

 rived from the Piedmont and Paleozoic formations to the west. 



The evidence for the source of the material is found in many dif- 

 ferent pebbles whose origin can be traced by their lithologic charac- 

 ter or the fossils they contain. In the vicinity of Washington many 

 of the gravel deposits contain fossils of Devonian and Carboniferous 

 age brought from regions beyond the Blue Ridge. These fossils show 

 that Potomac River had extended its drainage basin westward to 

 those regions. During the submergence beneath the Lafayette sea, 

 conditions were not uniform over the entire area, as gravel deposits 

 were forming in some places at the same time that the clay beds 

 were being deposited in others adjoining. Yet on the whole sedi- 

 mentation was remarkably uniform throughout the area, considering 

 the circumstances under which it took place. Over the former land 

 surface a fairly persistent capping of gravel was deposited. But land 

 movements were again taking place slowly. The velocity of the 

 streams was checked so that gravel could no longer be carried down 

 except in occasional freshets. Fine sand and loam were laid down 

 over the gravel which had been previously deposited. This loam, 

 which is so extensively developed over a large portion of Prince 

 George's County, marks the last period of Lafayette sedimentation. 

 It marks also the last time that the entire region was submerged 

 beneath the ocean waters. 



SEDIMENTARY RECORD OF THE PLEISTOCENE. 



At the close of the Pliocene epoch the region was raised again 

 and extensively eroded, and was then lowered and received the de- 

 posits which constitute the first member of the Columbia group. 

 The Sunderland, Wicomico, and Talbot formations, which make up 

 this group, are exposed as a series of terraces lying one above the 

 other throughout the I*^orth Atlantic Coastal Plain from Raritan 



