MARYLAND GEOLOGICAL SURVEY 109 



striking that the conclusion regarding a common origin of both is irre- 

 sistible, and there can be no reasonable doubt that the mode of formation 

 of the modern terrace furnishes the key to the interpretation of the 

 ancient. 



The subsidence of the Atlantic Coastal Plain, which carried down be- 

 neath the ocean level the entire surface of Calvert County, gave oppor- 

 tunity to the waves to finish the destruction of such portions of the 

 Lafayette formation as chanced to survive the erosive work of the streams. 

 As Calvert County sank slowly beneath the water, the shore of the ad- 

 vancing Atlantic gradually worked farther and farther landward until 

 it had passed beyond the bounds of Calvert County and finally came to 

 rest near the borders of the Piedmont far to the westward. Calvert 

 County at that time was being rapidly covered by an off-shore deposit of 

 mud, sand, gravel, and ice-borne boulders which were floated down the 

 rivers on huge ice-blocks. How long the sea remained in this posi- 

 tion is not definitely known, but it is certain that it remained 

 long enough for the waves of the Sunderland sea to cut a well-pronounced 

 scarp against the Lafayette and older formations north and west in 

 regions beyond the bounds of Calvert County. These ancient sea-cliffs 

 are to-day prominent features of the topography of southern Maryland 

 and may be mapped as easily as the sea-cliff which is now being cut by 

 the waves of Chesapeake Bay and its estuaries. 



While the Sunderland off-shore deposits were still in progress of for- 

 mation over the surface of Calvert County, the region rose again above 

 the surface of the water and erosion began vigorously to cut away the 

 loose sands and gravels which had been previously deposited. How 

 extensive this uplift was, it is now quite impossible to say. It is equally 

 difficult to determine its duration, but it was of sufficient length to per- 

 mit the destruction of a large portion of this Sunderland formation, for 

 many of the larger streams within Calvert County opened up deep val- 

 leys within it. As has been pointed out before, the streams which were 

 chiefly instrumental in this destructive work were Lyons, Hall, Hunting, 

 Fishing, Parker, Battle, Island, and St. Leonard creeks. The question 

 as to whether the Patuxent River first came into existence at this time or 



