MARYLAND GEOLOGICAL SURVEY 101 



various terraces of the Columbia group, the analogy is found to be so 

 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 

 beneath the ocean level the entire surface of St. ^Mary's County, gave 

 opportunity for the waves to finish the destruction of such portions of 

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

 streams. As St. Mary's County sank slowly beneath the water, the 

 shore of the advancing Atlantic gradually worked further and further 

 landward until it finally came to rest near Charlotte Hall. St. Mary's 

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

 of mud, sand and gravel. How long the sea remained in this position 

 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-line 

 against the Lafayette. 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 ofE-shore deposits were still in process of forma- 

 tion over the surface of St. ]\Iary's 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 deposited just before. 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 

 permit the destruction of a large portion of this Sunderland formation, 

 for many of the larger streams within St. IMary's County opened up deep 

 valleys within it. 



The question as to whether the Patuxent Eiver first came into exist- 

 ence at this point or previously in the erosive interval which followed 

 the uplift of the Lafayete formation has been discussed elsewhere in 

 this volume. 



After St. Mary's County had been subjected to erosion for a certain 



