MAKYLAXD GEOLOGICAL SURVEY 99 



the -uplift is not definitely known, but it is certain that the sea retreated 

 eastward considerably beyond its present shore-line. Stream erosion 

 at once began to attack this new land area and to cut it down to base 

 level, where it remained for a long time until the crystalline rocks of 

 the Piedmont Plateau were decayed to a gTeat depth below the surface. 

 The rocks of complex mineralogical composition were reduced to quartz 

 sand and a red clay, while the quartz veins were broken up and scattered 

 as angular pebbles over the surface. AVhen, at the beginning of the 

 Lafayete period, this land mass was tilted so as to elevate the Piedmont 

 and depress the Coastal Plain below ocean level, the waters of the 

 Lafayette sea advanced over the sinking surface and streams gorged 

 with detritus from the decayed, uplifted Piedmont above, rushed down 

 to the sea and poured their contents into the ocean. Either the waves 

 were weak or the sea advanced rapidly or this decayed material was 

 discharged in enormous quantities, for the sea was unable to cope with 

 the detritus poured into it and deposited it unsorted on the bottom. 

 The amount of this depression is not known, but it is certain that the 

 land was submerged to at least 500 feet below its present altitude. 



SEDIMENTARY EECOKD OF THE COLUMBIA GGOUP. 



The sedimentation of the Lafa3'ette formation was brought to a close 

 by the elevation of the region once more above the ocean. After an 

 extensive interval of erosion, during which the depressions of the prin- 

 cipal estuaries in the Coastal Plain were made, the country was again 

 lowered beneath the waves, and the deposition of the formations belonging 

 to the Columbia Group began. 



These formations, to which the names Sunderland, Wicomico, and 

 Talbot have been applied, are developed in terraces lying one above the 

 other in a vertical range from tide to an altitude of about 180 feet. 

 Beneath these three terraces, there is forming to-day a fourth which 

 extends from high-tide downwards beneath the waves. 



The key to their interpretation is secured by studying the manner in 

 which this recent terrace is forming. At the present time the waves of 

 the Atlantic Ocean and Chesapeake Bay are engaged in tearing away 



