122 THE PLIOCENE AND PLEISTOCENE DEPOSITS OF MARYLAND 



At the close of the Miocene period a great part of the Coastal Plain 

 and the adjacent borders of the Piedmont Plateau were lifted above the 

 ocean to form land. The full extent of this uplift is not definitely 

 known, but it is certain that the sea retreated eastward considerably be- 

 yond its present shore line. Stream erosion at once began to attack this 

 new land area and to cut it clown to base level where it remained for a 

 long period until the crystalline rocks of the Piedmont Plateau were 

 decayed to a great depth below the surface. The rocks of complex miner- 

 alogical 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. When, at the beginning of the Lafayette period, this land 

 mass was tilted so as to elevate the Piedmont region and to 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 de- 

 cayed, 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 de- 

 posited 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. In 

 the absence of a scarp-line or of a well-defined beach deposit, it is impos- 

 sible to locate the position of the Lafayette shore. 



In the accompanying diagram (Plate XXVII) an attempt has been 

 made to reproduce in a general way the old Lafayette shore line. This 

 map, however, should be considered as extremely hypothetical. The alti- 

 tude of 500 feet or a little more has been assumed as approximately coin- 

 cident with the shore line of the old Lafayette sea. This contour has 

 been followed around the elevations of Maryland, and all depressions be- 

 neath it regarded as submerged, while elevations rising above are repre- 

 sented as dry land areas during Lafayette time. It has not been practi- 

 cable to make allowances for possible deformations which may have taken 

 place since Lafayette time for the reason that these movements are not 

 sufficiently understood at present. The valleys in western Maryland then 



