214 PHYSIOGRAPHY AXD GEOLOGY OF THE COASTAL PLAIX PROVINCE. 



mediate contact with the crystalline rocks of the Piedmont. Near the shore 

 leaves from land plants carried out to sea by the streams were dropped and 

 their remains have been preserved in the fine-grained deposits of the Calvert 

 in the vicinity of Eichmond. 



Near the beginning of the Miocene submergence certain portions of the 

 sea bottom received little or no material from the land, and the water in 

 those places was well suited as a habitat for diatoms. These must have lived 

 in the waters in countless millions, and as they died their silicious shells fell 

 to the bottom and produced the diatomaceous or infusorial earth beds 

 which are so common in the lower part of the Calvert formation. Many 

 protozoa as well as higher forms of animal life, particularly mollusca, lived 

 in the same waters, and their remains are found plentifully distributed 

 throughout the beds. At certain times conditions must have been especially 

 favorable for animal life, as may be inferred from the great shell marl 

 deposits which are found in so many places throughout Virginia. 



x^Lfter the deposition of the Calvert formation the region was again 

 raised and subjected to erosion for a short period, and on sinking once more 

 beneath the sea, the Choptank formation, which has been differentiated 

 north of the Potomac Eiver, was laid down contemporaneously with the 

 advancing ocean. The Choptank formation lies unconformably on the Cal- 

 vert in Maryland and farther north transgresses it and rests upon the 

 Cretaceous deposits. During this last submergence conditions changed 

 somewhat so that materials of different lithologic character were brought in. 

 The sands of the Choptank in turn gave place to clay deposits alternating 

 with layers of sand. These latter constitute the St. Mary's formation which 

 in Virginia unconformably overlies the Calvert formation. These changes 

 in physical conditions of sedimentation were accompanied by slight changes 

 in the climate, which reacted upon the life forms inhabiting the ocean waters. 

 Consequently we find in the St. Mary's formation a somewhat different 

 assemblage of fossil forms. In general these are closely related to forms 

 which now exist in somewhat higher latitudes. They indicate a slightly 

 colder climate than had existed during the formation of the Choptank strata. 



After the deposition of the St. Mary's an uplift brought practically the 

 entire Coastal Plain of the State above the water, and a short period of 

 erosion was inaugurated. The Yorktown period of submergence followed. 

 During this period the ocean shore-line extended across the eastern part of 

 the Coastal Plain, the Eastern Shore of Chesapeake Bay and the counties 

 bordering Chesapeake Bay along the Western Shore were alone submerged. 

 Molluscan life was very abundant during this time, and along the low, sandy 



