106 the geology of calvert county 



Interpretation of the Geological Eecord. 

 sedimentary record of the nanjemoy formation. 



If the Nanjemoy formation in Calvert County sustains the same rela- 

 tions to the rocks which lie beneath it as it does farther to the north 

 where its base comes to the surface, it must rest conformably on the sur- 

 face of the Aquia formation, and the Aquia formation is believed to 

 rest unconformably on the eroded edges of the Upper Cretaceous. This 

 would indicate that, after the close of the Upper Cretaceous cycle of sedi- 

 mentation, Calvert County was raised above the ocean and extensively 

 eroded; it was then depressed and submerged beneath the Eocene sea 

 which planed down the somewhat irregular surface of Cretaceous rocks 

 and deposited on them the materials which compose the two Eocene 

 formations. The depth at which these deposits were laid down cannot 

 be definitely determined, but from a study of the materials which com- 

 pose them and the fossils which they contain, it is believed that the sea 

 attained a depth of 100 to 300 fathoms. Deposits vrere thus made at a 

 considerable distance from the shore line in quiet seas and probably 

 accumulated with great slowness. 



SEDIMENTARY RECORD OF THE CHESAPEAKE GROUP. 



The close of the Nanjemoy epoch was marked by an elevation of the 

 region which brought the Eocene deposits above the ocean and exposed 

 them to a prolonged attack of erosion. After the region had suffered 

 extensively from the work of waves and rivers, it was again submerged 

 beneath the ocean and the materials composing the Calvert formation 

 were deposited. As the Miocene sea advanced little by little on the 

 sinking surface of the mainland, the waves caught up and re-worked the 

 clays and greensands of the various Eocene beds. The more obdurate 

 fossils of the Eocene survived in a great measure the erosive work along 

 the old Miocene shore and were carried out and deposited in deeper 

 water. They may now be seen re-worked in the basal member of the 

 Calvert formation. The old shore line of the Miocene sea which was 

 formed during the Calvert epoch of sedimentation has nowhere been 



