98 THE GEOLOGY OF ST. MARY's COUNTY 



Interpretation of the Geological Eecord. 

 sedimentary record of the chesapeake group. 



The close of the Nanjemo}' 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 

 preserved in Maryland, but the materials which composed the Calvert 

 formation in this county were deposited in seas of moderate depth in 

 which an abundance of life was present, as is shown by the remains of 

 diatoms and the extensive beds of fossil mollusks. The remains of 

 whales and other cetaceans show that these vertebrates abounded in the 

 ocean, and the discovery of a bone belonging to a gannet indicates that 

 birds existed along the nearby shores. This particular form doubtless 

 sought its food in the sea as the modern fishing gannets do at the present 

 time. 



The Calvert epoch was brought to a close by the elevation of the region 

 once more above the level of the ocean. A period of erosion followed 

 which was probably of short duration and closed with the depression of 

 the region again beneath the sea. Then followed the deposition of the 

 Choptank and St. Mary's formations, in which conditions similar to 

 those just described for the Calvert were repeated. 



SEDIMENTARY RECORD OF THE LAFAYETTE FORMATION. 



At the close of the Miocene j^eriod St. Mary's County and adjoining 

 areas were lifted above the ocean to form land. The full extent of 



