68 THE GEOLOGY OF ST. MART'S CODNTT 



the Choptank in turn rests iinconformably on the Calvert, but passes 

 into the St. Mary's formation without a break. The materials of the 

 formations %vhieh compose the Chesapeake Group consist of marls, clays, 

 and sands. Each formation is abimdantly supplied with fossils. After 

 the St. Mary's formation was deposited, the region was elevated and 

 extensively eroded, and later submerged to receive the deposits of the 

 Lafayette formation. These consist of clay, loam,, sand and gravel and 

 are deposited in a terrace which is the oldest of the series described above. 

 A long period of erosion followed the deposition of the Lafayette terrace 

 and the subsidence which brought it to a close permitted the deposition 

 of the Columbia formations. These are, beginning with the oldest, 

 , Sunderland, Wicomico, and Talbot. They are all unconformable with 

 whatever lies beneath them, and they are also unconformable with each 

 other. They are developed in terraces, lying one above the other, and 

 separated by well defined scarp-lines. The materials which enter into 

 them are composed of clay, peat, sand, gravel, and ice-borne boulders. As 

 a group, they record what took place in St. Mary's County while the 

 regions to the north were covered with the great ice sheet. 



The Miocene. 

 The Chesapeake Group. 



The Miocene deposits of the Middle Atlantic slope have been de- 

 scribed under the name of the Chesapeake Group. In Maryland, the 

 materials which compose the formations of this group consist of clay, 

 sandy-clay, sand, marl and diatomaceous earth. The sand^'-clay mem- 

 bers are, when freshly exposed, greenish to greenish-blue but slowly 

 change under the influence of the weather to a slate or drab color. 



It has been found possible to separate the beds of the Chesapeake 

 Group into three formations, which are designated, beginning with 

 the oldest, the Calvert formation, the Choptank formation and the 

 St. Mary's formation. 



the CALVERT FORMATION. 



Calvert County has suggested the name for this formation because 

 of its typical development there. In the famous Calvert Cliffs along 



