112 THE GEOLOGY OF PKINCE GEORGE S COUNTY 



farther north the upper fossiliferous bed of the Choptank rests upon 

 the Calvert formation. How far this nnconfornnty continues down 

 the dip after the beds disappear from view is not known, as the data 

 from well records are too meager to permit any conclusion to be 

 drawn from them. Above the Choptank is the St. Mary's formation, 

 which is not represented in this county. 



Subdivisions. — The Choptank formation is subdivided into five 

 zones, which are distinguished from one another by the character of 

 material and the fossils they contain. These zones, together with 

 their fossil contents, have been fully described in the State report 

 on the Miocene of Maryland. 



The Pltocexe ( ?). 



The Lafayette Formation. 



The name Lafayette was proposed by Hilgard in 1891^ to replace 

 the term Orange sand, used in Tennessee and Mississippi, and the 

 term Appomattox, which had been applied to the deposits of the 

 Atlantic Coast. The name is derived from Lafayette County, Miss., 

 a region where the formation is well developed. The exact correla- 

 tion of the formation has not been definitely settled, as its meager 

 fauna has furnished little clue to its age. Its general character, 

 firmly indurated layers, and occasional greatly decomposed pebbles 

 suggest a formation much older than any known Pleistocence deposit 

 of the province, and hence furnish evidence for a provisional ref- 

 erence to the Pliocene. 



Areal Distribution. — The Lafayette forms the surface cover over 

 the principal stream divide of the southern portion of the county. 

 At one time, however, it probably formed a mantle over the entire 

 area, for outliers are found north of Washington. If such was the 

 case, the Lafayette must originally have rested upon the exposed 

 edges of all the earlier formations represented in this region, but 

 erosion has so reduced its area that it is now in contact principally 

 with the Miocene, although in small areas it rests upon the Patuxent 



lAm. Geologist, vol. VIII, 1891, pp. 129-131. 



