maryland geological survey 151 



The Potomac Group. 1 



The western boundary of the Potomac group of Cecil county is 

 a sinuous and interrupted line extending from the vicinity of "Wood- 

 lawn through Theodore, Bay View, Laurel Hill and Cherry Hill to 

 Barksdale. Along this line the formation often consists of isolated 

 outliers resting on ancient crystalline rocks. The eastern boundary 

 of the Potomac area is much more regular and extends from the 

 mouth of the Sassafras river to the western end of the Chesapeake 

 and Delaware Canal. The Potomac beds, which are composed of 

 clay, sand and gravel, are characterized by extreme diversity in com- 

 position, texture and color, and by sudden and oft-repeated changes 

 in the same, both horizontally and vertically. They are, on the 

 whole, rich in iron oxide, and by it the sand and gravel beds are 

 often locally indurated to sandstone and conglomerate. 



The fossils consist almost exclusively of plant remains, among 

 which may be mentioned leaf impressions of ferns, cycads, conifers, 

 monocotyledons and dicotyledons and wood altered to lignite or re- 

 placed by various minerals. Semitransparent pellets of amber are 

 occasionally found. 



The thickness of the beds has been estimated as about 600 feet at 

 the mouth of the Sassafras and 420 feet near Chesapeake City. The 

 strike runs from northeast to southwest, and the dip varies between 

 30 and 60 feet per mile to the southeast. 



Only three of the four formations of the Potomac group are posi- 

 tively recognized in Cecil county; these are the Patuxent, Patapsco 

 and Karitan. The Arundel formation, which is so well developed 

 farther south, if present here, is only slightly represented. 



THE PATUXENT FORMATION. 



The Patuxent formation is the basal member of the Potomac 

 group, so called from its typical locality along the Patuxent river 

 in Southern Maryland. It was formerly thought to be Lower Cre- 

 taceous in age, but of late years it has been provisionally referred 



1 The discussion of the Potomac group is based od the field notes and manuscript 

 of Mr. A. Bibbins, who placed them at the disposal of the writer. 



