62 PHYSIOGRAPHY AND GEOLOGY OF THE COASTAL PLAIN PROYIXCE. 



The flora of these deposits is a highly varied one consisting of equiseta, 

 numerous ferns, cycads and conifers, a few monocotyledons, and a con- 

 siderable variety of dicotyledons especially in the upper beds. The known 

 fauna is a meagre one including a few pelecypods and gastropods of brack- 

 ish water or estuarine habitat, a single fish and a considerable number of 

 reptiles, especially dinosaurs. From the Arundel formation in the Maryland 

 area both gigantic and diminutive forms of this order have been collected 

 as well as the remains of stegosaurs and crocodilians. 



That the Potomac sediments were laid down as terrestrial, lacustrine 

 and fluviatile sediments, combined with contemporaneous deposits in shallow 

 water along the Lower Cretaceous shore-line, is indicated by the absence of 

 any strictly marine fossils and sediments, and by the presence of a few 

 estuarine species of shells, and by the abundance of delicate plant remains 

 often but slightly if at all triturated. The fossil wood is also frequently 

 silicified or lignitized without having suffered much from decay. The 

 coarseness of a large proportion of the materials, the frequency of marked 

 current bedding, and the presence of a large amount of coarse gravel or 

 even cobbles, effectually confirms the littoral and estuarine conditions 

 under which the Potomac formations were deposited. 



The deposits constituting the Potomac group were at first thought to 

 constitute a single formation and in 1886 Dr. "W J McGee* applied the 

 name Potomac to them ^because of their extensive development in the 

 Potomac Eiver basin near Washington. Later investigations in Maryland 

 have clearly demonstrated the fact that these deposits do not constitute a 

 single formational unit, and in 1897 Clark and Bibbins^ proposed a four- 

 fold classification which has now come to be generally accepted. The forma- 

 tional names in order from the oldest to the youngest were the Patuxent, the 

 Arundel, the Patapsco, and the Earitan. More recently the uppermost of 

 these formations, the Earitan, has been referred to the Upper Cretaceous. 



The Patuxent is the most constant from Maryland southward. The 

 Arundel attains a considerable development in central Maryland, but dis- 

 appears to the northward in northeastern Maryland and to the southward 

 in southern Maryland. The Patapsco is largely developed from Delaware 

 to the vicinity of Fredericksburg, Virginia, where it disappears. The known 

 Earitan extends from the islands off the southern coast of New England to 

 southern Maryland where it is transgressed by much later deposits of Ter- 

 tiary age. 



aReport of Health Officer, D. C, 1884-5 (1886), p. 20. 

 bJour. of GeoL, vol. v, pp. 479-506. 



