58 THE PLIOCENE AND PLEISTOCENE DEPOSITS OF MARYLAND 



to be Jurassic while the two latter are referred to the Lower Cretaceous. 

 These four formations taken together constitute the Potomac group. This 

 group represented by either one or more of its members extends along the 

 Atlantic coast nearly parallel to the present shore line almost continuously 

 from Gay Head to the Gulf of Mexico. Throughout the northern portion 

 of this area are found certain deposits, which have been called by Ward 

 the Island series and which are believed by him to be younger than the 

 rest of the series. They have suffered greatly from erosion and are con- 

 sidered by him to be limited to Marthas Vineyard, Block, Long, and 

 Staten Islands. From Earitan Bay southward to Washington City the 

 beds of the Potomac group are typically developed in a continuous belt, 

 but south of the Potomac river they have not been thoroughly studied and 

 their distribution is not so well known. 



The Patuxent period of sedimentation was ushered in by the seaward 

 tilting of a previously base-leveled land surface. The proximity of this 

 formation to the ancient shore line is indicated in the arkosic character 

 of its rocks. The distribution of the arkosic materials seems to depend 

 on that of the felspathic rocks for it increases in their vicinity and de- 

 creases rapidly, or is altogether absent, when removed from their presence. 

 The cross-bedded character of the strata shows that deposition was rapid. 

 A pronounced elevation closed the Patuxent epoch, revived the rivers and 

 brought about a large amount of erosion. This was followed by a subsi- 

 dence in which many of the stream valleys but lately eroded were occupied 

 for a portion of their courses by bogs and swamps. In these marshes 

 there was an extensive development of plant life, and in them also were 

 deposited those iron ores which are now considered of such great value. 

 The presence of Teredo-boared wood indicates that there was some con- 

 nection between these swamps and the ocean. After the deposition of the 

 Arundel formation the region was again elevated, eroded and finally 

 depressed to receive the sediment of the Patapsco sea. This formation 

 was apparently deposited in quieter and deeper waters than the one just 

 preceding. A period of elevation and erosion succeeded the deposition 

 of the Patapsco formation and this in turn was followed by another period 

 of subsidence during which the Earitan formation of clay and sand was 

 deposited. 



