TAXONOMY OF THE PAMUNKEY FORMATION. 443 



Taxonomy. — The paleontologic evidence in regard to the age of the Pa- 

 munkey formation establishes its equivalency with the Eocene as recognized 

 by Rogers and Conrad half a century ago. It is not possible as yet to defi- 

 nitely state its precise relative position in the Eocene, or to correlate it with 

 other North American deposits. It is not known whether its basal and sur- 

 face planes are approximately parallel throughout, either to each other or to 

 the bedding of the formation. 



CHESAPEAKE FORMATION. 



' Distribution and Characteristics. — This formation occupies a belt com- 

 prising nearly the entire width of the coastal plain region in Virginia and a 

 wide area in southeastern Maryland. All of the water-courses of the region 

 cut more or less deeply into the formation, and it frequently constitutes high 

 bluffs along the larger streams. In Maryland it lies east of the Potomac 

 river, and on the " western shore " its northern termination is in a series of 

 outliers midway on a line connecting Washington and Annapolis. Its 

 northern limit on the " eastern shore " is indicated approximately by the 

 dotted line on the map (plate 16), but the details of its distribution in that 

 region are not yet determined. 



The formation is diverse in composition, consisting of sands, clays, marls, 

 diatomaceous beds, and shell fragments, in all several hundred feet in thick- 

 ness. The lower beds consist mainly of dark-colored clays and fine, mealy 

 sands containing the extensive and well-known diatomaceous deposits. These 

 are succeeded by lighter-colored clays and sands, with occasional local in- 

 clusions of blue marl. The upper beds are coarser-grained, and consist 

 chiefly of white beach sands containing shells and deposits of shell frag- 

 ments, and occasional argillaceous members. These three series intergrade 

 in zones, which vary somewhat in stratigraphic position and vertical extent, 

 and all the members rapidly thicken seaward, apparently reaching a thick- 

 ness of nearly 1,000 feet at Fort Monroe. 



The lower beds of the formation occupy a broad, irregular belt extending 

 through Virginia into Maryland along the western part of the coastal plain 

 region. Its finest exposures are at Richmond, at Petersburg, on the Rap- 

 pahannock river below Fredericksburg, on the Potomac river at Pope's 

 creek (Maryland), on the Patuxent river near Nottingham, and at Herring 

 bay on Chesapeake bay. On the "eastern shore" it is seen near Wye Mills 

 (Maryland), and on Little Duck creek, south of Clayton (Delaware) ; and 

 it is found in the deep artesian wells at Atlantic City, New Jersey. On the 

 "western shore" of Maryland I have found that it extends northwestward 

 to Washington, being conspicuous in the high terraces overlooking the city 

 from the east, and represented by an isolated patch lying on the Potomac 

 sands and crystallines just outside of West Washington. 



