TAXONOMY OF THE SEVERN FORMATION. 439 



formation near Lloyd's creek. On the northern side of the Sassafras, again 

 near its head, and in the Bohemia creek depression, the formation is also ex- 

 posed at intervals. In Delaware the exposures in the Chesapeake and Dela- 

 ware canal cuts are well known, and in this region the formation comprises a 

 series of more or less distinctly separable beds, which, by Chester, have been 

 correlated with the various members of the New Jersey greensand series. 



' The thickness of the formation gradually decreases southward ; in the 

 lower Sassafras and Round bay regions it is nearly 100 feet ; northeast of 

 Washington, 10 to 30 feet; opposite Alexandria, 25 feet; along Henson 

 creek, 20 feet ; and in the Fort Washington bluff, 18 feet. 



StratigraphiG Relations and Equivalency. — The Severn formation is fre- 

 quently exposed in contact with the Potomac clays, and lies on an uncon- 

 formity always sharply defined by great contrast in material and considera- 

 ble local irregularity of surface. It is in turn unconformably overlain by 

 the Pamunkey formation, from which it is widely separated structurally and 

 faunally. 



The Severn formation is the continuous southern extension of the New 

 Jersey Cretaceous greensand series, but whether it represents all or part of 

 these members is not as yet determined. In Maryland it is a stratigraphic 

 unit, distinctly separable from the New Jersey series as a whole by its 

 homogeneity of constitution ; and it is with this restriction that the term 

 " Severn " is applied. 



PAMUNKEY FORMATION. 



Distribution and Characteristics: General Features. — This formation occu- 

 pies a belt of considerable width extending through Maryland and Virginia 

 above tide-level with a length of about 200 miles. The greater part of its 

 area is buried beneath younger formations, but it is exposed extensively in 

 each of the larger depressions, where it is a conspicuous member of the 

 coastal plain series. 



The formation consists of a homogeneous sheet of fine-grained materials, 

 glauconitic sands mainly, usually profusely fossiliferous. Excepting a few 

 local beds of clay, secondary limestones, and some gravels at its base, the 

 formation does not comprise stratigraphic components. Wherever the for- 

 mation has been bared of overlying formations its glauconitic constituent is 

 either weathered out, leaving fine light-colored sands, or decomposed and the 

 iron redeposited as a red or brown stain and in crusts and concretions. This 

 weathered phase is general in the northern part of the region beyond the 

 edge of the overlyin<^ Chesapeake formation, along the western margin in 

 Virginia, and in all old outcrops. 



In describing the distribution and noteworthy exposures of the formation 

 it will be convenient to briefly consider each general area separately. 



LXIV— Buij,. Gkoi,. Soo. Am., Vor,. 2, 18!)(). 



