438 N. H. DARTON LATER FORMATIONS OF VIRGINIA AND MARYLAND. 



The erosion epoch between the Potomac formation and the Cretaceous 

 greensand is one of considerable moment, but the gap between the Potomac 

 and New^ark represents one of the great periods of uplift and erosion wdiich 

 is second only to the gap between the Paleozoics and the Newark formation. 



SEVERN rORMATIOX. 



Distribution and Characteristics. — In 1888, Clark identified the Cretaceous 

 formation on the " western shore " of Maryland by the discovery of typical 

 molluscan casts at Round bay on the Severn river, on Magothy river, at 

 Millersville and Collington, and at Fort AV^ashington, and so set at rest any 

 question in regard to the southward extension of the formation, at least 

 through Maryland. The writer has found that the formation is a continuous 

 sheet, clearly defined in its stratigraphic relations, and finally disappearing 

 near the latitude of Marshall Hall, on the Potomac river, south of Wash- 

 ington. 



The formation consists throughout almost entirely of fine black sand, more 

 or less flecked wdth scales of mica, very sparingly but irregularly glauco- 

 nitic, and usually containing considerable carbonaceous materials. The 

 finest exposures are in the high cliflTs at Round bay, on the Severn river, a, 

 locality to which Clark and Uliler have paid considerable attention. There 

 it is exposed lying on an irregular surface of a local coarse gray sand bed 

 in the Potomac formation ; and back from the river, on some of the higher 

 lands, it is in turn capped by weathered beds of the Pamunkey formation, 

 beneath which it also disappears down the river toward Annapolis. South- 

 ward there are frequent exposures in road, railroad and stream cuts, in a 

 narrow belt which extends continuously nearly to Washington ; thence its 

 edge is cut out for a few miles by an overlap of the Chesapeake formation, 

 but it comes out again opposite Alexandria and is exposed in considerable 

 force in the gullies along the face of the terrace fronting the Potomac and 

 in some of the side drainage ways. At Fort Washington it is exposed in 

 the "bluff," lying on gray lignitic clays of the Potomac formation, and 

 capped by a few feet of weathered Pamunkey deposits. 



At every exposure organic remains are found, commonly in the forms of 

 casts or impressions. At several localities east and southeast of Washington 

 and a short distance from the city I have found fossil shells occurring abun- 

 dantly in the formation, notably Exogyra costata and a large Cyprimeria, 

 like densata. 



Northward from Round bay the formation is exposed in the low^er Magothy 

 river for the last time on the " western shore." Ou the eastern side of Chesa- 

 peake bay the black carbonaceous sands are again exposed, lying on the 

 Potomac sands and clays at Howell's point, and thence occupying the high 

 banks of the low'er Sassafras river, finally sinking beneath the Pamunkey 



