the Middle Atlantic Slope. 329 



colored clay or sand, sometimes interbedded with gravel or 

 interspersed with pebbles. Perhaps the best exposure is at the 

 " Crater " (a pit formed by the explosion of 8,000 pounds of 

 powder in a mine carried by Federal engineers beneath a Con- 

 federate fort, July 13, 1864). Here the principal material 

 is a dense, tenacious clay, orange, gray, pink, reddish, and 

 mottled in color, plastic yet firm when wet, and so hard and 

 tough when dry that medalions stamped from it as souvenirs are 

 as durable as rock — indeed the well known strategetic measure 

 to which the " Crater" is due was rendered successful by the 

 firmness and tenacity of the clay through which the entire 

 mine was excavated save where it barely touched the subja- 

 cent fossiliferous glauconitic sands of the Miocene. At But- 

 terfield's bridge in the southwestern part of Petersburg the 

 railway cutting exposes some 20 feet of plastic clay (like that 

 found at the " Crater "), pebbly and sandy clay, and cross-lami- 

 nated clayey sand, all predominantly orange-colored, in alter- 

 nating beds ; and it is noteworthy that here, as at some other 

 points, flakes and lines of white plastic clay similar to those of 

 the Potomac arkose are occasionally included in the formation. 



The formation continues to thicken and expand south of the 

 Appomattox river, until it forms the surface everywhere in the 

 vicinity of the fall-line save where it is cut away by erosion or 

 concealed beneath the Columbia deposits. Typical- exposures 

 occur along the Atlantic Coast Line railway at several points, 

 notably on the Roanoke opposite Weldon, 1ST. C, where a few 

 pebbly bands are intercalated within the regularly stratified 

 orange-colored clays and sands. . 



In brief the inland margin of the Appomattox formation, as 

 exposed north of Roanoke river, is a moderately regularly strati- 

 fied sand or clay with occasional intercalations of fine gravel, 

 generally of pronounced orange hue, and without fossils ; it 

 reaches a thickness of probably 50 to 100 feet and forms the 

 predominant surface formation over a zone 40 or 50 miles wide 

 on the Roanoke, but attenuates and narrows northward, finally 

 disappearing at Potomac creek, 4 or 5 miles north of Freder- 

 icksburg ; and although it appears to thicken seaward it soon dis- 

 appears beneath tide level and newer deposits. 



Stratigraphic Relations. — At Fredericksburg the formation 

 reposes, sometimes xjnconf ormably and again without visible un- 

 conformity, upon the lower member of the Potomac, and like 

 relations are frequently exhibited in the vicinity of Richmond 

 and Petersburg ; in the bluffs of the Taponi generally, and of 

 the Pamunkey two or three miles north of Hanover Court 

 House, it rests unconf ormably upon fossiliferous Eocene beds ; 

 at the " Crater " and at a number of other localities in the 

 vicinity of Petersburg it rests without visible unconformity 



Am. Jour. Sci— Third Series, Vol. XXXV, No. 208.— April, 1888. 

 20 



