442 N. H. DARTON — LATER FORMATIONS OF VIRGINIA AND MARYLAND. 



cially well exhibited at Georgetown and Fredericktowu. The Pamunkey 

 area on the " eastern shore " is indicated on the map (plate 16) by a broken 

 line, which, I should add, is only approximately accurate. 



Thickness. — The thickness of the Pamunkey formation along its southeast- 

 ern outcrops is about 150 feet from the South river to James river, and is 

 apparently quite uniform throughout. Northwestward the thickness dimin- 

 ishes to a mere feather edge lying on the Potomac formation and usually 

 overlapped by the Chesapeake or Appomattox formation. 



Stratigraphic Relations. — North of the Potomac river the Pamunkey for- 

 mation lies between the black sands of the Severn formation and the diato- 

 mous beds of the Chesapeake formation, separated in each case by a wide 

 structural and paleontologic gap. Exposures of unconformity with the 

 Severn formation are not abundant, and, owing to close similarity and in- 

 termingling of materials at the contact, are not always distinct. Along 

 some of the headwaters of the Northwest branch of the Patuxent and again 

 in the bluff at Fort Washington the weathered red beds of the Pamunkey 

 are at several points exposed lying on a slightly irregular surface of the 

 unchanged black sands of the Severn formation, and contacts are also 

 occasionally observed on the South, Severn and Magothy rivers. A fine 

 exposure of the contact is displayed in a road cutting a few hundred yards 

 south of Buena Vista, Prince George county, Maryland. 



At Glymont, Maryland, and thence southward through Virginia, the 

 Pamunkey formation lies directly on an irregular surface of the Potomac 

 formation, usually including more or less numerous Potomac. pebbles in its 

 lower beds. In the vicinity of Acquia creek, exposures of contacts are fre- 

 quent, and along the railroad a few rods south of the bridge there is an 

 exposure, referred to by McGee, in which the base of the Pamunkey forma- 

 tion is seen occupying an old ravine in the Potomac surface. At many 

 points near Fredericksburg and Brooke station, as well as at Richmond and 

 Deep Bottom on the James, and at Petersburg and Boilings bridge, the 

 basal pebble bed of the Pamunkey and the contact with the Potomac are 

 exposed with uniform relations throughout. 



The unconformable superposition of the Chesapeake on the Pamunkey 

 formation was frequently referred to by Rogers, and I have traced it through 

 hundreds of exposures in Virginia and through Maryland. 



The surface is usually relatively smooth, but there is a sharp contrast in 

 materials and usually a streak of pebbles of quartz or of Eocene fossil casts 

 in the overlying beds. 



Along the western border of the coastal plain region at some localities, 

 and in the higher river terraces, the Appomattox formation lies directly on 

 the Pamunkey formation, and in the lower river terraces the latter is over- 

 lain by the Columbia deposits. 



