570 THE JOURNAL OF GEOLOGY. 



TERTIARY BASE-LEVELS. 



It is now very clearly recognized that the Piedmont plateau 

 is a peneplain of Tertiary age. This plain extends eastward 

 over the Coastal plain region, and with gradually decreasing 

 altitude finally passes beneath tide water level not far from the 

 present coast line. In the Piedmont region, the plain has been 

 deeply trenched by drainage ways, but wide areas are preserved 

 in the divides. On the Coastal plain, it is overlain by the 

 Lafayette formation by which it is largely preserved to the 

 southward, but in northern Maryland, Delaware, and New Jersey, 

 this formation has been removed and the peneplain gives place 

 to rounded hills or to later terrace levels. The Tertiary pene- 

 plain extends over the Piedmont plateau to the foot of the 

 mountains and through their gorges into the great Appalachian 

 valley. Its altitude near the Blue Ridge west of Washington is 

 about 600 feet, but it rises gradually along the foot of this range 

 to 900 feet on James river. A short distance west of Washington, 

 its altitude is 400 feet, and a few miles to the eastward, where it 

 is widely overlain by the Lafayette formation, it is 270 feet. At 

 the shore of Chesapeake Bay, east of Washington, it is 110 feet, 

 and near the mouth of the Potomac river, 90 feet. At Rich- 

 mond, it is 200 feet, and it passes beneath tide level near Norfolk. 

 These altitudes indicate a gradual slope to the east, and to the south 

 along the Blue Ridge near James river. There is, also, a gen- 

 eral increase of altitude to the northward along the Coastal 

 plain, which has resulted in the wide removal of the Lafayette 

 formation and degrading of the underlying peneplain in that 

 direction. In figure 5, an attempt has been made to represent 

 the contour of the peneplain by one hundred foot contours, 

 restoring the portions which have been degraded. The steep 

 slopes in the Washington-Baltimore region are due mainly to the 

 inter-Columbia tilting. A portion of the general tilt to the east, 

 especially to the southward, was probably pre-Lafayette, and the 

 peneplain had originally, of course, a moderate seaward slope. 

 The surface contour of the peneplain is quite smooth. On the 



