Epetrogenic Movements. 89 
McGee for their interpretation and the determination of their 
bearing on Appalachian history. The conclusions will be stated 
briefly without attempting to give the evidence on which they 
are based, although some of it is contained a subsequent page. 
The series of oscillations occurring since the close of the Ter- 
tiary period of baseleveling consists, first, of a depression which 
allowed the waters of the ocean and the Mississippi embayment 
to advance inward far beyond their previous margin.* Fol- 
lowing this came an elevation of the entire province that again 
started the streams in a career of great activity, and the sea re- 
treated probably beyond the present shoreline. These broad 
movements may properly be termed epeirogenic, as they affected 
the entire province, but in every case the movements culminated 
along certain axial lines and produced decided local or orogenic 
warping. In the subsidence the greatest depression was along 
the cross-axis A B, but in the subsequent elevation the greatest 
movement was along the main longitudinal axes. A period of 
comparative quiescence followed, during which the land stood 
somewhat higher than at present and much higher than during 
the Tertiary baseleveling period. It was during this interval 
that the rivers of the eastern coast carved their broad outer val- 
leys, now almost completely submerged beneath the waters of 
the Atlantic, and the Mississippi corraded its broad valley from 
Cairo to the Gulf. 
In very recent geologic time these oscillations have been re- 
peated in the same order and with a similar effect. The land 
first subsided and the Columbia sediments were laid down; then 
it arose to its present position and the modern gorges mark the 
duration of the present high level attitude of the land. 
INTERRELATIONS OF THE TWO PENEPLAINS. 
The greatest divergence in altitude between the two deformed 
peneplains is in the northern portion of the province. This 
great pre-Tertiary elevation is somewhat dome-shaped and at- 
tains its maximum elevation of 2,400 feet about 30 miles north- 
west of Harrisonburg, Virginia; from this point it descends quite 
rapidly in all directions, but shows a partial agreement with the 
axes O Dand E F (plate 5). Toward the west the actual coin- 
cidence of the two plains cannot be determined, but they appear 
*The Lafayette Formation, by W J McGee: 12th Ann. Rep. U.S. Geol. 
Survey, 1890-’91, pp. 508, 509. 
