National Geographic : 1894 May 23
Epeirogenic 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 C D and 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.
1894 Jun 22
1894 Apr 25