Dates of Orogenic Movements
The uplift which elevated the Tertiary peneplain to an altitude of 2,500 feet, as shown in plate 6, was confined almost entirely to the axis K L. This axis crosses the river in its lower course, but the river had sufficient volume to hold its antecedent position across the rising fold. In doing so it has cut a narrow, rugged gorge 1,500 feet deep, and is still actively corrading its channel. The movement along the axis must have been practically continuous from the completion of the Tertiary peneplain down to the present. The region northeast of New river, in which rise branches of the Potomac, the James, the Kanawha and the Monongahela, has probably been an area of continuous uplift during, every period of orogenic activity affecting the province. The Cretaceous peneplain, of which only a few doubtful remnants exist, was elevated at least 2,400 feet and Tertiary erosion was proportionally stimulated. It succeeded, however, only in reducing to baselevel and slightly broadening the valleys of the larger streams. A post-Tertiary elevation of 1,600 feet has renewed their activity, so that it has been continued with scarcely a pause from the close of the Cretaceous period down to the present. The result of this almost continuous downward stream cutting has been to produce the most sharply cut region in the Appalachian province. The slopes are steep and generally uniform from the highest points, which may represent the surface of the earlier peneplain, down to the present streams, with only an occasional trace of terracing to mark the Tertiary baselevel. The elevation of the Tertiary peneplain along the eastern border of the province has been only moderate, and the streams have accomplished correspondingly little erosion upon its surface. The Roanoke, the James and the Potomac have cut’ rather narrow and shallow valleys across the piedmont plain. These become shallow gorges in the broad baseleveled valleys west of the Blue ridge.
