National Geographic : 1894 May 23
92 Hayes and Campbell-Appalachian Geomorphology. rugged gorge 1,500 feet deep, and is still actively corrading its channel. The movement along the axis must have been practi cally 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 Creta ceous peneplain, of which only a few doubtful remnants exist, was elevated at least 2,400 feet and Tertiary erosion was propor tionally 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 Appa lachian 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 sur face. 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. RELATIVE DATES OF THE OROGENIC MOVEMENTS. Before closing this portion of the paper it is perhaps advisable to review hastily, as far as the evidence will admit, the succes sion of oscillations in post-Paleozoic time. As already stated, the determination of the character of these movements is one of the most important results derived from this study, since the en tire physiography of the region, including the arrangement of its drainage systems, has been modified to a great extent by them. Movements in the Tertiary Cycle.-It is not advisable at present to go farther back in geologic time than to the close of the Cre taceous period of baseleveling, although there are traces of sim similar movements in the preceding ages of post-Paleozoic time.
1894 Jun 22
1894 Apr 25