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Campbell — Changes in the Drainage of Virginia. 21 



Art. Y. — Tertiary Changes in the Drainage of Southwestern 

 Virginia ;* by Marius K. Campbell. 



It has been recognized for a long time that isostatie move- 

 ments have been active in the Appalachian region throughout 

 post-Paleozoic time. Their exact character has never been 

 worked out, nor has their influence on the physiography and 

 drainage of the region been fully appreciated. Certain phe- 

 nomena have, by a few workers, been attributed to movements 

 of this kind but in such cases they have been referred to broad 

 epeirogenic rather than local orogenic movements. The writer 

 is fully convinced that the local movements have been more 

 important in shaping both the physiography and the drainage 

 of the region than the broader and more general movements 

 and offers the following observations concerning a reversal of 

 drainage, in support of the statement. 



Along the northwestern border of the Appalachian valley, 

 from Cumberland gap northeast, there is a great synclinal 

 trough of Coal Measures, bounded on the northwest by Pine 

 mountain and on the southeast by Stone mountain or the 

 Cumberland escarpment. The edges of the trough are formed 

 by the Carboniferous conglomerate, a hard and resisting stra- 

 tum which has been chiefly instrumental in turning the streams 

 to subsequent courses. The change in drainage, here described, 

 occurred within this syncline, about TO miles northeast of 

 Cumberland gap. 



The region in question is drained by Guest and Powell 

 rivers which rise in the Black mountains and flow south in 

 parallel courses to Norton, where they encounter the conglom- 

 erate rising on the southern rim of the syncline. To avoid 

 this, Powell river turns west, whereas Guest river turns east. 

 They are but a mile apart at these bends and separated by a 

 very low divide. Powell river flows westward within the 

 synclinal rim to Big Stone gap which it has carved through 

 the conglomerate barrier ; through this gateway it enters the 

 head of a broad, open valley eroded in the great Silurian 

 limestones which it follows until it joins the Clinch river 

 15 miles north of Clinton, Tenn. Guest river turns eastward 

 and flows around the end of Powell mountain, joining the 

 Clinch river a few miles below St. Paul, Ya. 



In the basin of Powell River (fig. 1) the drainage appears 

 normal, showing that mature adjustment to structural features 

 which is found only in systems of great antiquity. The 

 changes occurring in the past have been so remote that all 

 traces are obliterated, both in the physiography and in the 



* Published by permission of the Director of the IT. S. Geological Survey. 



