26 Campbell — Changes in the Dramage of Virginia. 



river. The chances are in favor of the former stream recap- 

 turing some of its lost territory. 



This career of conquest so well begun by Guest river was 

 stopped short by an obstruction in its pathway. The river 

 soon removed the soft shales upon which it was originally 

 located and encountered the massive, horizontal conglomerate 

 which soon absorbed all of its energies. This work has greatly 

 retarded the development of its upper course which has re- 

 mained almost stationary and has retained all of its inherited 

 characteristics. In the great bend below Yellow creek, this 

 massive conglomerate has again obstructed the course of the 

 stream and still more retarded the development of its head 

 branches. 



The reversal was accomplished after the Tertiary period of 

 base leveling, for the uplift along the axis C D warped the 

 Tertiary peneplain as described above and hence must have 

 occurred after the completion of that topographic feature. The 

 amount of corrasion accomplished by Guest river since the 

 change, requires a large allowance of time ; therefore it seems 

 probable that the change occurred soon after the completion of 

 the base-leveling period or in late Tertiary time. 



Altogether the evidence seems quite conclusive (1) that such 

 a change occurred ; (2) that it was accomplished soon after the 

 completion of the Tertiary peneplain ; and (3) that its cause 

 was the orogenic uplift along the axis C D. 



In connection with the case above described, the writer 

 wishes to call attention to another probable adjustment of 

 drainage clue to the same orogenic uplift ; a change which 

 accounts for some peculiar topographic features now apparently 

 anomalous. 



Cumberland gap, that historic gateway between the east and 

 the west, is but a wind-gap in the Cumberland escarpment and 

 is remarkable chiefly on account of the depth to which it is 

 cut. That its history is different from the history of the ordi- 

 nary wind-gaps in the ridge is apparent, but the only explana- 

 tion yet offered regarding its origin is that by Shaler who 

 attributes it to retrogressive erosion along a local cross-fault.* 

 That a fault of this character determined the location of the 

 gap seems quite probable but it is not so clear that sub-aerial 

 erosion could reduce it to its present condition. The writer is 

 familiar with several such cross-faults, but knows of no case in 

 which a remarkably low gap has been produced by them unless 

 corraded by a stream of water. If we assume for the present 

 that Cumberland gap is due to stream corrasion, then there are 

 three questions which must be answered. (1) What stream 



* Kentucky Geological Survey, vol. iii, New Series, pp. 9S-99. 



