Campbell — Changes in the Drainage of Virginia. 29 



deviating from its course, it leaves the soft Chickamauga 

 (Trenton) limestone and cuts across the siliceous Knox dolo- 

 mite to a point about four miles from the gap and on the axis 

 of the anticline ; here it makes another sudden change and 

 follows the axis to the southwest. This peculiar bend in the 

 river cannot be explained by existing conditions ; it appears 

 to be an inherited course from an older river which turned 

 at this point and flowed northwestward through Cumberland 

 gap. 



The Cumberland river was probably one of the principal 

 streams draining the Appalachian highlands before the close 

 of the Paleozoic era and it held its antecedent course long 

 after the Cumberland plateau was elevated above sea-level. 

 The great river flowing southwest in the Appalachian valley 

 encroached upon the headwaters of the Cumberland and 

 gradually diverted its upper tributaries. In Cretaceous time 

 Powell river was the sole surviving member of the Cumber- 

 land river system within the Appalachian valley. It held its 

 antecedent course to the westward during the elevation which 

 followed the Cretaceous period of baseleveling and carved 

 deep gorges in Pine and Cumberland mountains. It main- 

 tained this course while the surface was again reduced to 

 a peneplain, but in the succeeding uplift, it was diverted. 

 Thus Cumberland river lost its last hold on the Appalachian 

 valley. 



The axis of orogenic uplift which affected the headwaters of 

 Powell river so disastrously passes just west of Cumberland 

 gap. Previous to the uplift, Cumberland river was barely 

 holding its own against the encroaching Appalachian river ; 

 when the uplift occurred athwart its course it was forced to 

 yield its last tributary to its more favorably located rival, 

 Clinch river. A small branch working back along the axis of 

 the Powell Yalley anticline tapped Powell river above the 

 rising fold and turned its waters into the Tennessee system. 



Thus in all probability a slight orogenic uplift was the direct 

 cause of a complete change in the drainage of this region, — a 

 change radical in its effect but now indicated alone by the de- 

 serted gorge of Cumberland gap. 



