28 Campbell — Changes in the Drainage of Virginia. 



could have occupied the gap : (2) what conditions changed 

 its course ; and (3) when did this change occur \ 



The accompanying map shows in a general way, by means 

 of 500 foot contours the configuration of the country. The 

 great valley bordering the Cumberland escarpment on the 

 southeast is called Powell valley and in this during the Ter- 

 tiary period the limestones were reduced almost to baselevel. 

 Since then the plain has been tilted so that at the northeastern 

 corner of the area, covered by the map, its present elevation is 

 about 2,000 feet, sloping gradually to about 1,500 feet at Cum- 

 berland gap and 1,200 feet in the extreme southwestern corner 

 of the sheet at Clinton, Tenn. The Cumberland river valley 

 was partially baseleveled at about this same elevation, but the 

 rocks are harder and the reduction was not so complete. A 

 very significant fact about Cumberland gap is that its eleva- 

 tion (1,3S3 feet)* is less than the Tertiary baselevel peneplain 

 on either side. This shows that, after the completion of the' 

 peneplain, the stream occupied the gap long enough to reduce 

 it at least 100 feet below the former baselevel. An uplift 

 following a period of base-leveling would stimulate erosion 

 very much and the' streams would rapidly deepen their chan- 

 nels, but on the other hand the lower courses of a stream must 

 be cut before it can deepen its channel near headwaters, so it 

 is probable that considerable time elapsed between the com- 

 pletion of the peneplain and the diversion of the stream that 

 carved the gap. This agrees with the probable date of change 

 in Guest river and indicates that the same uplift is responsible 

 for both. 



Thus we can with considerable confidence establish the date 

 at which this change occurred. Now it remains to be seen 

 whether we can locate the stream that was then diverted from 

 the gap. Did Powell river then turn north and join Cumber- 

 land river or did the Cumberland flow south and join Powell 

 river? The headwaters of the Cumberland probably did not 

 flow south, for the gorge through Pine mountain must have 

 been occupied since Cretaceous time ; therefore no change 

 could have been made in that stream as late as the Neocene. 

 In regard to Powell river there are some facts that support 

 the idea of a northward flow. This stream as already de- 

 scribed follows the Powell valley throughout most of its course. 

 From Jonesville, Va., to a point opposite Cumberland gap, 

 the river flows closely along the southeastern side of the valley 

 in a normal course, but at the latter point it turns abruptly 

 and flows toward the gap for a distance of five miles. In thus 



* Elevation given by the profile of the Cumberland gap extension of the Louis- 

 ville and Nashville Railroad. 



