226 DOUGLAS WILSON JOHNSON 



under the conditions preceding the supposed capture. It is difficult 

 to conceive that a small stream like the Sequatchie, having its very 

 beginning less than 70 miles to the northward, and flowing over 300 

 miles to the sea without receiving the waters of any large stream, 

 could have reduced its upper course to a lower level than the middle 

 course of a neighboring great Appalachian river. It is true that 

 the smaller stream would have had some advantage of more direct 

 outlet to the sea, as the region of the gorge is something over 300 

 miles from the mouth of the Sequatchie and about 400 miles from 

 the mouth of the Appalachian, as represented on Hayes and Camp- 

 bell's map of the Tertiary peneplain. It is also possible that there 

 is some difference in the character of the rocks over which the streams 

 were flowing, which was. to the advantage of the smaller stream, 

 although both were developed largely on soft rocks. But, after 

 making all due allowance for these advantages, it is still impossible 

 to admit that, whereas the great Appalachian River still had a fall 

 of 3 inches per mile throughout its middle and lower courses, the 

 comparatively small Sequatchie had reduced its gradient almost 

 to zero even in its upper portion, as has been represented. 



The fact that the Tertiary peneplain is somewhat lower in the 

 Sequatchie valley than in the valley east of the ridge, while presenting 

 grave difficulties in the way of a satisfactory explanation on the basis 

 of the theory of capture, is wholly in accord with the alternative 

 theory. We should expect the peneplain to be higher on the eastern 

 side of the ridge, for two reasons : it is there farther upstream ; and, 

 what is probably of greater importance at this point, the stream in 

 flowing from east to west passes through a mountain barrier which 

 sheds abundant waste into its waters and obstructs its passage with 

 coarse debris, compelling the maintenance of a steeper gradient 

 throughout 25 miles or more; and when it is remembered that nearly 

 or quite to the close of the Tertiary period the mountain served as 

 a sandstone barrier in the path of the stream., it is seen that the base- 

 leveling process must necessarily have gone on at a higher level 

 in the valley in soft rocks east of the ridge. The observed difference 

 in elevation is not greater than we should expect under the conditions 

 referred to, and is not believed to be sufficiently great to account 

 for the proposed capture, even were it possible for this difference to 



