TERTIARY HISTORY OF THE TENNESSEE RIVER 221 



Character of stream-dissection on Walden Ridge and Sand Moun- 

 tain. — The streams occupying the open longitudinal valleys on either 

 side of the ridge belong to that type known as subsequent streams, 

 since they must have gained their present positions by a process of 

 adjustment to weak structures from some former position more 

 directly consequent upon the initial folding of the region. The 

 branches of the subsequent streams which head up the ridge belong 

 to the group of streams termed obsequent, since they have grown 

 headward against the dip of the beds which they cut. It is one of 

 the characteristics of obsequent streams that they are not located, 

 so far as is known, by any specific structural factors, but develop at 

 more or less regular intervals, as is required by the removal of rainfall. 

 Over an area where practically uniform conditions prevail the 

 development of such streams will be roughly uniform. We should 

 not expect one stream of this class to dissect an area deeply, while 

 similar adjacent streams had barely begun their work. 



An examination of the Chattanooga and Stevenson topographic 

 sheets shows that the dissection on Walden Ridge and Sand Mountain, 

 north and south of the Tennessee gorge, has progressed at a fairly 

 uniform rate. It also appears that the dissection is of no great extent, 

 the integrity of the plateau remnant being quite well preserved in 

 either direction. Only at the gorge is the flat-topped mountain cut 

 wholly in two. Now, the theory of capture necessitates the assump- 

 tion that one of the obsequent streams grew headward entirely through 

 the mountain and cut down to the level of the main stream in the 

 broad valley to the east, during the same time that its neighbors were 

 able to make but a small beginning on the dissection of that moun- 

 tain — an assumption which it seems difficult to accept. If one of 

 the obsequent streams was able to cut its ways entirely through the 

 ridge, then we should expect to find the ridge more or less nearly 

 breached at various other points. This is distinctly not the case. 

 The marked contrast between the amount of dissection at the gorge 

 and elsewhere points to an entirely different origin for the gorge from 

 that of breaching by an obsequent stream. 



According to the suggested procedure of the supposed capture, 

 the branch from the Sequatchie River to the west was aided in the 

 breaching of the ridge by several small obsequents working back 



