TERTIARY HISTORY OF THE TENNESSEE RIVER 201 



and Coosa -Alabama drainage basins; (2) a comparison of the amount 

 of material eroded from the Appalachian valley with that of the 

 sediment deposited at the mouth of the Coosa- Alabama River; (3) 

 the character of the gorge of the Tennessee below Chattanooga. 



Evidence from the Coosa-Tennessee divide. — Regarding the char- 

 acter of the divide between the Tennessee and Coosa- Alabama basins, 

 Hayes and Campbell point out the fact that the divide is low and 

 indistinct, and that it occurs as a less prominent feature in the well- 

 defined, broad, open valley which is continuous to the northeast with 

 the valley of the present Tennessee above Chattanooga, and to the 

 southwest with the valley of the present Coosa and Alabama Rivers. 

 The well-marked character of this broad valley, which continues 

 directly across the dividing line between the two river basins, is 

 thought to indicate the former presence of a large northeast-southwest 

 stream, the Appalachian River. This being true, the present posi- 

 tion of the large stream must have resulted from river-capture, and a 

 diverting stream must have led the upper Appalachian westward 

 through the new-made gorge. 



It does not seem to me that the baseleveled character of the divide 

 alone necessitates the conclusion that a large stream once flowed 

 across it. In order to make such a conclusion necessary, we should 

 have to assume that every such broad valley developed across a low 

 divide must have been carved by a large stream — an assumption 

 which does not seem to be warranted. The region is one of parallel 

 bands of alternating hard and soft rocks. (This parallelism of 

 structure would prevent the development of dendritic drainage, the 

 absence of which is noted by Hayes and Campbell.) The soft bands 

 are rapidly eroded, while the hard bands remain as intervening 

 ridges. Thus the region is characterized by broad, northeast- 

 southwest valleys, many of which are occupied by very insignificant 

 streams between whose headwaters are inconspicuous divides. Nor 

 does there seem to be any need of assuming the former presence of 

 larger streams, since the softness of the rocks and their easy solubility 

 sufficiently account for the phenomena observed. Indeed, there are 

 cases in which it seems practically certain that no drainage readjust- 

 ments have taken place in recent geological time. Hayes and Camp 

 bell recognized this fact, and, after stating the argument and their 



