218 DOUGLAS WILSON JOHNSON 



amount of material eroded. A study of Walden gorge and the Scotts- 

 boro valley shows the invalidity of any argument in favor of the 

 theory of capture based on a comparison of the two valleys, and 

 satisfactorily explains the narrowness of the gorge on the basis of 

 an alternative theory. The supposed evidence of the Unionidae in 

 favor of the theory of capture is seen to be based upon assumptions 

 which do not seem to be warranted by due consideration of the 

 various means of molluscan dispersal or by the facts of Unionidae 

 distribution. It is therefore believed that the arguments brought 

 forward in support of the theory of capture are far from conclusive, 

 and that the history of the Tennessee is still open to consideration. 

 Certain lines of evidence, some of which have hitherto been uncon- 

 sidered, appeal to the writer as weighing strongly against the theory 

 of capture, and as strongly indicating the continued presence of the 

 Tennessee River in its present path across Walden Ridge since 

 Cretaceous time. 



EVIDENCE IN PAVOR OF THE ALTERNATIVE THEORY 



The evidence against the proposed theory of capture, and in favor 

 of the long continuance of the Tennessee in its present course through 

 Walden Ridge, is derived from a consideration of (i) the winding 

 character of the gorge; (2) the character of stream-dissection on the 

 ridge both north and south of the gorge; (3) the character of the 

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

 Chattanooga; (4) the lack of sufficient inequality of levels between 

 the valleys east and west of Walden Ridge; (5) the absence of the 

 Tertiary river gravels from the supposed former southward course 

 of the river. 



Before considering these lines of evidence which appear to bear 

 directly on the present problem, it is desirable to note one line of 

 evidence which might be available in certain cases, and to make 

 clear my reasons for not applying it in the present instance. When 

 a mature stream has developed a broad, open valley, the floor of that 

 valley will have a gentle slope downstream, represented by the line 

 AD in Fig. 7. Now, if capture of this stream takes place at a point 

 B, the portion of the stream AB will intrench itself, developing a 

 profile A'B' . From the point of capture, B, the divide will be pushed 



