212 



DOUGLAS WILSON JOHNSON 



The degree of similarity between these gorges (Fig. 6) is what we 

 should expect when the conditions under which they were carved were 

 for so long a time so much alike; and inferences drawn from such a 

 comparison seem more reliable than those from a comparison with 

 a valley where the conditions were for so long a time so radically 



different. No compari- 

 son between the gorge 

 through Walden Ridge, 

 and the broad longitudi- 

 nal valleys east and west 

 of the ridge, can be made, 

 since the latter were 

 carved entirely on broad 

 bands of soft rocks. 



It is believed, then, 

 that when the geological 

 conditions at the Walden 

 gorge and the Scottsboro 

 valley are carefully con- 

 sidered, and the com- 

 parison between the two 

 made on the same time 

 basis, the difference in 

 form between the two 

 will afford no evidence in 

 favor of the recent diversion of the Tennessee River to a westward 

 course. It is further believed that these considerations fully explain 

 the present narrowness of the gorge on the basis of the alternative 

 theory, the youthful features being just what we must expect under 

 the conditions that have controlled erosion at this point since Creta- 

 ceous time. 



Evidence from the distribution of the Unionidcs. — Six years after 

 the publication of the paper by Hayes and Campbell giving the 

 above lines of evidence, and the conclusions to which they led, Mr. 

 Simpson reached the same conclusions from a study of the fresh- 

 water mussels of the region. The facts that Mr. Simpson brought 

 forth are as follows: Pleurobema, a genus of Unio, has its metropolis 



Fig. 6. — Profiles of (A) Susquehanna Water- 

 gap above Harrisburg through Second Mountain, 

 (B) Delaware Watergap, and (C) Tennessee gorge. 



(Horizontal scale, i inch = i mile; vertical scale, i 

 inch = 2,000 feet.) 



