TERTIARY HISTORY OF THE TENNESSEE RIVER 209 



river. The limitation of the limestone to the bottom of the gorge 

 is also indicated by the presence of sandstone in ledges apparently 

 in place well out on spurs projecting forward from the main base of 

 the mountain. These spurs occupy a position which does not seem 

 to admit of their sandstone being interpreted as debris that had 

 been let down from above by the undermining of the limestone. 



The geological map accompanying the Chattanooga folio rep- 

 resents a larger portion of the gorge as being in limestone. Thus 

 the thickness of the limestone exposed in the gorge is shown as over 

 600 feet in places, and as less than 300 in others. Throughout a 

 large part of the central portion of the gorge the 1,000-foot contour 

 line is taken as the upper limit of the limestone, and the indicated 

 thickness of between 300 and 400 feet of limestone may be taken 

 as a fair average figure as given by this line of evidence. This 

 leaves, as the map shows, a cap of between 600 and 700 feet of sand- 

 stone. 



It appears, then, that if we take the more conservative estimates 

 of the thickness of sandstone in the Walden gorge, the sandstone cap 

 at that locality is still over three times as thick as the similar cap 

 in the Scottsboro valley. If a thickness of sandstone be taken nearer 

 that indicated by the sections given in a preceding paragraph, the 

 contrast is still more marked. Such a difference in geological struct- 

 ure must result in a marked difference in form between the two 

 valleys. The Scottsboro stream, having the thin cap of hard rock 

 to cut through must have reached the underlying soft rock, and so 

 have had ample opportunity to widen its valley under favorable 

 conditions, while the stream through Walden gorge, having the 

 thick cap of hard rock to contend with, still continued for a long time 

 to carve a narrow gorge in the resistant beds. The great degree of 

 contrast to be expected in this particular case will be more apparent 

 after a consideration of the next point. 



A second serious objection to the comparison between the valley 

 southwest of Scottsboro and Walden gorge is found in the fact that 

 it is throughout a comparison between the former as it was at the end 

 of Tertiary times, and the latter as it is at present. As Hayes and 

 Campbell have shown, the Scottsboro valley was completed at the 

 end of the Tertiary period, and the valley floor forms a part of the 



