210 DOUGLAS WILSON JOHNSON 



Tertiary peneplain. It has not been materially modified since, and 

 therefore it is truly a Tertiary valley. On the other hand, while 

 the Tennessee gorge had a certain form at the close of the Tertiary 

 period, post-Tertiary cutting has lowered the gorge 250 feet, almost 

 obliterating all traces of the old Tertiary valley floor at the higher 

 level. Manifestly, then, the two valleys as they are at the present 

 cannot be properly compared. The comparison becomes of value 

 only if we conceive the post-Tertiary cutting in the gorge to be re- 

 placed, filling up the bottom of the gorge to the level of the Tertiary 

 peneplain. We may then compare the two valleys as they were at 

 the close of the long period of Tertiary baseleveling. Such a com- 

 parison shows a more marked contrast in geological conditions in 

 the two valleys than that already indicated. 



Hayes and Campbell have shown that the Tennessee River at 

 Chattanooga has cut down its channel 250 feet below the level of 

 the Tertiary peneplain, while the distribution of the Tertiary gravels, 

 to be considered later, indicates that the gradient of the river through 

 the gorge was then much the same as now. Hence filling up the gorge 

 to that old level, and thus restoring the Tertiary peneplain, means 

 a reduction of 250 feet in the extent to which the river had cut into 

 the limestone then as compared with now. And since the vertical 

 extent of limestone now shown in the gorge is between 300 and 400 

 feet, according to the larger estimates, this means that at the close 

 of the Tertiary baseleveling period the Tennessee had cut into the 

 limestone underlying the massive sandstone cap only between 50 

 and 150 feet; while if we accept the smaller estimates for the thick- 

 ness of the limestone exposed at present throughout the main portion 

 of the gorge, which seems to the writer to be nearer the actual con- 

 ditions, we find that at the close of the Tertiary the river was in places 

 still flowing wholly in sandstone, not having yet reached the under- 

 lying limestone. At this same time the stream southwest of Scotts- 

 boro had cut through the thin sandstone cap of that region and over 

 400 feet into the underlying limestone. The following figures 

 illustrate graphically the different conditions obtaining in the two 

 areas at the close of the Tertiary baseleveling period. 



These diagrams are based on the more conservative figures given 

 above. It is believed that the actual conditions at the close of the 



