Fig. 5. — Section of Tennessee gorge show- 

 ing relations of sandstone cap and underlying 

 limestone at close of Tertiary cycle. 

 (Scale, same as in Figure 4.) 



TERTIARY HISTORY OF THE TENNESSEE RIVER 211 



Tertiary period would be more accurately represented by making 

 the sandstone cap over the limestone of the Scottsboro valley (Fig. 

 4) thinner than it is shown, and the sandstone of the gorge (Fig. 5) 

 enough thicker to show the gorge entirely in sand- 

 stone. 



But even 

 on the basis 

 of the more 

 conservative 

 representa- 

 tion, the con- 

 trast in con- 

 ditions under 



which the two valleys were carved is such as to 

 explain satisfactorily the difference in form which 

 has been noted. Indeed, it is difficult to conceive 

 how the gorge of the Tennessee could be otherwise 

 than narrow and steep-sided as compared with the 

 Scottsboro valley, since it was carved almost 

 wholly, or possibly entirely, in hard sandstone 

 throughout the long period of Tertiary baselevel- 

 ing, and only during the comparatively very short 

 post-Tertiary trenching have favorable valley- 

 widening conditions existed; whereas the Scotts- 

 boro valley was carved largely in easily erodible 

 limestone, favorable conditions certainly existing 

 a vastly longer period in this case. It appears, 

 then, that the gorge of the Tennessee might be 

 more properly compared with other gorges cut 

 through sandstone ridges since the Cretaceous 

 period, such as those of the Delaware and Susque- 

 hanna Rivers through the Pennsylvania ridges; 

 for while the present conditions at Walden gorge 

 and the latter localities are different, they were 

 much the same until comparatively recent times, 

 barring the greater width of the Walden ridge, and 

 a possible difference in hardness of the sandstone. 



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