206 DOUGLAS WILSON JOHNSON 



it is continuous with the Tertiary peneplain, and hence was complete at the close 

 of the Tertiary baseleveling period; and at the close of that period it was deserted 

 by the stream which carved it. The conditions under which this valley was cut 

 are practically the same as those now prevailing in the gorge through Walden 

 plateau. In both cases the rocks are nearly horizontal, heavy sandstones capping 

 the plateau, with easily erodible Carboniferous limestones beneath. Such condi- 

 tions are highly favorable for rapid corrasion of a river channel. The sandstone 

 cap is undermined, and its debris rolls down and forms a talus on the lower slopes. 

 The rate at which the cliffs recede depends largely on the rate at which the sand- 

 stone talus is removed from the slopes and the limestone is exposed to erosion. 

 No conditions could be more favorable for this rapid removal of the protecting 

 debris than those now present in the Walden gorge, where the base of the slope is 

 washed by a stream competent to remove all talus from the cliffs above, the 

 coarsest as well as the finest. Certainly the conditions in the gorge are fully as 

 favorable as they were in the valley west of Scottsboro when that was being cut, 

 and the stream which flowed in that valley was probably smaller than the present 

 Tennessee; therefore, if under the same conditions a smaller stream than the 

 present Tennessee could cut so broad a valley as it did in northern Alabama 

 during the Tertiary cycle, the conclusion seems inevitable that the present gorge 

 through Walden plateau has been occupied a very much shorter time, and hence 

 the Appalachian drainage was not diverted to its present westward course till 

 after a part or the whole of the Tertiary cycle (pp. 113, 114). 



While a comparison of the Tennessee gorge with some valley of 

 known age may be expected to give us our best light on this particular 

 phase of the question, two very serious criticisms must be urged 

 against the comparison given above. In the first place, sufficient 

 account is not taken of the details of geological structure in the two 

 regions compared, although these details are features of prime im- 

 portance in the present connection. In the second place, the com- 

 parison is one between the Tertiary valley southwest of Scottsboro 

 and the present gorge of the Tennessee. The references are always 

 to "the present gorge through Walden plateau," the conditions 

 "now prevailing in the gorge," etc. A proper comparison can only 

 be made between the Scottsboro Tertiary valley and the gorge of 

 the Tennessee as it also appeared at the close of the Tertiary period. 



When the geological formations in the region of the Scottsboro 

 valley and in that of the Tennessee gorge are compared, it is found 

 that the conditions under which the two valleys were carved were 

 far from being the same. It is true that in each case we have soluble 

 limestone capped by hard sandstone, but the sandstone cap in the 



