Walden Gorge and Tennessee Valley. 118 
has oceupied this narrow gorge throughout the entire period 
during which the enormous erosion of the Appalachian valley 
was accomplished. That a peneplain should have been devel- 
oped from 20 to 40 miles in breadth and from central Virginia 
to northern Georgia by the same river in the same time that the 
insignificant strips of flood-plain in the gorge were being cut is 
quite improbable. It is true, the conditions of erosion in the 
two cases were not the same. The Tertiary peneplain in the 
Appalachia valley is developed only on areas of soft rocks 
which are generally steeply inclined ; but, even allowing the 
greatest possible weight to the different conditions of erosion, 
the discrepancy in amount of erosion requires some further ex- 
planation, if the time were the same in both cases. 
While a direct comparison cannot be made between the Wal- 
den gorge and the upper Tennessee valley on account of differ- 
ence in conditions, such a comparison can be made between the 
gorge and a valley in northern Alabama, extending from Scotts- 
boro southwestward to the mouth of Flint river. <A portion of 
it is shown on plate 4. It is nowhere less than six miles broad, 
and its floor is very regular, forming a portion of the Tertiary 
peneplain. The age of this valley is easily determined; it is 
carved in the Cretaceous peneplain ; therefore it is more recent 
than the Cretaceous; it is continuous with the Tertiary pene- 
plain, and hence was completed at the close of the Tertiary base- 
leveling 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 pre- 
vailing 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 conditions are highly favorable for rapid corrasion of a 
river channel. The sandstone cap is undermined and its débris 
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 
sandstone 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 débris 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 
