1877. ] The Mountain Axes near Cumberland Gap. 389 
anticlinal, have gone back as much as six or eight miles from the 
top of the arch, while the lateral escarpment of Pine Mountain 
has not retreated more than half a mile from its original place. 
After making all due allowance for possible differences in ero- 
sion rate of the two forces, it will be impossible to believe that 
erosion has acted on these two faces for equal lengths of time. 
There can be no doubt left in the mind of any one who studies 
these escarpments and satisfies himself of their relation to the 
geology of the neighboring districts, that their outcrops were 
made at periods widely remote from each other. 
If there should be any need of accumulating proofs on this 
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(Fie 70.) DIAGRAMMATIC SECTION ACROSS THE CUMBERLAND SYN-ANTICLINAL. 
connected with them. The Pine Mountain fault, for instance, is 
characterized by a wonderfully rectilinear front, being hardly 
swerved from a straight line in fifty miles of its length. Through- 
out this distance the fault that made the escarpment is hardly 
half a mile from the summit of the crest. The Cumberland 
Mountain, which is an anticlinal escarpment, is an exceedingly 
irregular line, often departing as much as a mile from a direct 
course, and cut through and through at many points by streams. 
These irregularities in the one case and the regularity in the other 
attest the difference in the age of the two escarpments. There 
is still other evidence, the nature of which, however, it is not 
easy to make plain in a few words. This evidence may be 
briefly stated as follows: The streams on the west which head 
against the Pine Mountain are generally characterized by a 
