RUSSELL FORK FAULT OF SOUTHWEST VIRGINIA 353 
the great overthrust of Pine Mountain suddenly becomes very 
much less severe at Skegg Gap, and from there northeastward the 
structure is essentially a low anticline broken by a minor over- 
thrust which decreases rapidly in extent of thrust and comes to 
an end a few miles into Buchanan County. He considered that 
the principal Pine Mountain overthrust was cut off at the north- 
east end by the Skegg Gap fault which he mapped as far as Russell 
Fork. Between this point and Big A Mountain he has mapped a 
number of narrow areas of faulted and buckled rocks which he 
describes in some detail and in explanation of which he postulates 
lateral shearing with the southwest side moving northward with 
some overthrusting and buckling against the northeast side. He 
states that succeeding this movement there was normal faulting 
along this line in which the southwest side was downthrown. His 
evidence for this belief is not clear, and his several areas of dis- 
turbed rocks are separated by areas in which he found no evidence 
of movement. 
DESCRIPTION OF THE CUMBERLAND BLOCK’ 
The structure of the area concerned in this paper has been 
described in considerable detail at many points by previous writers.” 
It is not the writer’s purpose to give here a thorough description 
of the structure or topography but rather to point out briefly their 
alient features. 
The “remarkable quadrilateral block’? whose southwestern 
extremity was first recognized by Safford and described in detail 
by Keith extends from the valley of Cove Creek in Campbell 
County, northeastern Tennessee, northeastward for one hundred 
and twenty-five miles to the valley of Russell Fork of Big Sandy 
River in Dickinson and Buchanan counties, Virginia. It is sur- 
prisingly uniform in width, averaging about twenty-five miles, is 
bounded on the northwest by the Pine Mountain fault and on the 
southeast by the Hunter Valley fault and the closely associated 
Wallen Valley fault, which is developed only from near Big Stone 
t This name is here applied for the first time. 
2 J. M. Safford, Geology of Tennessee (1869); M. R. Campbell, Geologic Folios 
12 and 59, Arthur Keith, Geologic Folios 33 and 75, G. H. Ashley and L. C. Glenn, 
Prof. Paper 49, all of the U.S. Geol. Survey. 
