72 Hayes and Camphell—Appalachian Geomorphology. 
formed and so excellently preserved. It can be traced continu- 
ously from an altitude of 600 feet in central Alabama to 2,000 
feet at the Tennessee-Alabama line, and thence holding about 
the same altitude, with slight irregularities, to Cumberland gap. 
North of the Kentucky-Tennessee line the identification of this 
peneplain becomes a much more difficult matter, for elevation 
has been greater and erosion more rapid. The rocks are gen- 
erally soft and have been unable to preserve any extent of level 
surface ; hence the plain is almost wholly destroyed. Neverthe- 
less, upon careful study of a wide area, it is seen that along north- 
east-southwest lines there is a marked uniformity in the altitude 
of the summits, and on transverse lines an extremely regular in- 
crease in their elevation toward the interior. ‘This gentle but 
regular slope bears apparently no relation to the structure, and 
there seems no other explanation but to regard this as an almost 
completely dissected peneplain whose surface is represented ap- 
proximately by the summits of the isolated knobs. The alti- 
tudes of these remnants of the plain vary from 1,300 or 1,400 
feet near the mouth of the Big Sandy river to 4,000 feet near the 
central portion of the Virginia-West Virginia line. Above this 
inclined peneplain no summits rise until well toward the interior 
of the region, where their occurrence seems to be due to the same 
causes which produced monadnocks further southward, viz., un- 
favorable location with reference to the main drainage lines. This 
is well exemplified in the Big Black mountain on the state line 
between Kentucky and Virginia. This irregular mountain mass 
near Big Stone gap is composed of upper Coal Measures, and has 
an altitude of 4,100 feet, while Pine mountain, but a few miles 
northwestward, is finely baseleveled at about 2,500 feet. True, 
there is a great difference in attitude of the strata in these two 
mountains, for in Pine mountain the dip is about 30° south- 
eastward, while in Big Black mountain the rocks are horizontal ; 
but the former is made up of 1,200 to 1,400 feet of hard con- 
glomerate, interbedded with shales and sandstones, whilé the 
latter is composed of the ordinary shales and sandstones of the 
upper Coal Measures. Apparently Big Black mountain owes 
its preservation to the presence of Pine mountain on its north- 
western side, which acted as a barrier against erosion from that 
direction. 
Valley Ridge Type.—In the Appalachian valley the type is more 
uniform throughout the whole extent of the province and con- 
