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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 approximately by the summits of the isolated ‘knobs. The altitudes 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., unfavorable 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 southeastward, while in Big Black mountain the rocks are horizontal; but the former is made up of 1,200 to 1,400 feet of hard conglomerate, interbedded with shales and sandstones, while 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 northwestern 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 consists of the even crested ridges similar to those of Pennsylvania which have been so well described by Davis.* As a rule the ridges of the southern Appalachian valley are remarkably even crested and are unquestionably the remnants of a plain. In many cases, however, more or less wide variations from the type are found.