Cumberland Mountain is one of those “even-crested” Appalachian ridges whose summits, in general, are commonly supposed to represent a peneplain—the Schooley, or “Cretaceous,” peneplain—which once covered the region. Cumberland Gap is a notch, cut to a depth of about 600 feet in that ridge, but not now occupied by a stream.
If the apparent evenness of the ridge crest is due to peneplanation, one would expect the crest to have essentially the same altitude on the two sides of the gap. As a matter of fact, the ridge crest on the southwest side lies about 300 feet lower than on the northeast side. This difference in level on the two sides of the gap is a striking feature, which immediately attracts the attention of the physiographer (figures 1 and 2 and profile, figure 3). Why should this marked change in the elevation of the ridge crest be . . .