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National Geographic : 1894 May 23
Contents
Extent of the Peneplain. to the Coosa at Rome, and thence to its junction with the western branch. DIVERSION OF THE APPALACHIAN RIVER. It is stated above that the drainage of the Appalachian valley was southwestward, from the New-Kanawha basin to the sea margin, until the close of the Tertiary baseleveling period. Since the date of diversion of this drainage is an extremely im portant point in the history of the region and since the above statement is liable to be questioned, the grounds on which it is based will be given in some detail. The evidence is derived from (1) the perfectly baseleveled divide between the Tennessee and Coosa river basins; (2) a comparison of the volume of ma terial eroded from the. Appalachian valley with that of the Tertiary sediments in central Alabama; and (3) the immaturity of the Tennessee gorge through the plateau below Chattanooga. Evidence from the Coosa-Tennessee Divide.-As already stated, a peneplain, extending from the Cumberland plateau on the north west to the Great Smoky mountains on the southeast, stretches from the head of the Holston and Clinch rivers to the edge of the Tertiary sediments in central Alabama. This peneplain is well shown in the photograph of the relief map of this region reproduced as plate 4. It is as perfectly developed across the Coosa-Tennessee divide as elsewhere, and shows no perceptible variation in the two basins except the gradual southward de scent shown in plate 6 and due to subsequent differential eleva tion. It extends across the Appalachian valley from Pigeon mountain to the base of the Cohutta mountains, a distance of 40 miles, interrupted only by the valley ridges of hard sand stone or by low knobs of silicious Knox dolomite. Since the peneplain is developed only on soft rocks, it is possible that the divides might have been cut down to their present altitudes by backward erosion of headwaters while the streams occupied their present courses; but while the altitude of the divides is not conclusive evidence that the main streams have flowed across them, the breadth of the valley upon the divide materially strengthens the evidence. By the backward cutting of streams at their headwaters a characteristic dendritic, inosculating drain age is developed, and it seems improbable that the divides should have been maintained in their present position throughout the Tertiary cycle without producing this characteristic surface, which is conspicuously absent. 109
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