Fig. 16.—Section of anticlinal valleys and synclinal mountains.
This reversion of what would have been ridges and troughs had there been no erosion, is illustrated by the following cross-section through Lookout Mountain in Alabama, which is an example of what is known as a synclinal mountain. Many such synclinal mountains or plateaus, separated by narrower anticlinal valleys, occur throughout the Appalachians.
The characteristics in the present topography of the Appalachians just considered are but a minor portion of the great changes that have resulted from erosion. The history of the system has not been the same in this connection throughout, but retains evidences of successive upward movements with long periods of erosion intervening which have produced certain striking differences in its northern and southern portions. These differences are so well marked that it is convenient to divide the system into two portions, termed the northern Appalachians and southern Appalachians. The most conspicuous difference between the two is shown by the direction of flow of the larger rivers. At the north, the principal rivers—the Delaware, Susquehanna, Potomac, and James—rise well to the west of the mountains and flow southeast athwart the numerous folds, and after crossing the Piedmont plateau and coastal plain discharge into the Atlantic. At the south, however, the rivers, particularly New River and the Tennessee, rise on the eastern border of the Appalachians and flow westward, cutting through the Alleghany plateau, and are tributary to the Mississippi and the Gulf of Mexico. The somewhat arbitrary dividing line between these two
provinces follows the divide to the north of New River, or in a general way, as has been stated by C. W. Hayes, is marked by a line drawn from the most easterly point of Kentucky southeastward to Cape Fear, on the Atlantic coast.
The fact that several large rivers rising to the northwest of the northern Appalachians flow directly across or through the numerous ridges composing the system in deep, narrow valleys, and the similar behaviour of the streams rising on the eastern border of the southern Appalachians, but flowing westward, are among the most interesting features of the entire region. Why is it that the mountains have not formed a divide or water-parting so as to force all of the streams having their sources on its west side to take what would seem the easier course, and to flow to the Gulf of Mexico, and cause the waters falling on its eastern slopes to flow to the Atlantic? The answer to this apparently puzzling question has been furnished by Davis, Willis, Hayes, Campbell, and others, who have shown that the mountains were not raised all at once, but experienced upward movements at widely separated intervals, with intervening periods of rest during which the elevations previously produced were more or less completely planed away by erosion. During one of these intervals the north Appalachians more especially were worn down to approximately sea-level and a gently sloping plain produced across which the larger rivers flowed to the Atlantic. This peneplain was later upraised into a plateau and its downward inclination towards the east increased. The streams were thus given greater energy and began again to deepen their channels. They held their right of way acquired on the featureless erosion plain and cut deep trenches through the edges of the hard layers which crossed their courses. At the same time lateral branches were developed which followed the outcrops of the less resistant beds and eroded them away so as to leave the hard beds in bold relief. As the edges of the more resistant beds became more and more prominent the eastward-flowing streams cut deeper and deeper into them. The even summits
of the ridges, one of the most striking features in the beautiful scenery of the Appalachians, still mark the position of the elevated erosion plain.
In the southern Appalachian the old erosion plain formed nearly at sea-level was tilted gently westward, and the streams flowing over its surface given initial courses in that direction, which were maintained as they deepened their channels, and on account of increased energy originating from the upraising of the region drained by them, developed lateral branches, as is the case of the more northern streams just referred to, and the process of carving away the land to sea-level was again renewed.
Portions of the original upland or mountain mass left unconsumed during the long period of planation, which reduced most of the region nearly to sea-level, still remain in eastern Tennessee, western North Carolina, and northern Georgia, and form the highest and most picturesque portion of the Appalachians.
After the upraised peneplain from which the long, even-crested ridges of the Appalachians were produced by the excavation of the bordering valleys had been deeply dissected and the valleys broadened, another upward movement took place and the streams again deepened their valleys. This is the stage in which we now find the mountains. The crests of the ridges, characteristically displayed in eastern Pennsylvania, are portions of the first peneplain of which a definite record is preserved, while the broad valleys with sharply cut channels in their bottoms represent the much less complete second stage of planation.
The two ancient peneplains referred to above, the histories of which are recorded in the topography, have received definite names in order that they may be readily designated. The older and higher one is termed the Schooley peneplain[2] on account of the preservation of a typical portion at Schooley Mountain in New Jersey, while the lower one, represented by the broad valley through