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DISSECTED PENEPLAINS. When a rise of the land brings one cycle to an end and begins another, the characteristic land forms of each cycle are found together and the topography of the region is composite until the second cycle is so far advanced that the land forms of the first cycle are entirely destroyed.
Revived by the uplift, the streams of the area trenched it as deeply as its elevation permitted, and reaching grade, opened up wide valleys and new peneplains in the softer rocks. The Connecticut valley is Tertiary in age, and in the weak Triassic sandstones has been widened in places to fifteen miles.
For the sake of clearness we have described oscillations, foldings, and fractures of the crust as separate processes, each giving rise to its own peculiar surface features, but in nature earth movements are by no means so simple, they are often implicated with one another: folds pass into faults; in a deformed region certain rocks have bent, while others under the same strain, but under different conditions of plasticity and load, have broken; folded mountains have been worn to their roots, and the peneplains to which they have been denuded have been upwarped to mountain height and afterwards dissected, as in the case of the Alleghany ridges, the southern Carpathians, and other ranges, or, as in the case of the Sierra Nevada Mountains, have been broken and uplifted as mountains of fracture.
Thus many peneplains which have been elevated and dissected have been recognized by the remnants of their ancient erosion surfaces, and the length of time which has elapsed since their uplift has been measured by the stage to which the new cycle has advanced.
Where peneplains have been uplifted to great height and have since been well dissected, and where they have been upfolded and broken and uptilted, their recognition becomes more difficult. THE SOUTHERN APPALACHIAN REGION. We have here an example of an area the latter part of whose geological history may be deciphered by means of its land forms.
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