The lesser mountains which stand along the northern border of the great limestone Alps, those known as the Prealpes, present the strange characteristic of resting upon materials younger than themselves. Such mountains as the remarkable-looking Mythen, near Schwyz, for instance, are weathered from masses of Triassic and Jurassic rock, and repose on the much more recent Flysch. In sharp contrast to the Flysch scenery, they stand as abrupt and gigantic erratics, which have been transported from the central zone of the Alps lying far to the south. They are strangers petrologically,
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stratigraphically, and geographically,[1] to the locality in which they now occur. The exotic materials may be dolomites, limestones, schists, sandstones, or rocks of igneous origin. They show in every case traces of the severe dynamic actions to which they have been subjected in transit. The igneous, like the sedimentary, klippen, can be traced to distant sources; to the massif of Belladonne, to Mont Blanc, Lugano, and the Tyrol. The Prealpes are, in fact, mountains without local roots.
In this last-named essential feature, the Prealpes do not differ from the still greater limestone Alps which succeed them to the south. These giants, e.g. the Jungfrau, Wetterhorn, Eiger, etc., are also without local foundations. They have been formed from the overthrown and drawn-out anticlines of great crust-folds, whose synclines or roots are traceable to the south side of the Rhone Valley. The Bernese Oberland originated in the piling-up of four great sheets or recumbent folds, one of which is continued into the Prealpes. With Lugeon[2] we may see in the phenomenon of the formation of the Prealpes a detail; regarding it as a normal expression of that mechanism which has created the Swiss Alps. For these limestone masses of the Oberland are not indications of a merely local shift of the sedimentary covering of the Alps. Almost the whole covering has
[1] De Lapparent, Traite de Geologie, p. 1,785.
[2] Lugeon, Bulletin Soc. Geol. de France, 1901, p. 772.
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been pushed over and piled up to the north. Lugeon[l] concludes that, before denudation had done its work and cut off the Prealpes from their roots, there would have been found sheets, to the number of eight, superimposed and extending between the Mont Blanc massif and the massif of the Finsteraarhorn: these sheets being the overthrown folds of the wrinkled sedimentary covering. The general nature of the alpine structure
{Fig. 8}
will be understood from the presentation of it diagrammatically after Schmidt of Basel (Fig. 8).[2] The section extends from north to south, and brings out the relations of the several recumbent folds. We must imagine almost the whole of these superimposed folds now removed from the central regions of the Alps by denudation,
[1] Lugeon, loc. cit.