Fig. 483.—The Mythen, composed of Jurassic and Cretaceous sediments, and resting upon softer Tertiary formations. View from a balloon (after a photograph by C. Schmidt).
When, as sometimes happens, an older and likewise more resistant bed has been folded back upon younger and softer formations, an isolated remnant may be found “unrooted” to its base, upon which it appears as though floating within a billowy sea of the softer formations ([Fig. 483]).
The mark of the rift in the eroded mountains.—Applying the term “mountain” in its collective sense for a circumscribed area of uplifted crust, whether represented to-day by a folded or a faulted complex, a lava mass, or a granite dome; the period of uplift has marked the beginning of the activity of sculpturing agencies. By these the mass is pared down as it is shaped into a more or less intricate design of component and essentially repeating units. In the vernacular the word “mountain” is applied to these units into which the larger mountain mass is subdivided.
Fig. 484.—The battlement type of erosion mountains. Die Drei Zinnen (Three Battlements) in the Dolomites (after Marr).
It has been one of the main objects of this work to point out that the peculiar shapes of these elementary mountains are each characteristic of the erosive agents which produced them, and that each surface has marks which may be recognized in those lines of profile which recur within the landscape—the character profiles. In the subdivision of the larger mass—the genetical mountain—to form the numerous smaller masses—the erosional or circumvallational mountains—there is disclosed a pattern of fractures which has guided the erosional agents in their incisional operations (see Chapter XVII). In high altitudes, where the action of frost is so potent in prying at the wider fractures, this subdivision of the mass may be revealed by the sculpturing of squared towers or battlements ([Fig. 484]).
Fig. 485.—Symmetrically formed low islands repeated in ranks upon Temagami Lake, Ontario.
For other examples in which the sculptured surface is largely the handiwork of a single erosional agent, as over vast areas in the Canadian wilderness, the revelation of the fracture design is no less apparent. Here a series of crystalline rocks underlie broad expanses of territory and are without noteworthy variations of hardness and almost bare of surface débris. Sculptured beneath a mantling ice sheet, excavation has naturally been concentrated above the more widely gaping fissures of the joint-fault system, doubtless already marked out in the river network which the glacier overrode. The result has been a division of the surface into a series of low, oval ridges or hummocks, which over vast areas are repeated with monotonous regularity. Wherever the lower levels have been flooded, symmetrical low islands of nearly uniform elevation rise from the expanse of water and may be counted by thousands. Though the smaller islands have notably regular shore lines, the larger ones disclose their composition from smaller units by the breaking of their shores into similar bays spaced with regular intervals ([Fig. 485], and [Figs. 243] and [245], [p. 229]).