Earth Features and Their Meaning / An Introduction to Geology for the Student and the General Reader

by William Herbert Hobbs

Fig. 481.—Ideal sections of laccolite and bysmalite.

When subjected to long-continued erosion, the generally fissured granitic core of the laccolite weathers in a wholly different manner from the bedded sediments which surround and still in part mount over it. The former usually presents a more or less jagged surface which contrasts sharply with the gently sloping tables of the latter ([Fig. 479]). About the high granite core of the mountain, the several strata of the uptilted formations present each a steep slope toward this higher land, and a gentler slope in the opposite direction. Such unsymmetrical ridges which surround the mountain area are often referred to as “hog backs” ([plate 12 B]). The arrangement of the strata in the hog backs thus presents an overlapping series like the shingles upon a roof, except that the overlapping is here from the bottom instead of the top, and the exposed ends thus face toward the crest. Unlike a shingle roof the hog backs do not shed the water which descends to them from the higher levels, but, on the contrary, they cause it to flow in troughs parallel to the base of the slope except where outlets are found through them.

Mountains carved from plateaus.—In the mountain types thus far discussed, the local uplifting of the land has itself developed features which in the aggregate may be referred to as mountains, even though the characters of the original surface are soon destroyed by erosive processes of one sort or the other. Erosive processes are, however, quite competent to produce mountain forms from a featureless plateau, and particularly through the incision by streams of running water, the best studied process of mountain sculpture (see [Chapters XI-XIII]). This process of throwing valleys about an elevated section of the earth’s surface, and so carving out mountains, is sometimes described as circumvallation; and if the term “mountain” be applied in its ordinary sense to describe an individual feature, it is clear that most mountains have been formed in this way.

To discuss the characteristic shapes of such mountains would be largely to review the contents of this book, and especially those portions which discuss the character profiles resulting from the action of each sculpturing or molding agent. The work of frost and other weathering agencies, of running water, of mountain and of continental glacier, would all have to be considered in order to evolve the history of each mountain.

In addition to discovering the agents which were chiefly responsible for the shaping of the mountain, we may, further, in many cases determine at what stage the work of one agent has been succeeded by that of another, and at least at what stage of its complete cycle of activity the latest agent is now at work.

Fig. 482.—The gabled façade so largely developed in desert landscapes and sharply contrasted with the recurring curves in the landscapes of humid districts (from a painting of the Grand Cañon of the Colorado by Moran).

The climatic conditions of the mountain sculpture.—Since the different geological agencies operate either in a different manner or with differences in vigor according to the varying climatic conditions, the mountains of arid regions may in most cases be readily differentiated from those of the more habitable humid sections of country. In broad lines these differences may be summed up in the greater prevalence of the curving line within the landscapes of humid districts. This may be largely ascribed to the influence of the mat of vegetation, which protects the rock surface from more rapid mechanical degeneration, and arrests the sliding movements within the already loosened rock débris. In place of the reversed curves of the lines of beauty, so generally observed in the landscapes of well-watered regions, the desert lands present ever a repetition of the vertical cliff alternating with a sort of many gabled façade which is occasionally due to truncation of mountain spurs by the waves of former lakes, but far more often the outlines of débris cones built up beneath each prominent joint of the cliff walls ([Fig. 482]).

The effect of the resistant stratum.—In a striking manner mountain landscapes may disclose the influence of the diversified rock materials and of the rock structures as well. After prolonged erosion there is likely to be little correspondence between the positions of the anticlinal folds and the crests of the higher mountains. Such mountains are, in fact, much more likely to rise over synclines than upon the site of anticlines. The traveler who enters the Alps by any of the several railways, or who journeys by steamer over the beautiful lake of Lucerne, has a most favorable opportunity to study the position of the rock folds in the mountain sections that are unrolled in succession before him. Rarely indeed will he find a definite anticline in correspondence with a mountain peak, for the layers which are most resistant have developed the peaks, and it is because the outer layers of the anticlines open by local tension (see [Fig. 26], [p. 45]) that they were first cut away by erosion, so that the hard layers within the synclines are likely to constitute the peaks within the existing surface.