REPEATING PATTERNS IN THE EARTH RELIEF

The weathering processes under control of the fracture system.—In an earlier chapter it was learned that the rocks which compose the earth’s surface shell are intersected by a system of joint fractures which in little-disturbed areas divide the surface beds into nearly square perpendicular prisms (Fig. 36, p. 55), more or less modified by additional diagonal joints, and often also by more disorderly fractures. Throughout large areas these fractures may maintain nearly constant directions, though either one or more of the master series may be locally absent. This distinctive architecture of the surface shell of the lithosphere has exercised its influence upon the various weathering processes, as it has also upon the activities of running water and of other less common transporting agencies at the surface.

Within high latitudes, where frost action is the dominant weathering process, the water, by insinuating itself along the joints and through repeated freezings, has broken down the rock in the immediate neighborhood of these fractures, and so has impressed upon the surface an image of the underlying pattern of structure lines (plate 10 A).

In much lower latitudes and in regions of insufficient rainfall, the same structures are impressed upon the relief, but by other weathering processes. In the case of the less coherent deposits in these provinces, the initial forms of their erosional surface have sometimes been determined by the dash of rain from the sudden cloudburst. Thus the “bad lands” may have their initial gullies directed and spaced in conformity with the underlying joint structures (Fig. 238).

In such portions of the temperate regions as are favored by a humid climate, the mat of vegetation holds down a layer of soil, and mat and soil in coÖperation are effective in preventing any such large measure of frostwork as is characteristic of the subpolar regions or of high levels in the arid lands. In humid regions the rocks become a prey especially to the processes of solution and accompanying chemical decomposition, and these processes, although guided by the course of the percolating ground water along the fracture planes, do not afford such striking examples of the control of surface relief.

Those limestones which slowly pass into solution in the percolating water do, however, quite generally indicate a localization of the solution along the joint channels (Fig. 239 and plate 6 B). Though in other rocks not so apparent, yet solutions generally take their courses along the same channels, and upon them is localized the development of the newly formed hydrated and carbonate minerals, as is well illustrated by the phenomenon of spheroidal weathering (Fig. 155, p. 150).

The fracture control of the drainage lines.—The etching out of the earth’s architectural plan in the surface relief, which we have seen begun in the processes of weathering, is continued after the transporting agents have become effective. It is often easy to see that a river has taken its course in rectangular zigzags like the elbows of a jointed stove pipe, and that its walls are formed of joint planes from which an occasional squared buttress projects into the channel. This structure is rendered in the plan of the Abisko CaÑon of northern Lapland (Fig. 240). We are later to learn that another great transporting agent, the water wave, makes a selective attack upon the lithosphere along the fractures of the joint system (Fig. 250, p. 233 and Fig. 254, p. 235).

Where the scale of the example is large, as in the cases which have been above cited, the actual position and directions of the joint wall are easily compared with the near-by elements of the river’s course, so that the connection of the drainage lines with the underlying structure is at once apparent. In many examples where the scale is small, the evidence for the controlling influence of the rock structure in determining the courses of streams may be found in the peculiar character of the drainage plan. To illustrate: the course of the Zambesi River, within the gorge below the famous Victoria Falls, not only makes repeated turnings at a right angle, but its tributary streams, instead of making the usual sharp angle where they join the main stream, also affect the right angle in their junctions (Fig. 241).

The repeating pattern in drainage networks.—It is a characteristic of the joint system that the fractures within each series are spaced with approximation to uniformity. If the plan of a drainage system has been regulated in conformity with the architecture of the underlying rock basement, the same repeating rectangles of the master joints may be expected to appear in the lines of drainage—the so-called drainage network.

Such rectangular patterns do very generally appear in the drainage network, though they are often masked upon modern maps by what, to the geologist, seems impertinent intrusion of the black lines of overprinting which indicate railways, lines of highway, and other culture elements. On river maps, which are printed without culture, the pattern is much more easily recognized (Figs. 242 and 243). Wherever the relief is strong, as is the case in the Adirondack Mountain province of the State of New York, individual hills may stand in relief between the bounding streams which compose the rectangular network, like the squared pedestals of monuments. Such a type of relief carved in repeating patterns has been described as “checkerboard topography.”

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The dividing lines of the relief patterns—lineaments.—The repeating design outlined in the river network of the Temiskaming district (Fig. 243) would appear in greater perfection if we could reproduce the relief without at the same time obscuring the lines of drainage; for where the pattern is not completely closed by the course of the stream, there is generally found either a dry valley or a ravine to complete the design. If these are not present, a bit of straight coast line, a visible line of fracture, a zone of fault breccia, or the boundary line separating different formations may one or more of them fill in the gaps of the parallel straight drainage lines which by their intersection bring out the pattern. These significant lines of landscapes which reveal the hidden architecture of the rock basement are described as lineaments (Fig. 82, p. 87). They are the character lines of the earth’s physiognomy.

It is important to emphasize the essentially composite expression of the lineament. At one locality it appears as a drainage line, a little farther on it may be a line of coast; then, again, it is a series of aligned waterfalls, a visible fault trace, or a rectilinear boundary between formations; but in every case it is some surface expression of a buried fracture. Hidden as they so generally are, the fracture lines must be searched out by every means at our disposal, if we are not to be misled in accounting for the positions and the attitudes of disturbed rock masses.

As we have learned, during earthquake shocks, as at no other time, the surface of the earth is so sensitized as to betray the position of its buried fractures. As the boundaries of orographic blocks, certain of the fractures are at such times the seats of especially heavy vibrations; they are the seismotectonic lines of the earthquake province. Many lineaments are identical with seismotectonic lines, and they therefore afford a means of to some extent determining in advance the lines of greatest danger from earthquake shock.

The composite repeating patterns of the higher orders.—Not only do the larger joint blocks become impressed upon the earth relief as repeating diaper patterns, but larger and still larger composite units of the same type may, in favorable districts, be found to present the same characters. Attention has already been more than once directed to the fact that the more perfect and prominent fracture planes recur among the joints of any series at more or less regular intervals (Fig. 40, p. 57, and Fig. 41, p. 58). Nowhere, perhaps, is this larger order of the repeating pattern more perfectly exemplified than in some recent deposits in the Syrian desert (plate 10 B). It is usually, however, in the older sediments that such structures may be recognized; as, for example, in the squared towers and buttresses of the Tyrolean Dolomites (Fig. 244). Here the larger blocks appear in the thick bedded lower formation, the dolomite, divided into subordinate sections of large dimensions; but in the overlying formations in blocks of relatively small size, yet with similarly perfect subequal spacing.

The observing traveler who is privileged to make the journey by steamer, threading its course in and out among the many islands and skerries of the Norwegian coast, will hardly fail to be struck by the remarkable profiles of most of the lower islands (Fig. 245). These profiles are generally convexly scalloped with a noteworthy regularity, and not in one unit only, but in at least two with one a multiple of the other (Fig. 246). As the steamer passes near to the islands, it is discovered that the smaller recognizable units in the island profiles are separated by widely gaping joints which do not, however, belong to the unit series, but to a larger composite group (Fig. 246 b). Frostwork, which depends for its action upon open spaces within the rocks, has here been the cause of the excessive weathering above the more widely gaping joints.

High northern latitudes are thus especially favorable for revealing all the details in the architectural pattern of the lithosphere shell, and we need not be surprised that when the modern maps of the Norwegian coast are examined, still larger repeating patterns than any that may be seen in the field are to be made out. The Norwegian coast was long ago shown to be a complexly faulted region, and these larger divisions of the relief pattern, instead of being explained as a consequence solely of selective weathering, must be regarded as due largely to fault displacements of the type represented in our model (plate 4 C). Yet whether due to displacements or to the more numerous joints, all belong to the same composite system of fractures expressed in the relief.

Reading References for Chapter XVII

William H. Hobbs. The River System of Connecticut, Jour. Geol., vol. 9, 1901, pp. 469-485, pl. 1; Lineaments of the Atlantic Border Region, Bull. Geol. Soc. Am., vol. 15, 1904, pp. 483-506, pls. 45-47; The Correlation of Fracture Systems and the Evidences for Planetary Dislocations within the Earth’s Crust, Trans. Wis. Acad. Sci., etc., vol. 15, 1905, pp. 15-29; Repeating Patterns in the Relief and in the Structure of the Land, Bull. Geol. Soc. Am., vol. 22, 1911, pp. 123-176, pls. 7-13.