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).
[Illustration:
FIG. 238.—Rain sculpturing under control by joints. Coast of southern
California (after a photograph by Fairbanks).]
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).
[Illustration:
FIG. 239.—Outcrop of flaggy limestone which shows the effects of
solution along neighboring joints in a sagging of the upper beds (after
Gilbert, U. S. G. S.).]
=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).
[Illustration:
FIG. 240.—Map of the joint-controlled Abisko Cañon in northern Lapland
(after Otto Sjögren).]
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).
[Illustration:
FIG. 241.—Map of the gorge of the Zambesi River below the Victoria
Falls (after Lamplugh).]
=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.”
[Illustration:
FIG. 242.—Controlled drainage network of the Shepaug River in
Connecticut.]
[Illustration:
FIG. 243.—A river network of repeating rectangular pattern. Near Lake
Temiskaming, Ontario (from the map by the Dominion Government).]
=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.
[Illustration:
FIG. 244.—Squared mountain masses which reveal a distribution of the
joints in block patterns of different orders of magnitude. The Pordoi
range of the Sella group of the Dolomites, seen from the Cima di Rossi
(after Mojsisovics).]
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.
┌──────────────────────────────────────────────────────────────────────┐
│ PLATE 10. │
│ │
│ [Illustration: _A._ View in Spitzbergen to illustrate the │
│ disintegration of rock under the control of joints. │
│ (_Photograph by O. Haldin._)] │
│ │
│ [Illustration: _B._ Composite pattern of the joint structures within │
│ recent alluvial deposits. │
│ (_Photograph by Ellsworth Huntington._)] │
└──────────────────────────────────────────────────────────────────────┘
[Illustration:
FIG. 245.—Island groups of the Lofoten archipelago off the northwest
coast of Norway, which reveal repeating patterns of the relief in two
orders of magnitude (after a photograph by Knudsen).]
[Illustration:
FIG. 246.—Diagrams to illustrate the composite profiles of the islands
on the Norwegian coast. _a_, distant view; _b_, near view, showing the
individual joints and the more widely gaping fractures beneath each sag
in the profile.]
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.