THE FORMS CARVED AND MOLDED BY WAVES
=The motion of a water wave.=—The motions within a wave upon the
surface of a body of water may be thought of in two different ways.
First of all, there is the motion of each particle of water within
an orbit of its own; and there is, further, the forward motion of
propagation of the wave considered as a whole.
[Illustration: FIG. 247.—Diagram to show the nature of the motions
within a free water wave.]
The water particle in a wave has a continued motion round and round its
orbit like that of a horse circling a race course, only that here the
track is in a vertical plane, directed along the line of propagation of
the wave (Fig. 247). Each particle of water, through its friction upon
neighboring particles, is able to transmit its motion both along the
surface and downward into the water below. The force which starts the
water in motion and develops the wave, is the friction of wind blowing
over the water surface, and the size of the orbit of the water particle
at any point is proportional to the wind’s force and to the stretch
of water over which it has blown. The wind’s effect is, therefore,
cumulative—the wave is proportional to the wind’s effect upon all
water particles in its rear, added to the local wind friction.
The size or _height_ of the wave is measured by the diameter of the
orbit of motion of the surface particle, and this is the difference in
height between trough and crest. The distance from crest to crest, or
from trough to trough, is called the _wave length_. Though the wave
motion is transmitted downward into the water there is a continued
loss of energy which is here not compensated by added wind friction,
and so the orbital motion grows smaller and smaller, until at the
depth of about a wave length it has completely died out. This level
of no motion is called the _wave base_. In quiet weather the level of
no motion is practically at the water’s surface, and inasmuch as the
geological work of waves is in large part accomplished during the great
storms, the term “wave base” refers to the lowest level of wave motion
at the time of the heaviest storms. Upon the ocean the highest waves
that have been measured have an amplitude of about fifty feet and a
wave length of about six hundred feet.
=Free waves and breakers.=—So long as the depth of the water is below
wave base, there is obviously no possibility of interference with the
wave through friction upon the bottom. Under these conditions waves are
described as _free waves_, and their forms are symmetrical except in so
far as their crests are pulled over and more or less dissipated in the
spray of the “white caps” at the time of high winds.
[Illustration: FIG. 248.—Diagram to illustrate the transformation of a
free wave into a breaker as it approaches the shore.]
As a wave approaches a shore, which generally has a gentle outward
sloping surface, there is interposed in the way of a free forward
movement the friction upon the bottom. This friction begins when the
depth of water is less than wave base, and its effect is to hold back
the wave at the bottom. Carried slowly upward in the water by the
friction of particle upon particle, the effect of this holding back
is a piling up of the water, which increases the wave height as it
diminishes the wave length, and also interferes with wave symmetry
(Fig. 248). Moving forward at the top under its inertia of motion and
held back at the bottom by constantly increasing friction, a strong
turning motion or couple is started about a horizontal axis, the
immediate effect of which is to steepen the forward slope of the wave,
and this continues until it overhangs, and, falling, “breaks” into
surf. Such a breaking wave is called a “comber” or “breaker” (plate 11
B).
┌─────────────────────────────────────────────────────────────────┐
│ PLATE 11. │
│ │
│ [Illustration: _A._ Ripple markings within an ancient sandstone │
│ (courtesy of U. S. Grant).] │
│ │
│ [Illustration: _B._ A wave breaking as it approaches the shore. │
│ (_Photograph by Fairbanks._)] │
└─────────────────────────────────────────────────────────────────┘
[Illustration:
FIG. 249.—Notched rock cliff cut by waves and the fallen blocks
derived from the cliff through undermining. Profile Rock at Farwell’s
Point near Madison, Wisconsin.]
=Effect of the breaking wave upon a steep rocky shore—the notched
cliff.=—If the shore rises abruptly from deeper water, the top of
the breaking wave is hurled against the cliff with the force of a
battering ram. During storms the water of shore waves is charged with
sand, and each sand particle is driven like a stone cutter’s tool
under the stroke of his hammer. The effect is thus both to chip and
to batter away the rock of the shore to the height reached by the
wave, undermining it and notching the rock at its base (Fig. 249).
When the notch has been cut in this manner to a sufficient depth, the
overhanging rock falls by its own weight in blocks which are bounded
by the ever present joints, leaving the upper cliff face essentially
vertical.
[Illustration:
FIG. 250.—A wave-cut chasm under control by joints, coast of Maine
(after Tarr).]
=Coves, sea arches, and stacks.=—It is the headland which is most
exposed to the work of the waves, since with change of wind direction
it is exposed upon more than a single face. The study of headlands
which have been cut by waves shows that the joints within the rock
play a large rôle in the shaping of shore features. The attack of the
waves under the direction of these planes of ready separation opens
out indentations of the shore (Fig. 250) or forms _sea caves_ which, as
they extend to the top of the cliff by the process of sapping, yield
the _coves_ which are so common a feature upon our rock-bound shores
(Fig. 259, p. 238). With continuation of this process, the caves formed
on opposite sides of the headland may be united to form a _sea arch_
(Fig. 251).
[Illustration:
FIG. 251.—The sea arch known as the Grand Arch upon one of the
Apostle Islands in Lake Superior (after a photograph by the Detroit
Photographic Company).]
A later stage in this selective wave carving under the control of
joints is reached when the bridge above the arch has fallen in, leaving
a detached rock island with precipitous walls. Such an offshore island
of rock with precipitous sides is known as a _stack_ (Fig. 252), or
sometimes as a “chimney”, though this latter term is best restricted to
other and similar forms which are the product of selective weathering
(p. 300).
[Illustration: FIG. 252.—Stack near the shore of Lake Superior.]
Whenever the rock is less firmly consolidated, and so does not stand
upon such steep planes, the stack is apt to have a more conical form,
and may not be preceded in its formation by the development of the
sea arch (Fig. 260, p. 239). In the reverse case, or where the rock
possesses an unusual tenacity, the stack may be largely undermined
and stand supported like a table upon thick legs or pillars of rock
(Fig. 253). In Fig. 254 is seen a group of stacks upon the coast of
California, which show with clearness the control of the joints in
their formation, but unlike the marble of the South American example
the forms are not rounded, but retain their sharp angles.
[Illustration:
FIG. 253.—The Marble Islands, stacks in Lake Buenos Aires, southern
Andes (after F. P. Moreno).]
=The cut rock terrace.=—When waves first begin their attack upon a
steep, rocky shore, the lower limit of the action is near the wave
base. The action at this depth is, however, less efficient, and as the
recession of the cliff is one of the most rapid of erosional processes,
the rock floor outside the receding cliff comes to slope gradually
downward from the cliff to a maximum depth at the edge of the terrace,
approximately equal to wave base (Fig. 255). This cut terrace is
extended seaward or lakeward, as the case may be, in a _built terrace_
constructed from a portion of the rock débris acquired from the cliff.
[Illustration:
FIG. 254.—Squared stacks which reveal the position of the joint planes
which have controlled in the process of carving by the waves. Pt.
Buchon, California (after a photograph by Fairbanks).]
[Illustration:
FIG. 255.—Ideal section of a steep rocky shore carved by waves into a
notched cliff and cut terrace, and extended by a built terrace.]
The broken wave, after rising upon the terrace under the inertia of
its motion until all its energy has been dissipated, slides outward
by gravity, and though checked and overridden by succeeding breakers,
it continues its outward slide as the “undertow” until it reaches the
end of the terrace. Here it suddenly enters deep water, and losing its
velocity, drops its burden of rock, and builds the terrace seaward
after the manner of construction of an embankment. As we are to see,
the larger portion of the wave-quarried material is diverted to a
different quarter.
[Illustration:
FIG. 256.—Map showing the outlines of the Island of Heligoland at
different stages in its recent history. The peripheries given are in
miles.]
To gain some conception of the importance of wave cutting as an eroding
process, we may consider the late history of Heligoland, a sandstone
island off the mouth of the Elbe in the North Sea (Fig. 256). From a
periphery of 120 miles, which it possessed in the ninth century of the
Christian era, the island has reduced its outline to 45 miles in the
fourteenth century, 8 miles in the seventeenth, and to about 3 miles at
the beginning of the twentieth century. The German government, which
recently acquired this little remnant from England, has expended large
sums of money in an effort to save this last relic.
[Illustration:
FIG. 257.—Cut and built terrace with bowlder pavement shaped by waves
on a steep shore formed of loose materials.]
=The cut and built terrace on a steep shore of loose materials.=—In
materials which lack the coherence of firm rock, no vertical cliff can
form; for as fast as undermined by the waves the loose materials slide
down and assume a surface of practically constant slope—the “angle of
repose” of the materials (Fig. 257). The terrace below this sloping
cliff will not differ in shape from that cut upon a rocky shore; but
whenever the materials of the shore include disseminated blocks too
large for the waves to handle, they collect upon the terrace near where
they have been exhumed, thus forming what has been called a “bowlder
pavement” (Fig. 258).
[Illustration:
FIG. 258.—Sloping cliff and terrace with bowlder pavement exposed at
low tide upon the shore at Scituate, Massachusetts.]
The edge of the cut and built terrace is, as already mentioned,
maintained at the depth of wave base. If one will study the submerged
contours of any of our inland lakes, it will be found that these
basins are surrounded by a gently sloping marginal shelf,—the cut and
built terrace,—and that the depth of this shelf at its outer edge is
proportioned to the size of the lake. Upon Lake Mendota at Madison,
Wisconsin, the large storm waves have a length of about twenty feet,
which is the depth of the outer edge of the shore terraces (Fig. 267,
p. 242). The shelf surrounding the continents has, with few local
exceptions, a uniform depth of 100 fathoms, or about the wave base of
the heaviest storm waves.
=The work of the shore current.=—In describing the formation of
the built terrace, it was stated that the greater part of the rock
material quarried upon headlands by the waves is diverted from the
offshore terrace. This diversion is the work of the shore current
produced by the wave.
[Illustration: FIG. 259.—Map to show the nature of the shore current
and the forms which are molded by it.]
At but few places upon a shore will the storm waves beat
perpendicularly, and then for but short periods only. The broken wave,
as it crawls ever more slowly up the beach, carries the sand with it in
a sweeping curve, and by the time gravity has put a stop to its forward
movement, it is directed for a brief instant parallel to the shore.
Soon, however, the pull of gravity upon it has started the backward
journey in a more direct course down the slope of the terrace; and
here encountering the next succeeding breaker, a portion of the water
and the coarser sand particles with it are again carried forward for a
repetition of the zigzag journey. This many times interrupted movement
of the sand particles may be observed during a high wind upon any sandy
lee shore. The “set” of the water along the shore as a result of its
zigzag journeyings is described as the _shore current_ (Fig. 259),
and the effect upon sand distribution is the same as though a steady
current moved parallel to the shore in the direction of the average
trend of the moving particles.
=The sand beach.=—The first effect of the shore current is to deposit
some portion of the sand within the first slight recess upon the shore
in the lee of the cliff. The earlier deposits near the cliff gradually
force the shore current farther from the shore and so lay down a sand
selvage to the shore, which is shaped in the form of an arc or crescent
and known as a _beach_ (Fig. 259 and Fig. 260).
[Illustration: FIG. 260.—Crescent-shaped beach formed in the lee of
a headland. Santa Catalina Island, California (after a photograph by
Fairbanks).]
[Illustration: FIG. 261.—Cross section of a beach pebble.]
=The shingle beach.=—With heavy storms and an exceptional reach of the
waves, the shore currents are competent to move, not the sand alone,
but pebbles, the area of whose broader surface may be as great as the
palm of one’s hand. Such rock fragments are shaped by the continued
wear against their neighbors under the restless breakers, until they
have a lenticular or watch-shaped form (Fig. 261). Such beach pebbles
are described as _shingle_, and they are usually built up into distinct
ridges upon the shore, which, under the fury of the high breakers, may
be piled several feet above the level of quiet water (Fig. 262). Such
storm beaches have a gentle forward slope graded by the shore current,
but a steep backward slope on the angle of repose. Most storm beaches
have been largely shaped by the last great storm, such as comes only at
intervals of a number of years.
[Illustration:
FIG. 262.—Storm beach of coarse shingle about four feet in height
at the base of Burnt Bluff on the northeast shore of Green Bay, Lake
Michigan.]
=Bar, spit, and barrier.=—Wherever the shore upon which a beach is
building makes a sudden landward turn at the entrance to a bay, the
shore currents, by virtue of their inertia of motion, are unable longer
to follow the shore. The débris which they carry is thus transported
into deeper water in a direction corresponding to a continuation of
the shore just before the point of turning (see Fig. 259, p. 238).
The result is the formation of a _bar_, which rises to near the
water surface and is extended across the entrance to the bay through
continued growth at its end, after the manner of constructing a railway
embankment across a valley.
[Illustration: FIG. 263.—Spit of shingle on Au Train Island, Lake
Superior (after Gilbert).]
Over the deeper water near the bar the waves are at first not generally
halted and broken, as they are upon the shore, and so the bar does not
at once build itself to the surface, but remains an invisible bar to
navigation. From its shoreward end, however, the waves of even moderate
storms are broken, and the bar is there built above the water surface,
where it appears as a narrow cape of sand or shingle which gradually
thins in approaching its apex. This feature is the well-known _spit_
(Fig. 263) which, as it grows across the entrance to the bay, becomes a
_barrier_ or _barrier beach_ (Fig. 264).
The continuation of the visible in the usually invisible bar, is at the
time of high winds made strikingly apparent, for the wave base is below
the crest of the bar, and at such times its crescentic course beyond
the spit can be followed by the eye in a white arc of broken water.
[Illustration:
FIG. 264.—Barrier beach in front of a lagoon on Lake Mendota at
Madison, Wisconsin. The shallow lagoon behind the barrier is filling up
and is largely hidden in vegetation.]
The construction of a barrier across the entrance to a bay transforms
the latter into a separate body of water, a lagoon, within which
silting up and peat formation usually lead to an early extinction
(p. 429). The formation of barriers thus tends to straighten out
the irregularities of coast lines, and opens the way to a natural
enlargement of the land areas. While the coasts of the United Kingdom
of Great Britain have been losing some four thousand acres through wave
erosion, there has been a gain by growth in quiet lagoons which amounts
to nearly seven times that amount. As evidence of the straightening
of the shore line which results from this process, the coast of the
Carolinas or of Nantucket (Fig. 459) may serve for illustration.
=The land-tied island.=—We have seen that wave erosion operates to
separate small islands from the headlands, but the shore currents
counteract this loss to the continents by throwing out barriers which
join many separated islands to the mainland. Such land-tied islands are
a common feature on many rocky coasts, and upon the New England coast
they usually have been given the name of “neck.” The long arc of Lynn
Beach joins the former island of Nahant, through its smaller neighbor
Little Nahant, to the coast of Massachusetts. A similar land-tied
island is Marblehead Neck. The Rock of Gibraltar, formerly an island,
is now joined to Spain by the low beach known as the “neutral ground.”
The Spanish name, _tombola_, has sometimes been employed to describe an
island thus connected to the shore.
[Illustration: FIG. 265.—Cross section of a barrier beach with lagoon
in its rear.]
=A barrier series.=—The cross section of a barrier beach, like that of
a storm beach upon the shore, slopes gently upon the forward side, and
more steeply at the angle, of repose upon the rear or landward margin
(Fig. 265). The thinning wedge of shore deposits which the barrier
throws out to seaward raises the level of the lake bottom (Fig. 266),
and when coast irregularities are favorable to it, new spits will
develop upon the shore outside the earlier one, and a new bar, and in
its turn a barrier, will be found outside the initial one, taking a
course in a direction more or less parallel to it (Fig. 267).
[Illustration: FIG. 266.—Cross section of a series of barriers and an
outer bar.]
[Illustration:
FIG. 267.—Formation of barrier series and an outer bar in University
Bay of Lake Mendota, at Madison, Wisconsin. The water contour interval
is five feet, and the land contour interval ten feet (based on a map by
the Wisconsin Geological Survey).]
[Illustration: FIG. 268.—Series of barriers at the western end of Lake
Superior (after Gilbert).]
So soon as the first barrier is formed, processes are set in operation
which tend to transform the newly formed lagoon into land, and so with
a series of barriers, a zone of water lilies between the outer barrier
and the bar, a bog, and a land platform may represent the successive
stages in this acquisition of territory by the lands. A noteworthy
example of barrier series and extension of the land behind them, is
afforded by the bay at the western end of Lake Superior (Fig. 268).
[Illustration: FIG. 269.—Character profiles resulting from wave action
upon shores.]
=Character profiles.=—The character profiles yielded by the work of
waves are easy of recognition (Fig. 269). The vertical cliff with notch
at its base is varied by the stack of sugar-loaf form carved in softer
rocks, or the steeper notched variety cut from harder masses. Sea caves
and sea arches yield variations of a curve common to the undercut
forms. Wherever the materials of the shore are loosely consolidated
only, the sloping cliff is formed at the angle of repose of the
materials. The barrier beach, though projecting but a short distance
above the waves, shows an unsymmetrical curve of cross section with the
steeper slope toward the land.
READING REFERENCES FOR CHAPTER XVIII
G. K. GILBERT. The Topographic Features of Lake Shores, 5th Ann. Rept.
U. S. Geol. Surv., 1885, pp. 69-123, pls. 3-20; Lake Bonneville, Mon.
I, U. S. Geol. Surv., 1890, Chapters ii-iv, pp. 23-187.
VAUGHAN CORNISH. On Sea Beaches and Sand Banks, Geogr. Jour., vol. 11,
1898, pp. 528-543, 628-658.
F. P. GULLIVER. Shore Line Topography, Proc. Am. Acad. Arts and Sci.,
vol. 34, 1899, pp. 149-258.
N. S. SHALER. The Geological History of Harbors, 13th Ann. Rept. U. S.
Geol. Surv., 1893, pp. 93-209.
SIR A. GEIKIE. The Scenery of Scotland, 1901, pp. 46-89.
W. H. WHEELER. The Sea Coast. Longmans, London, 1902, pp. 1-78.
G. W. VON ZAHN. Die zerstörende Arbeit des Meeres an Steilküsten nach
Beobachtungen in der Bretagne und Normandie in den Jahren 1907 und
1908, Mitt. d. Geogr. Ges. Hamb., vol. 24, 1910, pp. 193-284, pls.
12-27.