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.

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.

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).

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.

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).

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).

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.

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.

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.

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.

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).

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.

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).

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.

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.

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.

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.

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).

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).

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.

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