CHAPTER IV.

ACTION OF THE SEA ON COASTS.-DIFFERENCE IN TIDAL AND TIDELESS SEAS. UNEQUAL ABRASION OF COASTS.-SHINGLE BEACHES.-CHESIL BANK.-COAST SAND-HILLS.

BEFORE We consider the accumulations effected in the sea, it is desirable to call attention to the action of the sea on coasts, since that action often contributes, in no small degree, to the matter of which such deposits are formed.

The sound produced by the grating and grinding of the pebbles of a shingle beach, even when the breakers on shore are comparatively unimportant, can scarcely have escaped the attention of those who have, even for a short time, visited coasts where such beaches, and they are common, are to be found. It will soon be apparent, that this friction, if continued for ages, must not only wear down the pebbles to sand, but grind away and smooth off even the hard rocks exposed to such powerful action. It is, however, when the observer sees the huge masses of rock moved by breakers arising from a heavy gale of wind, blowing on shore from over a wide spread of open sea, or from the long lines of wave known as a ground swell, that he not only learns to value the force of the water taken by itself, thus projected against a coast, but also the additional power it possesses of abrading the cliffs which may be opposed to the breakers by the size and abundance of the shingles they can then hold in mechanical suspension.

Properly to appreciate the power of breakers, a geologist should be present on an exposed ocean coast, such as that of Western Ireland, the Land's End (Cornwall), or among the Western Islands of Scotland, during a heavy and long-continued gale of wind from the westward, and mark the effects of the great Atlantic waves as they break and crash upon the shore. He will generally find in such situations that, though the rocks are scooped and hollowed into the most fantastic forms, they are still hard rocks; for no others could long resist the breakers, which, with little intermission, act

upon them. Not only blocks of rock resting on the shore are driven forward by the repeated blows of such breakers, but those also firmly bolted down on piers are often thrown off and driven aside in far more sheltered situations. The history of many a pier harbour is that of the destructive power of breakers, and those who have witnessed a breach made in such a harbour during a heavy gale of wind, are not likely to remain unimpressed with the importance of breakers in the removal of land.*

Slight attention to the manner in which waves break on a coast will soon show that, upon the prevalent winds and the proportion of those which force the greatest waves, or seas, as they are generally termed, on shore, will depend, other things being equal, the greatest amount of destructive action. Thus, on a coast on which western winds prevail, and there is sufficient extent of open sea before it, we should expect to discover the greatest loss of land, the force of the breakers being there the greatest and most incessant. As a whole, the coasts of the British Islands are exposed to the heaviest and most incessant breakers from winds ranging from the N.W. to the S.W., and but slight acquaintance with our coasts will soon satisfy the geologist, that if the other coasts of our islands were exposed to an equal amount of abrading force, a large portion of them would soon be cut away at a far more rapid rate than at present.

With regard to the force of breakers on the coasts of the British Islands, Mr. Stevenson has found by experiments at the Bell Rock and Skerryvore lighthouses, † that while the force of the breakers on the side of the German Ocean may be taken at about a ton and a half upon every square foot of surface exposed to them the Atlantic breakers fall with about double that weight, or three tons to the square foot. Thus a surface of only two square yards would sustain a blow from a heavy Atlantic breaker equal to about fifty-four tons.

Taking an equal amount of prevalent winds and of open sea over which they may range, it will soon be observable that the abrasion

* During a heavy gale in November, 1824, and also in another at the commencement of 1829, blocks of limestone and granite, from two to five tons in weight, were washed about at the breakwater, Plymouth, like pebbles. About 300 tons of such blocks were borne a distance of 200 feet, and up the inclined plane of the breakwater. They were thrown over it, and scattered in various directions. In one place a block of limestone, seven tons in weight, was washed a distance of 150 feet. We have seen blocks of two or three tons, torn away by a single blow of a breaker and hurled over into a harbour, and one of one and a half or two tons, strongly trenailed down upon a jetty, torn away and tossed upwards by the force of another.

+ Proceedings of the British Association for the Advancement of Science, 'dinburgh, 1850.

of rocks, of equal hardness and similar position, is modified according as the adjoining seas are tidal or tideless. In the latter case, though no doubt the pressure of the wind upon water raises it to levels above those which it commonly occupies, the difference is not so considerable as to bring any large faces of cliff exposed to the action of the breakers. A beach, moreover, piled in front of a cliff is, in such seas, as rarely passed and the cliff attacked. In tidal seas, on the contrary, many feet are vertically exposed to the fury of the breakers as the tide rises and falls; and beaches piled up in moderate weather are, in fitting situations, removed by the return action of the breakers, so that the cliffs are again open to abrasion. Moreover, the rocks are exposed to greater decomposition from being alternately wet and dry, a consideration of some importance in many climates, particularly in those where the temperature falls below the freezing point of water during certain seasons of the year. It should not, nevertheless, be forgotten that coasts, where breakers reach the cliffs at high water, are frequently protected by beaches at low water; and that, therefore, they are moved from the abrading power of the waves during all the time that they fall on the protecting beaches-a time which changes with the varying state of the tides and of the weather generally.

Attention will not long have been given to the abrading action of breakers on coasts before it will be seen that there are many circumstances modifying the effects which would be otherwise produced. It will be observed that the wearing away of coasts is, among the softer rocks more especially, often much accelerated by land-springs, which, as it were, shove portions of the cliffs into the power of the breakers by so moistening particular beds or portions of them, that much of the cliff loses its cohesion, and is launched seaward. The loss thus sustained in some coasts is very considerable.

So far from being thus brought by, so to speak, inland influences within the reach of the sea, in other situations we find the higher parts of cliffs protruding over the sea beneath, as in the previous sketch (fig. 40), when we suppose the parts of the rock to be so coherent that the breakers have been enabled to excavate the lower part of the cliff in the manner here represented. The same action continuing, a time must come when the weight of the overhanging portion will outbalance the cohesion of the rock, and the mass above will fall. Breakwater, as it then becomes to a part of the cliff, much will depend as to the length of time it may so act, according to the manner in which it has fallen, particularly if stratified. If composed of beds of rock, and the slope of these beds face the sea, as in the following sketch (fig. 41), the breakers will have less power

to act upon them, than if the edges of the strata were presented to the sea, as represented beneath (fig. 42), in which position they offer the least resistance to the destructive action of the sea.

It will be sometimes found, that a hard rock constitutes the high

E

part of a cliff, while the lower portion is composed of a softer substance, such as a clay or marl, and that masses of the harder rock falling from above afford protection, for a time, to the lower part of the cliff. Thus, let a in the annexed section (fig. 43) reFig. 43.

b

d,

C

present the upper portion of a cliff formed of hard beds of rock, such as sandstone, while b is a marl or clay, then the action of the sea, upon the cliff would undermine it, and cause the fall of masses of the hard rock, c, which, accumulating at its base, would tend to protect it according to the quantity of fallen rock, the size of the masses, and their hardness. It will be found that cliffs composed as a whole of somewhat soft rocks, and clays, marls, or slightly indurated sandstones, are often protected at their bases by an accumulation of indurated portions of these rocks. Thus let the accompanying section (fig. 44) represent a clay in which there are Fig. 44.

α

b

nodules of argillaceous limestones, as a a (and those of septaria in clays are often large), which, when washed out by removal of the clay, accumulate on the beach b. These then tend to protect the base of the cliff from the destructive action of the breakers. The study of any extended line of coast composed of horizontal or slightly-inclined beds of rocks of unequal hardness will present abundant examples of the modified protection afforded to the base of cliffs from the accumulation of masses derived from them.

Striking examples are often to be found on our shores of the wearing away of the land by the action of the breakers, so that rocks stand out in the sea detached from the main body of the land, but which once evidently formed part of it. Perhaps the