Imágenes de páginas

In addition to the effect of the horizontal movement of the wave, there is also a vertical action of the water on the face of the beach, which has a material effect in its disintegration. When the waves strike a cliff or sea-wall and the water is deflected upwards, in its descent it cuts out and loosens the beach.

Assuming that half the volume of the 10-feet wave described above to be thus deflected, a column of 213 tons of water would be projected upwards a mean height of 5 feet, the summit of the column reaching 10 feet above the beach. The weight of this column of water would exert a statical pressure on each square foot of the beach of 0.28 ton; and falling from a mean height of 5 feet, with a velocity of 18 feet a second, would produce an impact of 5:18 tons on each square foot. If this calculation be reduced in the same proportion as the other result obtained by the dynamometer, the blow would still be equal to about 12 tons on the square foot, repeated with each wave, at the rate probably of ten or twelve a minute.

Approximately, and under ordinary conditions, breaking waves are reflected from a vertical face upwards equal to the height of the wave.

During very heavy ground-swells, with a long fetch in exposed positions, this limit is very considerably exceeded. Mr. Stevenson has given an instance at the Bell Rock Lighthouse, where the water was thrown upwards 106 feet, and instances have occurred at the Skerries, where the water and spray have been projected 60 feet upwards, carrying with it pieces of stone on to the lighthouse roof, 240 yards from the face of the rock, one of these going through the roof, which was 50 feet above the sea.

At the breakwater at Alderney the water from the waves breaking on it is reported to have been thrown upwards to a height of 200 feet.

Even when the depth of water in front of a sea-wall or cliff is only that due to the rise of the tide, water from waves that break is thrown to very great heights. Thus at Hastings, where the beach at the foot of the sea-wall is dry at low water, and the depth of the water is only that due to a rise of 15 feet at high water, during a heavy gale in the winter of 1898 the broken water was thrown as high as the top of a large hotel, as shown in the frontispiece, which is taken from a photograph made by Blomfield of Hastings—and shingle was lifted off the beach and carried across the promenade into the bedrooms of the houses fronting the sea. At Peterhead, as already mentioned, the water due to a rise of tide on the foreshore of only 7 or 8 feet has been known to strike the wall with such force as to be thrown upwards 100 feet.



The term “ littoral drift ” is intended to describe the movement of the material, that is always taking place along the shores of a tidal coast, due to the action of the waves.

This material may be classed as rock fragments, boulders, shingle, sand, and alluvium. The two former, in course of time under the action of the waves, become reduced to shingle, and this finally to sand of varying degrees of fineness.

This material is sorted by wave-action, the stones and pebbles being deposited at the top of the beach above the level of mean bigh water, the sand being placed below this, and the alluvial matter carried to the depths of the sea. Owing to the sorting action of the waves, material of different sizes, shingle and sand, may be carried on the same beach and at the same time in opposite directions, the former towards the land and the latter seaward ; while the finer alluvial matter will remain in suspension until it is carried seaward beyond the reach of wave-action.

Although shingle and sand have accumulated in some places in great and apparently unlimited quantities, it must be borne in mind that these accumulations are the product of a vast period of time, and that the great bulk of the material was placed under different conditions from those that now prevail. An examination of the coasts of this country will show that the quantity of shingle drifting along the beach is by no means inexhaustible, but, on the contrary, is limited in quantity; and that the wear and tear of the sea-cliffs does little more now than supply the waste which is always going on from the perpetual action of the waves.

Source of Supply.— The present shape of the cliffs and their

| Further particulars as to the movements of beach material will be found in a paper by Dr. Vaughan Cornish in the Geographical Journal of May and June, 1898.

steep escarpments are testimonies to the continual wasting action of the sea. Originally these cliffs must have descended to the bed of the sea, with the same slopes as characterize their land faces, and were then washed by the deep water of the sea without the intervention of the sand beaches which now, in most cases, stretch from them.

The form of the cliffs as they exist is due to the destructive action of the waves, to landslips, and weathering from the effect of rain and frost.

The wearing away has not been regular. Headlands composed of hard rocks project out boldly to low-water mark and beyond, while the softer rocks, which at one time adjoined them and formed a continuous line, have been gradually worn away, leaving indents and bays of various shapes and depths.

The fight between sea and land is continuous and unceasing, with the result that the sea has advanced and the land receded; the area of this country being gradually reduced.

In the few instances where land has been reclaimed, such land consists of alluvial matter brought down by the rivers, and deposited in the comparatively quiet water of an estuary, or on some sheltered part of the coast; or else, as where seaports formerly washed by the tides are now some distance inland, as is the case with Rye and Winchelsea, this has been due to the closing up of an estuary, or the indent of the coast on which they were situated, with shingle drifted along the beach.

The large tract of land gained from the sea which constitutes the greater part of the United Netherlands, has been formed by the transportation, by three large rivers, of detritus, wasted from the land through which they flow, and its discharge on to a coast, where, owing to the small rise and fall of the tides, the conditions approach those under which Deltas are formed in tideless seas; and which now is only maintained from the encroachments of the sea by a constant struggle between the operations of nature and the works of man.

A careful consideration of all the circumstances that attach to beaches can only lead to the conclusion that the results which have been attained must be due to other and mightier forces than those now in existence. These forces may be ascribed to the same agency that gave to this country the shape which it now assumes, and by which the valleys and rivers were scooped out.

[ocr errors]

At the close of the great Ice Age the melting of the vast bed of ice and snow, estimated at several hundred feet in thickness, which then covered this country, must have led to immense torrents of water escaping seaward, carrying with them the débris from the rock-surface, disintegrated by frost, and broken up by ice sheets and glaciers; and besides leaving deposits of gravel in the valleys and on the surface of the land, carried the degraded material to the sea-bed, and there formed a talus at the level of the water. This deposit, after the wear and tear caused by the waves during long ages, resulted in the formation of sand beaches.

As the sea erodes the land on its margin, the glacial deposits of stones and gravel deposited on the cliffs, falling on to the beaches, afford a constant, though limited, supply of shingle.

There is every indication that at one time the space that is now occupied by the English Channel, about 30 miles in width, was solid land, and that a deep depression has at some time been produced by the scouring away and waste of the chalk, leaving the steep cliffs which exist on both sides. M. Lamblardie,1 in his treatise on the coasts of Normandy, estimated that about seven per cent. of the eroded cliffs consisted of flints, producing five billion cubic fathoms (40 billion cubic yards) of sand and two billions of shingle, and that this accounts for the vast beds of shingle that are found along the coasts of the Channel and which choke the entrance to the harbours, and the sand which constitutes the long stretch of dunes along the coast of Flanders.

It is certain that the enormous mass of sand, which now covers the littoral of the sea and the beds of estuaries, cannot have been deposited by existing agencies. The degradation of the cliffs that takes place is quite inadequate to account for its existence, more especially as the harder rocks alone have afforded the material of which the sand of the shore is composed; the softer detritus from the limestone, chalk and clay cliffs having been carried away in suspension to the depths of the ocean. And on many parts of the coast the sand has not been derived from the adjacent cliffs.

For example, on the West Coast, in Morecambe Bay, the sand-beds cover an area of 90,000 acres, dry at low water, and in the adjacent Duddon estuary they cover 9000 acres. It is impossible that this enormous mass of material can ever have been

| “Mémoire sur les Côtes de la Haute Normandie.” Par M. De Lamblardie. Havre, 1789.

« AnteriorContinuar »