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being 100 years ago almost in the identical position in which the Sailing Directions of to-day describe it.

As shown in Chapter II., waves, except when they break on the sea margin, are undulations, and, as such, have no progressive movement, and are not, therefore, capable of transporting material from the sea-bed to the shore.

Within the orbit of the wave, so long as it remains an undulation, in addition to the vertical movement of the particles of water which places the crest above the trough, there is an oscillating, horizontal movement alternately towards and away from the shore. Any material susceptible of movement lying on the bed of the sea, actuated by the waves, is moved alternately backwards and forwards, the mass of the substance and the distance over which it is moved depending on the height and on the length of the waves. As the beaches on which these waves act incline from the shore seaward, the retrograde action of the wave must be most effective in the movement of material, and the tendency be rather to drag it away from, than to push it towards, the shore. The slope of a beach being seaward; by the law of gravitation all material in movement has a natural tendency to work downwards unless prevented by some stronger opposing cause. Breaking waves, no doubt, have sufficient force, under certain conditions, to counteract this downward movement, but their general tendency is to aid the seaward movement by their undertow.

It is due to the seaward action of the undertow of the waves that bays and indents along the coast are kept open and free from accumulation of deposit.

It is true that stones and other substances of considerable size and weight, which have been buried in the sand for longer or shorter periods, are occasionally, in heavy gales causing high waves, lifted up, carried forward, and left stranded on the beach. These, however, are only rare and isolated events which occur during very heavy gales. Any material thus thrown up must have come within the range of the breaking waves, which in heavy gales operate in a much greater depth of water and a greater distance seaward than under normal conditions. The reason of these stones not being carried back by the waves is, that while the wave running up the beach does so with considerable momentum, and the substance is impelled by the thick end of the wedge of water, on the retreat it is only affected by the

thin edge of the wedge, and lacks the aid due to the momentum of the wave breaking on the beach, and so remains anchored to the spot to which it was carried.

The general tendency, therefore, of material coming within the range of littoral waves is to follow the ordinary law of substances oscillating on an inclined plane, and gradually to work down the sloping floor of the sea-bed towards the depths of the ocean, which tendency is only counteracted by the effect of the shallow shore waves breaking on the margin and pushing some portion of the rock detritus derived from falling cliffs above the level of high water.

Cliff-destruction. Where cliffs exist they supply material for littoral drift owing to their erosion, the degradation being more or less rapid, depending on whether the cliffs consist of the primitive rocks, or of those which have been at some time degraded and redeposited. The agents by which this operation is performed are waves due to wind-force and the tides.

The function of the wind-waves is to break down the cliffs, to sort the material displaced, and to grind up the larger rock fragments into sizes sufficiently small to be acted on by the tides, and to disperse material when, under tidal influence, it has collected in large quantities.

By the beating of the waves on the cliffs at high water in gales and storms, large masses of rock are dislodged, the fragments thus broken off being used as battering-rams in further work of disintegration. Incessantly rolled about by the waves, the disintegrated rock becomes ground to boulders, then to shingle, and finally to sand and alluvium.

The function of the tides consists in raising the water of the ocean sufficiently high to enable the waves to attack the cliffs; in assisting the grinding up of the reduced rock-fragments by their perpetual oscillating movement up and down the beach, and along the coast, until the particles become reduced in size; and finally in transporting the smallest particles to the bed of the sea. The latter operation is effected either in solution, in suspension, or by rolling along the surface.

The degradation of cliffs is further aided by rain and frost; and, where the drainage is imperfect, land-slips constantly occur and cause the wasting away of land bordering on the sea. The material thus eroded from the cliffs and broken up is continually in

motion, under the influence of the waves and tides, travelling along the coast in a direction determined by causes to be described. The power of waves in heavy storms breaking on the shore to move material is almost beyond comprehension, and has already been described in the chapter on wave-action.

The wear and tear of rock fragments and pebbles is due to the constant friction and rubbing to which they are exposed while being drifted backwards and forwards along the beach. Friction increases directly with the weight, and therefore as the cube of the diameter of the pebble, while the attrition to which the stone is exposed varies as the square of the diameter. The larger the fragment, so long as it is not too heavy to be drifted about, the more it suffers. Thus a pebble an inch in diameter would be worn as much in being drifted a yard as a grain of sand o inch in diameter would be in travelling over a distance of 5 miles.

Experiments made by M. Daubrée on the resistance of fragments of rock to prolonged friction in revolving cylinders of sandstone containing water, showed that not only did the fragments become rounded and diminished in size, but also underwent a certain amount of chemical decomposition. Angular fragments of granite, quartz, and felspar, after being made to slide over one another in a revolving horizontal cylinder for a distance equal to 15 miles at the rate of about 3 feet a second, resembled those found in a running watercourse, and lost of their weight, while rounded stones did not lose more than 180 to 400. The residue consisted of sand and fine alluvial matter and soluble salts, principally silicate of potash.1

Experiments made in 1897 by Mr. E. J. Lovegrove, M. Inst. C.E., for the purpose of ascertaining the wearing qualities of different kinds of road material, gave the following results. Four lbs. of the stone broken into cubes of about 2 inches were placed, with half a gallon of water, in a cylinder 11 inches internal diameter, and subjected to 8000 revolutions at a speed of 20 revolutions a minute. The cylinder had three 1-inch angleirons projecting from the face. The distance thus travelled by the stones was 4 miles. It will be noticed that after the stones had become rounded the wear was considerably diminished, as shown by the second journey of the flints.

1 Daubrée, "Géologie Experimentale."

Report in The Surveyor, November 5, 1897, and December 14, 1900.

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After the particles of rock fragments on a sea beach have reached a certain degree of fineness, further diminution in the size becomes a very slow process owing to the particles being separated from each other by a thin film of water, which prevents actual contact when they are in motion, and to this is no doubt due the small amount of wear that takes place in the sand grains on sea beaches.

Supply of Beach Material limited.-The supply of shingle and beach material being thus derived only from the destruction of the cliffs is not inexhaustible. If, therefore, the drift along any given part of the coast be stopped by groynes or other artificial works, the coast beyond becomes correspondingly denuded.

Several instances of such stoppage of supply may be quoted. On the south coast the extension of the west pier at Shoreham led to a stoppage of the shingle drifting to the east and the starving of the beaches in front of Hove and Brighton.

The supply from the west has also been materially affected by the projection of Dungeness Point, which has acted as a natural groyne, arresting the travel of shingle brought from the waste of the chalk cliffs at Beachy Head, which has led to the denudation of the shore in front of Dymchurch and Romney.

When the pier was projected from the shore of the English Channel at Folkestone, the shingle travelling along the beach. from the west was arrested and accumulated in a large bank; but the beach at Dover and eastward of it became in consequence denuded, and the safety of the cliffs was considerably endangered;

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and this effect was further enhanced after the Admiralty Pier at Dover was built, which completely stopped all eastward drift, the beach along St. Margaret's Bay consequently becoming much denuded.

The shingle derived from the waste of the boulder clay in the cliffs between Flamborough Head and the Humber travelled southwards along the beach to Spurn Point, where it accumulated. The removal, however, of large quantities of shingle for roadmending and concrete so denuded the beach at Spurn Point as to threaten its safety. The taking of the shingle was stopped, and the beach again accumulated, and the erosion of the cliffs diminished one-half.

The construction of groynes at Cromer for a time stopped the travel of shingle and led to the denudation of the beach at Overstrand and other places to the south.

Littoral Drift as affecting Harbours.-The question whether the supply of littoral drift is limited to the quantity supplied from the destruction of the cliffs is important as bearing on the construction of jetties for the improvement and protection of the entrances to tidal rivers and harbours on coasts subject to the drift of shingle and sand.

If the supply be derived from the sea as well as from the cliffs it would be practically inexhaustible, and the projection of jetties across the beach could only be regarded as a temporary expedient, which, to be effective, would require to be extended from time to time at comparatively short intervals as the material accumulated, involving great and continual expense.

If, however, the only source of supply be that now on the beaches or supplied from the erosion of the cliffs, the extent of the drift that would be arrested and accumulate at the projecting jetty may be estimated and provided for, the accumulation at the same time, by protecting the cliffs, cutting off the source of supply.

Projecting jetties act as groynes which arrest the drift of the material; but to be effective for the protection of a harbour from drifting material, it is essential that the jetty-head should be extended into deep water beyond the influence of wave-action. Experience shows that this depth should not be less than 15 feet at low water.1

1 "Littoral Drift in Relation to River Outfalls and Harbour Entrances," Min. Proc. Inst. C.E., vol. cxxv., paragraph "Harbours on Coasts subject to Littoral Drift."

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