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Shingle.—The supply and movement of this material is of much greater interest than either that of sand or alluvial matter, inasmuch as where it is forthcoming shingle forms one of the most important aids to coast protection.
The supply of shingle is obtained from the destruction of cliffs consisting of granite and similar rocks or hard carboniferous limestone, shale, chert and flints, from the chalk cliffs, pebbles from the gravel beds exposed on the cliffs, or boulders from the glacial drift. The fragments of the softer sandstone and limestone cliffs, although yielding a considerable quantity of material, rapidly become disintegrated and reduced to the original particles of sand or shells of which they were composed, the débris from limestone and chalk cliffs being broken up, and the components dissolved in the water.
Flint may be formed under other conditions than in chalk cliffs, from the remains of the shells of existing organisms. Fragments of granite and other rocks when undergoing decomposition on the sea-beach by the constant drifting action of the waves and tides leave a residue of silicate of potash, which may, by chemical precipitation in the sea-water, become aggregated round some organism, and thus form flint.
The flint in chalk cliffs occurs either in horizontal or vertical bands or pockets, and of various shapes and sizes, some of the larger flints measuring as much as from 2 to 3 feet across. The horizontal bands of the cliffs bordering the English Channel vary in thickness from 6 inches up to a foot or 18 inches, the average being about 12 inches, the bands being from 10 to 13 feet apart. The proportion of flint in these cliffs may be taken at from 7 to 10 per cent.
The quantity of shingle thrown on the beach of Normandy from the erosion of the chalk cliffs has been calculated at half a million cubic yards a year (see “ Coast of France," Chapter VIII.). M. Lamblardie estimated that two-thirds of the bulk of the flints was converted into sand by the action of the waves, and that the shingle found on the beach represented the other third.
Shingle becomes heaped up in banks on the shore, and is found, with very few exceptions, accumulated in a zone lying between high water of neap and that of spring tides, or above this level. The exceptions to this rule are where the accumulation has become so great, as, for example, at Dungeness and the
Chesil and Hurst Banks, that the material has been forced beyond the shore limits.
On beaches where shingle is mixed with sand and where the latter predominates, the shingle is prevented from collecting into a bank. The pebbles, in fact, become buried in the sand, and the surface movement is not sufficient to allow them to be moved upward by the wave action. An example of this may be found on the beach of Bournemouth Bay, on which there is a considerable quantity of shingle derived from the gravel-beds which cap the cliffs, which is buried under the sand which lies on the surface of the beach.
The banking up of the shingle, and also the travel along the shore, are due to tidal action. The travel of shingle is confined to the beach, and it is never found drifting where the water is deep. Evidences of this are afforded not only by natural projections beyond the line of low water, but in cases where groynes or harbours project out from the shore. Numerous examples of this will be found recorded in Chapter VII.
The slope of a beach where the quantity of shingle is not abundant, and where it is mixed with coarse sand, is about 5 degrees, or 1 in 10. Where, however, the quantity is abundant, the shingle is thrown up into banks or ridges of considerable size, the top being above the line of high water. The slope of the face in the upper part is sometimes as steep as 1 to 1, the general slope being from 2 or 3 to 1.
During on-shore gales the shingle is drawn down by the undertow of the breaking waves and scattered on the beach at a slope of from 7 to 10 to 1.
In calm weather the face of the bank becomes heaped up, assuming what is termed a “full,” or ridge and hollow, parallel to the coast. This is caused by the lower part of the wave striking the bank, curling round and cutting out a hollow in its face. Two such hollows will almost invariably be found in all shingle banks, one at the level of high, and the other at that of ordinary, spring tides, and a third lower down formed during neap tides, which, however, becomes obliterated by the following spring tides.
A breaking wave on striking a shingle bank divides into two parts, the crest running up its face carrying with it the pebbles with which it comes in contact, and at the same time pushing upward those that lie above the point to which the water reaches.
The pebbles in their movement are actuated by the momentum of the wedge-shaped mass of water, and are almost entirely waterborne. The lower part of the wave curls round and cuts out the hollow already described. A great part of the water soaks in amongst the interstices of the pebbles, and drains gradually down to the beach, so that the quantity of water in the return wave is less than that which was contained in the wave which broke on the bank. The wave returns down the line of quickest descent, or normal to the bank, and carries with it some of the pebbles; but the depth of water is not sufficient to remove all that were driven up, and a portion therefore remains stranded.
The pebbles near the top of the bank are frequently larger in size than those lower down. This is due to the fact that they
are carried there with a large volume of water, which, after their deposit, is dispersed partly by running further up the slope, and partly by filling in the interstices between the pebbles of the bank. The retreating water finds its way back due to the action of gravity, and while it lacks the energy due to the volume and momentum of the wave-action, retires at a more gradual rate than that at which it was driven up.
The vibratory motion given by the waves to the pebbles also has a tendency to bring the larger stones to the surface, in the same way as when a box containing pebbles is shaken the largest work to the surface. The large stones also being on the surface are the first to be caught by the waves and pushed up the bank.
The advancing waves have the greatest effect in drifting the material, along the shore when they strike the bank obliquely, the maximum effect in the forward movement of the material being attained when the angle of impact is 45 degrees.
Each successive wave acting in an oblique direction pushes the stones a certain distance forward along the face of the bank, one wave of each series, usually the tenth, rising higher, and acting with more force than the others. This process for ever acting is sufficient to account for the removal of an immense amount of material.
The top of a pebble ridge is generally above the line of high water, this varying from a foot, in sheltered positions, to 4 or 5 feet in ordinary cases, and reaching to as much as 40 feet. This height depends on the length of the fetch of the sea to which the bank is exposed, and whether it faces the direction from which the winds blow that make the highest tides and heaviest seas, and also to some extent on the range of the tide.
The stones of which a shingle bank is composed vary in size from pebbles hardly to be distinguished from sand, to boulders weighing from 50 to 100 lbs. The size depends on the age of the beach and the amount of oscillation to which the stones have been subject, the material of which they are composed, and the amount of exposure. The harder the material of which a pebble is composed the greater the distance it will travel, since the rate of wear varies directly as the softness of the stone.
The form of the pebbles is frequently a flat oval, showing that the motion to which they are subject is that of being pushed along, rather than rolled. Except in violent gales, the stones are not lifted bodily and hurled towards the shore to roll back with the receding tide, but are rather shoved along. Gravity tends to hold the rock fragments in one position, so that the wear is always greatest on the lower side, and this aids in giving them a flattened form.
It is difficult to trace the rate at which rock fragments are moved along a beach, but instances are given where pieces of brick have been traced to have moved at the rate of about half a mile in a day. The author has noted on a shingle beach on different occasions half bricks carried 25 to 30 yards in from 1} to 2 hours, the tide being half flood. Mr. De Rance records an instance of some encaustic tiles being drifted along the shore at Blackpool during a gale a distance of 1 mile in two tides.
The weight of pebbles varies with the density of the material of which they are composed; but approximately it may be taken that, for ovoid-shaped pebbles measured along the largest diameter,
Shaler, “Geol. History of Harbours.”
60 measuring a quarter of an inch, 20 half an inch, and 2 an inch, will weigh an ounce; and 14-inch pebbles, 1} ozs. ; 2-inch, 2} ozs.
The most remarkable accumulations of shingle are the Chesil Bank, which is 102 miles long, and has a width of 500 feet at the base, the top ranging from 20 to 50 feet above high water; the Hurst Bank in the Solent, which projects across the Channel for 1.4 miles in a mound varying from 3 to 100 yards wide, the top being 12 feet above low water, and extending below low water a distance of 3 miles, having a steep incline in a depth of water of from 20 to 70 feet;1 the Northam pebble ridge, which, commencing at the termination of the cliffs in Barnstaple Bay, extends as a bank 20 feet high for 2 miles, having a width of 180 feet at the base and 20 feet at the top, which is 6 feet above H.W.S.T. This bank is remarkable for the large size of the boulders of which it is composed, the largest of which exceed 12 inches in diameter, and weigh from 40 to 50 lbs., some at the foot of the bank weighing upwards of 150 lbs.?
At Lancing, on the Sussex coast, there is a bank of shingle about a mile long and 150 yards wide, and at Shoreham a bank 4 miles in length, and in places a quarter of a mile wide. On the East Coast the Aldborough bank extends southward from Orford Ness for 9 miles, having a width of 70 to 80 yards. On the Norfolk coast a bank extends from Weybourn in a westerly direction for 9 miles ; 4 and on the Yorkshire Coast a shingle-spit projects out into the mouth of the Humber for 3 miles, having a width of from 150 to 200 yards ;4 and a somewhat similar spit projects into Harwich harbour.
All these are banks standing out separate from the coast-line.
On the west coast of France, in the Bay of Audierne, Finisterre, there is a bank of shingle of a curved form 8 miles long, which extends in a southerly direction from the rocks in the middle of the bay at Penhors to Penmarck, abutting on the rocks by which the coast is bounded. The top of this bank is 16 feet above the beach, and behind the bank is a salt-water lake and salt marshes. In storms the sea occasionally breaks through the bank, but the gap is gradually restored by the drift of the shingle. The shingle is composed of pebbles of granite, gneiss, mica schist,
* See description of these banks in Chapter VII., “ South Coast.” ? See description of this bank in Chapter VII., “ West Coast." : See Chapter VII., “ South Coast." • Ibid., “East Coast."