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Art. LXXIII.—On the Formation of detached Shingle Beaches.
[Read before the Wellington Philosophical Society, 17th November, 1877.]
The travel of sand and shingle along a sea coast is not due to any currents which may obtain, but to the breaking of the waves on the beach. There is very seldom a littoral current strong enough to carry shingle or sand along with it, but where even a small wave breaks it has sufficient impetus to move shingle.
If the wave breaks square to the coast the shingle is thrown directly forward; when the wave recedes it carries the shingle back with it to the place it started from. In this case there is no travel of the shingle; it is thrown ashore and pulled back by the waves, time after time, over the same course until it is ground down into sand or mud. If, however, the waves break obliquely on the shore, the shingle is carried forward by the wave, but when the latter recedes it does not travel on the same track by which it rolled forward, but takes the line of quickest descent, which is at right angles to the beach. The shingle thus travels in a zig-zag path moving gradually along the shore in the direction of the waves. This causes a travelling beach.
The travelling shingle will often refuse to follow a sudden indentation of the coast and goes straight across the bay, forming a detached beach with a lagoon behind it. This may, I think, be explained in this way: Waves travel less quickly in shallow water than in deep; the inner end of a long oblique wave being in shallower water than the outer end is, therefore, checked as it approaches the shore and the wave takes a curved form. At the point forming the beginning of the bay this action is of course intensified, and the waves roll round the point in long curves which tend more and more to become perpendicular to the coast line. The varying speed of the waves, due to the varying depths and the different distances they have to travel, destroys the uniformity which obtained along the straight part of the coast; the waves cease to be continuous billows; those rolling on the outside of the point strike those breaking on the inside on the flank, and are tripped up and broken exactly as they would have been on a solid shore. The shingle is then deposited, and a spit is formed stretching out into the bay. When the spit reaches deep water the process becomes more simple; the waves rolling into the bay beyond the spit being in deep water do not break, but those striking the spit are broken and carry on the shingle exactly as was done on the straight coast line. In this manner the spit stretches at last quite across the bay, and becomes daily
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higher and wider as more shingle is thrown up it and over it by the waves.
An island lying off the coast on which there is a travelling beach also causes the latter to detach itself from the shore. The island forms a breakwater, and the shingle cannot pass across the still water behind it. It therefore collects on the windward side until it reaches out to the island, from the leeward side of which it continues its course as a detached beach.
Strong littoral currents and rivers entering the sea cause the waves to break before they reach the shore. If there is any travelling shingle it will be collected where the waves break, and will therefore form a detached beach.
In short, given a source of supply of shingle or sand, such as a large river or a cliff against which the sea beats, there will be a travelling beach wherever the average direction of the waves is oblique to the shore on which they break, and there will be a detached beach whenever, from any cause, the waves, under similar conditions, break before they reach the shore. The principal causes of this latter case are those above enumerated.
There are in New Zealand many examples of detached beaches. The spit of sand which ends Cape Farewell is an example on a very large scale.
The source of supply is in this case the whole west coast of the South Island with its many rivers. The débris formed by the denudation of the land is driven northwards by the prevailing south-westerly seas which break obliquely on the shore. At Cape Farewell the breaking of the waves against one another instead of on the shore caused the sand to be thrown down before reaching the shore on the east side of the Cape. The spit began to form and is still forming. Eventually, Golden Bay will become a lagoon if a sufficient supply of sand is kept up by the continued denudation of the west slope of the island.
Nelson Harbour is another case of a detached beach. The cliffs to the north of the harbour are being cut down by the sea, and the material, driven by the oblique north-westerly seas, which are here the most powerful, crosses the indentation of the coast on which Nelson is situated; the earthy matter contained in the cliff is washed away leaving only the mass of heavy boulders, which are all of the same material as those embedded in the cliff from which the boulder bank is derived.
Lake Ellesmere is another fine example. The coast of Canterbury consists of a travelling shingle beach on a scale of magnitude hardly equalled in the world. The great Canterbury rivers are nothing but mountain torrents on a magnified scale. They have their rise in ranges formed of clay-slates of a very friable nature, and they convey to the sea great quantities of shingle.
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The oblique south-easterly seas drive this shingle to the northward, spreading it along the Ninety-mile Beach.
The direct cause of the detached beach at Ellesmere is the Rakaia River, the delta of which forms a projecting point from which the Ellesmere beach starts on its detached course. Similar beaches have, no doubt, been formed at different times near the mouths of all the great rivers of Canterbury, forming lagoons which have been gradually filled up by material brought down from the mountains by the rivers, as Lake Ellesmere is now being filled up.