Imágenes de páginas

and quartz, derived from the erosion of the adjacent cliffs. The drift is from north to south, the same as that of the flood tide along the coast of the bay, the prevailing winds being from the south-west. The pebbles at the northern end are large, some being as much as 12 inches in diameter, but they diminish in size towards the southern end.?

Sand.—What is generally understood by the term sand is a material consisting of minute fragments of the harder rocks. On some beaches it is almost entirely composed of fragments of marine shells.

Sand is termed “coarse” when 50 particles are equal to 1 linear inch.

The specific gravity of sand composed of quartz or felspar is from 2:53 to 2:65. Shell sand of the same volume is rather heavier.

The particles of sand, such as are generally found on the seashore, vary from 100 to 150 to the lineal inch. The grains of blown sand, of which dunes are composed, and which have been blown off the shore by on-shore gales, run about 100 to the inch.

Sand consists almost entirely of grains of quartz, and even where flints abound in the cliffs, and in the shingle on the beach, silica, in the form of flint, is conspicuous by its absence in the sand on the shore. Felspar fragments, when subject to attrition in sea-water, do not result in sand, but are dissolved into mud.

The Dune Sands of Holland are almost entirely quartz grains, a few specimens of augite and hornblende being found.

After rock fragments have become worn down to grains of sand of the size ordinarily found on a sea-beach, their further diminution, as already shown, is a very slow process, especially when the particles are drifted about in a state of semi-suspension.

Sand, if stirred up in water, settles again almost immediately, and does not cause it to become turbid.

With a tidal current, say of 3 to 4 knots, sand of this description will be moved along the bed from which it is displaced in a state of semi-suspension; and there is always a certain quantity oscillating up and down a sandy beach with the ebb and flow of the tides. Grains of sand of a diameter of }o inch will remain suspended in a slow current, or when the water is only feebly agitated. Smaller grains than this come under the class of alluvium or mud.

1 “ Leçons de Géologie Pratique." L. Elie de Beaumont. Paris, 1845.

Low, flat coasts, where there are no cliffs to afford a supply of fresh material, are generally bordered by sand-beaches, and attain an inclination which represents an equilibrium of the wave forces : and, speaking generally, these sandy beaches retain their shape and position for long periods without material alteration.

Where the sand is coarse, and some shingle mixed with it, the normal slope of the beach at the upper part is about 1 in 10 from below H.W.S.T. to mean tide level; below this, to L.W.N.T., 1 in 30; from this to L.W.S.T., 1 in 50; and 1 in 100 below this. Sandy beaches lying at a general slope of 1 in 30 or more above L.W. are more or less stable, and subject to very little alteration.

Sand is not banked up or drifted along the coast like shingle.

During on-shore gales the waves breaking on the beach towards the time of high tide, when the depth of water covering the surface is greatest, cut out the sand and form a gully, or low, running parallel with the coast, drifting the sand so cut out seawards. The depth of the low place so formed depends on the thickness of the sand covering the beach, and may vary from 1 foot to 4 feet or 5 feet. After the subsidence of the gale, the sand gradu. ally works its way back, due to the action of the flood tide, and the low becomes filled up again. When the gale has been severe, and the quantity removed great, the level surface of the beach remains disturbed for a considerable time, especially when there is a mixture of shingle with the sand. Thus in strong easterly gales the sand-beach at Yarmouth is occasionally cut out for a width of 30 yards, and to a depth of from 3 feet to 4 feet. As soon as the gale subsides, the sand, mixed with some shingle, begins to work back in a mound or ridge, slowly advancing up the beach until the low is levelled up. In the mean time, access to and from the low-water margin of the beach is considerably obstructed by the water in the low, which finds an entrance to it on the rising tide for some time before the beach is covered, and, on the ebb, this low place acts as a drain to collect the water off the adjacent sands.

In some cases a low thus created becomes permanent, forming a shallow creek running parallel with the shore, along which there is a constant run of water on the early flood, and after the rest of the foreshore is left dry by the receding tide. At seaside resorts such lows are sources of danger. Visitors amusing themselves at the margin of the sea, and gradually retreating landwards as the incoming tide rises, are unconscious of, or lose sight of, the fact that the low is rapidly filling with water, and thus cutting off their retreat to the shore.

The quicksands which are to be found on some beaches are caused by the presence of water contained between the grains reducing the sand to a state of semi-suspension. When water presses downward on sand it becomes firm and compact, but when the pressure is from below, the grains become separated and the sand quick. This may be caused by land-springs breaking up through the beach ; or, where the sand lies on an impervious substratum, in which are lows and hollows from which the water cannot escape, the contents become a mixture of sand and water. On a rising tide the sands near the water's edge become alive, owing to the upward pressure of the rising water, whereas, on the ebb, the sands become firmer as the water recedes. In a quicksand the velocity of the water passing through it is sufficient to balance the excess of weight of sand over that of the water, but not sufficient to carry the particles away. With sand such as is generally found on a sea-beach having 100 grains to the lineal inch, it has been ascertained, from experiment on filter-beds, that an upward velocity of only 3 inches a minute is sufficient for this purpose.

Sand-beaches are not subject to much littoral drift, and, except to the extent already mentioned, little or no change takes place in their condition.

From the south side of the river Humber to the Wash there extends for 25 miles a low tract of flat country, bordered by hills of blown sand. The beach consists of sand which extends out at a slope of about 1 in 30 to low water. On this beach there is no appreciable littoral drift or alteration in form. Sand does not accumulate against the piers or groynes which extend across the shore, and the general outline of the beach remains as it always has been, so far as any record exists.

On the north coast of France the chalk cliffs which run along the coast terminate at Sangatte, a little beyond Cape Blanc Nez, beyond which, to the Texel, is a low sandy shore, sloping at an angle of from 1 in 50 to 1 in 100, bordered by sand-dunes, this being the distinguishing feature of the coast of this part of the French, Belgian, and Dutch coasts. At the southern end of this stretch of coast the supply of material from the waste of cliffs to the west results in a considerable amount of drift-sand, which, travelling eastward with the flood-tide, accumulates in large

masses wherever its course is obstructed by groynes or piers, as at Calais and Dunkirk. Beyond these places the drift gradually dies out, till at Nieuwport, Wendyke, and Heyst there is no appreciable drift, and the beach is subject to very little change. Where piers have been constructed there is scarcely any accumulation. The groynes placed between Ostend and Heyst and beyond Wendyke show no indication of any accumulation of sand since they were constructed several years ago; and a comparison of surveys of the beach made in 1833 and 1870 shows that there has been no material alteration in the form of the beach.

The Dutch coast is bordered by a sand-beach from 100 to 150 yards in width, sloping seaward at an angle of 1 in 70. Careful measurements have been made of this beach during the last half-century, which show that there has been a gradual wasting away, low-water line having advanced from 30 to 50 yards more inland, and the foot of the dunes driven 100 yards inland. There has, however, been an accumulation of sand against the jetties at the entrance to the Ymuiden harbour, which projects 1 mile from the shore, and also against those of the entrance to the Maas, which projects 2 miles from the shore.?

The surface of a sandy beach frequently consists of a series of ridges and furrows, the lower side of the ridges having a steep, and the upper a flat slope, the flat slope being the direction from which the water is flowing off the beach. The furrows are frequently covered with a black deposit consisting of small fragments of carbonaceous matter derived from decayed seaweed.?

Sand-spits.—On some sandy coasts, where there is a predominant drift in one direction, the sand is formed into spits, consisting of long narrow banks, which, commencing at some salient point, run for a considerable distance in a direction parallel to the general coast-line, leaving a protected bay or harbour inside. These long narrow spits, forming natural breakwaters, although existing in great depths of water and exposed to the storms and waves of the ocean, maintain their position in

| Nature, June 1, 1899.

? Professor Osborne Reynolds on the “ Regime of Rivers,British Association Report, 1887, and Report of Committee on the Action of Waves on the Beds of Estuaries in 1889; and article by Dr. Vaughan Cornish in Nature, April 25, 1901, and in Geographical Journal, March, 1897, May and June, 1898, and August, 1901.

a remarkable manner, the waste due to storms being made up by fresh deposits of littoral drift.

They sometimes extend from 5 up to 100 miles in length, and are situated in depths of from 20 to 30 feet of water. In some cases, owing to the strong current caused by the contraction of the space through which the flood and ebb current runs to fill the embayment, the end of the spit is turned into the form of a hook.

On the North German coast, in the Baltic, are several large

[ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small]

FIG. 3.–Sand Spit, Gulf of Dantzig. salt-water bays or lakes protected by sand-spits, in which some of the principal ports of Germany are situated.

The Curische Haff is a large salt-water bay into which the river Niemen discharges, and in which the Port of Memel is situated. This bay is separated from the Baltic by a narrow sand-spit 55 miles long, and having about 20 feet of water on the Baltic side. The entrance is by a deep narrow passage at the east end, having from 25 to 30 feet of water.

A similar bay exists on the coast near Danzig, into which the

« AnteriorContinuar »