LITMIR.BIZ

Скачать книгу

cut off.

      Changes of this kind are common, but we do not know the precise causes. We know that in a certain storm such and such a spit was shortened, and we may make a shrewd guess as to what may have happened. But precisely what happened and why is quite another matter. Similarly, it is a well-known fact that many sand and shingle beaches, but especially shingle ones, are not merely a simple line of stones, but have running back from them branch ridges called laterals, or recurved ends. Figs. 25 and 26, show this. Each lateral ridge was at one time the distal or free end of the growing spit. Later, for some reason or other, conditions caused the main beach to lengthen again, and to continue to do so until another lateral was formed. Figures 25 and 26 show that the laterals often meet the main ridge at high angles, even at right angles. The newer laterals, especially those we may see forming over a period of years, may bend gently round. Hence, something has happened to bring about the abrupt junction of the older laterals with the main ridge. The main beach has been over-rolled on to them as is indicated in Fig. 24. A direct proof of this inrolling can sometimes be found if marsh deposits are exposed on the foreshore. These deposits can often be shown to have originated inside the spit, which because of the occasional overtopping by big waves has been rolled landwards. A misleading impression of stability is given if the main ridge is now dune covered.

      In another chapter some of the coastal districts in which these lateral ridges are well developed are analysed. In this, it is emphasised that each such ridge was built by wave action, and was therefore at one time the outer ridge, and it follows that if one ridge lies in front of, or cuts across another, it is the newer one. Thus by carefully mapping the ridges, and noting their relationships one with another, the evolution of the whole structure of a shingle foreland may be investigated. Difficulties occur, especially in places where the ridges are in groups, but the groups themselves separated from one another. It is, for example, far from clear just how the several groups which are truncated by the present beach ridge on the southward facing part of Dungeness are related to one another and to the formation of the whole foreland. Reconstructions that have been made are quite possible, even probable, but they remain hypothetical (see here).

      It may also be remarked that Benacre Ness has altered greatly in form since the current Ordnance maps were published. The Ness point is now considerably farther north, and the southern end near Benacre Broad has suffered severe erosion. To balance this, there has been accumulation at or near the sluice.

¹ At the northern part of Kessingland and again at the southern end of Aldeburgh, new groynes have recently been built. In both cases they have been successful in collecting some beach, and it is interesting to note that at both places, perhaps more markedly at Kessingland, a reversal of beach drift seems to have taken place. As a rule the beach is piled up on the south side of the groynes, pointing to a northerly movement. This seems to be quite local, but no ready and adequate explanation is forthcoming. It may also be remarked that Benacre Ness has altered greatly in form since the current Ordnance maps were published. The Ness point is now considerably farther north, and the southern end near Benacre Broad has suffered severe erosion. To balance this, there has been accumulation at or near the sluice.

² It is well known that large cobbles on which seaweeds grow can be carried long distances in this way. The remark in the text applies to bare stones and cobbles.

¹ Approximately the relation between the movement of the particles and the depth is: “If L is the length of the wave, the movement of the particles from the surface downwards decreases one half for each 1/9 L of depth; i.e., at a depth of 1/9 L the movement is 1/2 that at the surface; at 2/9 L it is 1/4 the movement at the surface, etc.” (R. S. Patton and H. A. Marmer, The Waves of the Sea, in Physics of the Earth; V, Oceanography, Bull. Nat. Res. Council, Washington, 1932.)

¹ J. S. Owens and G. O. Case, Coast Erosion and Foreshore Protection, Ch. iii, 1908.

² Geogr. Journ. 78, 1931, 131.

¹ W. V. Lewis, Proc. Geol. Assoc., 49, 1938, 107.

¹ See D. Haldane, Trans. Edin. Geol. Soc. 13, 1931–38, 442. The nullipore is a robust form of Lithothamnium calcareum, Aresch, and at low tide can be seen growing on the spit connecting Lampay Island with the shore. Nullipore sands also occur at Loch Bracadale, Morar, Arran, Lower Loch Fyne, South Bute, and Cumbrae.

       CHAPTER 3

      EROSION AND ACCRETION:

      EVIDENCE OF COASTAL CHANGES

      IN OTHER parts of this book there are references to recent changes in the coast, to erosion and deposition. In this chapter an attempt is made to try to bring together various lines of thought to show the close interlocking between physiographical studies of the coast, archaeology, and history.

      It is logical to work from early times up to the present, but that approach may be dangerous, since accounts of early changes are often based on too few facts but on considerable imagination. Changes today are taking place mainly by erosion or by deposition; the slow vertical movement of the land surface suspected in south-eastern England may be very important in the long run, but need not be discussed at the moment (see, however, here).

Consider first the question of the rate of erosion of cliffs. Erosion to some extent is taking place in many parts of our coasts, but it is most marked in softer rocks or where for some reason the structure of the cliffs favours it. (See Chapter 5.) The most spectacular example is the Holderness coast of Yorkshire. Recent analysis suggests there are three main factors associated with this erosion.¹ From Bridlington as far as Barmston the cliff was not suffering in 1947, but it was thought that a good deal of erosion was taking place on the sea floor for one or two miles outwards from the beach. The water is heavily charged with clay, and the staining thus produced extends in long tongues to the south-east. On the other hand between 1840 and 1890 erosion along this line of cliffs was severe. From Barmston to Spurn Head erosion is serious, and averages two to three yards a year, and in places a rate of five yards over a period of years has been reached. Most of the pill-boxes erected on the cliffs during the 1939–1945 war are now half submerged in the beach sands. Houses have been wrecked by the collapse of the promenade at Withernsea, and at Skipsea a farm building, which in 1945 was slightly damaged at one end by cliff falls, was in 1947 derelict, half of its 45 feet of length having disappeared.

      In Holderness, as often elsewhere, much of the erosion is attributed to the drainage of land water. Springs from interbedded gravels and water from land drains cut deep gullies in the cliffs. Other reasons, apart from the direct attack of the waves, include the loosely compacted nature of much of the boulder clay in the cliffs, the action of frost in cracks, the low equilibrium angle (c. 5°) of wet boulder clay and the fact that the waves sometimes build storm beaches of fine shingle on the beach. Behind these ridges water may be impounded and seems to have a softening effect on the base of the cliff.

      This is but the modern continuation of a long process. In 1895 the British Association for the Advancement of Science appointed a committee to inquire into the rate of coast erosion, which some years later produced the following figures for Holderness.

Скачать книгу