LITMIR.BIZ

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it will be convenient to discuss certain special conditions.

      If the land is formed of soft rocks and is low and flat, the waves will begin to cut a slight notch in it. The notch implies a cliff and a platform, which develop concurrently. If the initial slopes of the land are gentle, the cliffs are not likely to attain any great height, but the platform can widen considerably. But in order that it may widen, it must also slope seawards. In the early stages the cliff and platform are insignificant, and only those waves reach the cliff foot which can traverse the shallow water off shore. With the increasing width of the platform, the waves crossing it lose much of their power. At the same time the larger storm waves will break near the outer edge of the platform and gradually wear it down, thus enabling other waves to traverse it. A factor of great importance in this connection is the tidal range. At high tide the platform may be covered by several feet of water and the waves can run over it and erode the cliff. At low tide the platform may be wholly or partly exposed. This statement immediately poses a difficult question. At what level are platforms cut? There is no definite answer to this, and the difficulty is not lessened if we consider cliffs and platforms around our own coasts, because it is not always clear that they are entirely the product of present-day conditions—in fact we may be reasonably certain that they often have a long and complicated history associated with former shifts of sea level.

      Some writers argue that platforms are cut mainly at and near high water. In order to think about this matter it is advisable to disregard any existing platforms and consider what might happen in an ideal case. Suppose in Figure 9 AB represents average high-water level, and CD average low-water level. (This in itself is an improper simplification, since in a tidal cycle all levels between these two selected ones, as well as some above or below them, are reached). It is clear that the waves can erode effectively at all levels between the two lines. In early stages we may probably assume somewhat irregular erosion, but since it is only at and near the time of high water that the higher levels can be attacked, it is plain that erosion near the AB level is associated with high water. But at low water all or nearly all above the CD level is exposed and cannot then be eroded by the waves. With the rise of the tide, waves, at first small, but gradually increasing with the increasing depth of water, can traverse the slope DB and it seems likely that the most effective erosion of this slope must take place, other things being equal, at and about high water.

      FIG. 9 Cutting of Platforms

      The bigger waves can attack it in its outer parts and gradually wear it down, and smaller but still effective waves can traverse it and attack the cliff in the rear. In ordinary conditions the waves need not be large and can travel over it easily: the differentiation in the size of the waves really only applies to the contrast between storm and normal conditions—but it is under storm conditions that the real work of erosion is so often accomplished.

      It may, however, be argued that platforms are more easily cut below average low water level. It is true that below this somewhat arbitrary level the sea floor is nearly always covered with water and therefore in a position to suffer erosion. Moreover, at high water, the bigger waves can travel over it and add their quota to the erosion that takes place at (and near) low water. This may be true, but simple low water erosion presumably implies that the cliff foot should begin at that level and, except in so far as beach deposits may obscure it, at high water the cliff foot should be well submerged.

      Since we are uncertain about the levels at which platforms are cut, we must either ignore, or at least refer with care, to those around our own coasts, so that we do not beg the question. However, it is likely that in soft rocks existing platforms are largely, perhaps wholly, the result of present conditions, e.g. the Garstone platform off Hunstanton and the Chalk platform around parts of Kent and Sussex. If that assumption is correct, it is almost certain that the effective cutting takes place only above mean sea level and especially near the time of high water. Low water cutting of many existing platforms at springs is impossible, and even at neaps is negligible.

      Another point demands consideration by field workers. If it is assumed that an existing platform is of present-day origin and is formed mainly by waves at high water, is there any reason for thinking that waves at low water are forming a lower platform at the seaward front of the visible one? Or are waves at low water, or at mean sea level, merely rounding off the edge of this flat? These are easy questions to ask, but so far there appears to be no conclusive answer.

      This digression has taken us away from our original theme. Suppose the sea comes to rest against a strip of coast of hard rocks, and suppose that the land slopes steeply into deep water. The waves will cut a notch—but how? On the previous pages a soft rock coast is assumed and in consequence there would be plenty of fine material, mud, sand, and small stones, with which the waves could attack the cliff and platform. It is true that the direct attack of the waves on a hard coast may be considerable, but it will only be so if the rocks attacked are much weathered or riddled with holes, clefts, and crannies in which the air can be suddenly compressed and released and so lead to fracture of the rocks. Waves, without sand and other ammunition, which are attacking a hard rock coast sloping steeply into deep water will have but little effect. They are reflected back from the coast, and owing to the rise and fall of the tides, to say nothing of the variability in size of the waves, the rocks are attacked through a considerable vertical range, and NOT just along a narrow strip. Thus, there is no very good reason why a notch, gradually giving way to a narrow platform and small cliff, should be formed unless conditions remain static for a long period of time.

      This at once provokes the question—what is a cliff? Definitions vary, but the following is from the Shorter Oxford Dictionary: “A high steep face of rock; esp. (now) a steep face of rock on the seashore.” This is a good definition, since it does not specify origin, In the preceding paragraphs, and in many writings on physical geography or geology, sea cliffs usually imply marine erosion. If this is so, we are bound to meet difficulties sooner or later. It is clear that if we associate cliff and platform together, then we must imply that the cliff is an erosion feature. The point, however, is worth making, since it is difficult, perhaps impossible, to avoid ambiguity. We speak quite properly of the cliffs of Torridon Sandstone at, e.g. Handa Island, Point of Stoer, Rhu Coigach, and Greenstone Point, so that it is difficult to avoid speaking in the same (erosive) sense of the steep slopes of the Sound of Sleat or Loch Hourn, neither of which is or has been appreciably affected by marine erosion.

There remain, however, long stretches of the coast of Great Britain fringed by well-developed platforms and backed by fine cliffs. Perhaps the most beautiful examples are seen between Bude and Westward Ho! Many miles of platform, cut out of contorted rocks of varying hardness, are exposed at low water, but covered at high water, at which time erosion of the cliffs, at any rate in times of storm, may be severe. These platforms may be the work of waves at present sea level, but they may equally owe part or even most of their formation to conditions existing when the more recent raised beaches were formed. On the other hand, if the Patella¹ beach is studied in parts of south Cornwall and south Devon it will be noticed that there are often remains of a definite platform, frequently covered by Head. The Patella beach, however, is probably inter-glacial, and if the present rock platforms are not wholly of modern origin, they may perhaps be associated with more recent fluctuations of sea level than those applicable to the Patella beach,

      To return, however, to the cliffs. Their form varies with many factors. First of all there is the topography of the original land, which may be flat and low-lying, rugged, rolling, or mountainous. The sea will come to rest against the land at a given level, and the “high steep faces of rock” will depend first on what the land is like. Later marine erosion may modify the original slopes, provided that wave action can be effectively directed toward them.

      If waves attack a land formed of more or less horizontal rocks there is the likelihood of nearly vertical cliffs being produced. This is especially noticeable if the various strata are of unequal hardness. Good examples occur at Hunstanton, Lyme Regis, and on the coast of Glamorganshire. If the strata composing the cliff dip seawards, the cliff form is likely to vary greatly in detail. Sometimes it may overhang,

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