The Sea Coast

CHAPTER 4

THE COAST IN PROFILE AND PLAN

BEFORE discussing particular stretches of coastline formed by rocks of various types, a general review of the nature of cliffs and of the shore profile is desirable.

Let us assume that the sea comes to rest against the land at a certain level, and that that level remains unchanged for a long period of time. What will happen depends on a variety of factors, and to make the matter clear 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, but the general slope of the surface above the part directly attacked by the sea may well leave a stronger impression on one’s mind than does the lower part of the cliff, which is likely to be steep. If the strata are inclined away from the coast the cliff slope is as a rule moderate. These simple cases can easily be visualised. In nature the form and structure of cliffs are very variable. Chalk Cliffs (see here) are often nearly vertical independently of their structure. This is presumably the outcome of the effects of marine and subaerial erosion on a rock of great homogeneity, good jointing, and often marked bedding. The profile of the cliffs of north Devon varies a good deal, but is locally steep—immediately north of Hartland Quay there is a range of high and almost vertical cliffs which are made up of sharply folded beds. On the other hand, soft rocks often form steep cliffs. The boulder clay cliffs of Holderness and of north Norfolk, or again around Criccieth and Afon Wen are soft and easily eroded by the waves. For this reason they are undercut and steepened. But local composition and height play their parts. The glacial cliffs near Sheringham are c. 100 feet high and steep, often tumbled, and composed not only of boulder clay, but sand and gravels. The natural angle of slope of these materials, even if partly consolidated, will not allow of verticality. Farther north the lower and, on the whole, more homogeneous Holderness cliffs sometimes approach the vertical.

In parts of north Devon and Somerset the slope of the land to the sea is decidedly steep. It is not in any sense a cliff of marine erosion, and the effect of the action of the sea can be noted at the bottom of the slope, which has been cut back into a low cliff. Comparisons between various parts of the Cornish coast bring out differences of this sort clearly. The relatively sheltered parts of the southern coast are fringed by low cliffs, whereas on the exposed parts of the northern coast there are fine and bold ones. But here again it is worth pausing to consider their origin. It looks as if these cliffs are entirely of modern formation, i.e. since the present levels of land and sea existed. But is this so? In order to illustrate this point turn for a moment to the Gower Peninsula, where there are some fine limestone cliffs at the foot of which are many traces of raised beaches. These remnants, whilst commonest in sheltered spots, are not only found in such places. A similar raised beach platform may have existed along north Cornwall; the remains of beaches in the Camel estuary and on the south coast are suggestive. Thus, it may be that the existing Cornish cliffs are far from being wholly produced under modern conditions. It is more than probable that they are but the slightly modified forms of much earlier formed cliffs, just as are those in the Gower peninsula. The same is true of the magnificent Old Red Sandstone cliffs near Duncansby Head in Caithness. The Geological Survey Memoir notes that the stacks seem to rise from a low platform, part of which may be found at the cliff foot. These are merely examples; many others could be given. They serve to show that our knowledge of cliff form and evolution is still in an elementary condition. More studies, such as that by Miss Arber1 on the cliff profiles of Devon and Cornwall, are wanted before we can make any real advance in our knowledge of cliff scenery.

Before leaving this question, consider the western coast and islands of Scotland. In Chapter 8 it is shown that much of this area is all that remains of a former great plateau of lava. The disposition of the lavas and sills in Skye, Eigg, Mull, and other islands shows that these rocks once covered far greater areas. In Skye the cliffs are often largely or wholly formed of lavas and sills. No part of the island is fully exposed to the Atlantic, but nevertheless severe storms attack its shores. On its more exposed south-westerly facing side, there are some magnificent cliffs near Talisker Bay and Loch Brittle. Waterstein Head, farther north, is an imposing cliff of lavas and sills. At the foot of the major cliff is a low projection which is eroded by the waves; it does not follow that the major cliff above is similarly formed. In the comparatively calm waters of Loch Bracadale there are several instances of vertical cliffs to which it is impossible to ascribe an entirely marine origin. The east coast of Skye facing the island of Raasay is steep, and the cliffs are for the most part made of lava flows and sills resting on Mesozoic rocks, and fringed by a boulder beach. Raasay Sound is narrow and protected: it is difficult to suppose that cliffs along it are simply formed by wave action—in fact the green, steep slopes of the Mesozoic rocks of Skye imply that marine erosion is negligible. Judd many years ago showed that a fault probably runs along this Sound. It is known that faulting and foundering have broken up all this former volcanic area. Thus many lines of steep cliffs in lavas and sills are primarily fault-formed, and only secondarily modified by wave action. Since it is not for a moment supposed that the sides of sea lochs are the product of wave erosion, there is no reason for thinking that in places like the Sound of Sleat or the Sound of Mull, marine erosion has accomplished serious cliff erosion. The steep cliffs fringing so many parts of Skye and other islands, in relatively protected waters, are probably no more than the slightly (often very slightly) wave-modified slopes produced in some other way, in western Scotland sometimes by faulting.1

Another feature which throws light on this problem is often seen in Scotland. As shown in Chapter 9 the present coast may be fringed for miles by raised beaches, usually by the 25-foot beach. This bench, which may be two or three hundred yards or more wide, forms almost a promenade attached to the land, and is backed by the old grass-grown cliffs. It is being eroded by the waves today; only where it is poorly developed have the waves cut through it and are now attacking the cliff behind, which is in those places clearly a composite cliff. How long this raised beach platform took to form is not known; it has not been greatly worn away in recent times and the cliffs cut in it are merely low crags and quite unlike the old cliff behind the beach, or the often numerous stacks on the beach platform. It is true that the coastline of most of England and Wales fails to show raised beach remains like those in Scotland, but the evidence in Scotland, Cumberland, Gower, Cornwall and elsewhere, makes one hesitate in ascribing many other English cliffs wholly and automatically to modern conditions.2

Cliff scenery varies greatly from place to place. In Chapter 5 an analysis is given of certain lines of cliffs. Here there is only need to emphasise the importance of major structural lines. If a series of gentle folds runs more or less parallel with the coast, cliffs may be eroded along them, whilst at some other locality they cut across the folds (see here). The form of cliffs also depends largely on the structure and nature of the land in which they are being cut. Much of the beauty of a cliffed coast depends not merely upon the details of marine erosion in various rock types, but on the occurrence of rivers debouching into the sea. In Lyme Bay there are a number of small streams which reach the sea by percolating through the shingle. The valleys have been cut down to, or even below, sea level, but the present streams are not powerful enough to keep their mouths clear. The streams at Chideock, Charmouth, and Burton Bradstock are examples. On the other hand powerful rivers usually have wide open mouths. There are, however, other factors to consider. The Stour and Orwell in Essex, the Ouse and Arun in Sussex, the Tamar and Fal in Cornwall possess many similarities, but also exhibit important differences. It is true to say of all that the existing streams and conditions could not have produced the present features. In each case there has been a drowning of the lower part of the river, and since the river mouths may separate lines of cliff, they add enormously to the beauty of the coast. A walk along any cliffed coast may be made rather more strenuous by these breaks, but it is unquestionably more interesting.

If, however, the streams are small or if for some other reason their downward erosion has not been able to keep pace with either the change of level or with the inward erosion of the sea, the valley will have its mouth part way up the cliff face, and the stream in it will fall into the sea by one or more cascades. Coastal waterfalls are often extremely beautiful. The finest examples in England and Wales are between Boscastle and Westward Ho!1 The watershed along this coast runs close to the sea, so that all the streams are short. The nature of a fall depends mainly upon whether the stream is draining a flat-topped cliff or one of the hog’s-back type. The former type, under natural conditions, may be spectacular, the latter rather uninteresting, e.g. falls at Glenthorne and Woody Bay. Litter Water has a vertical fall of 75 feet, and illustrates the rapid incutting of the sea. But some examples of this type of fall have been altered as a result of landslips; Hobby Water and Cleave Fall have, in fact, been obliterated in this way. On the other hand, the dip, strike, and hardness of the rocks play an important part. Milford Water illustrates this extremely well. It is one of the bigger streams, and there are really five individual falls. The uppermost (see Pl. III) is a dip fall, at the bottom of which there is a right-angled turn and the stream flows along a gutter following the strike. There is then another right-angled turn to form a short steep fall running contrary to the dip (the one at the bottom right of Plate III) and beyond it are two smaller falls.

In one or two cases the streams used to run in their lower courses roughly parallel with the coast. Erosion cutting into the more vulnerable parts of the cliffs more quickly than in other places has succeeded in interrupting their courses, so that a stream may now cascade into the sea rather higher up its valley than was originally the case. The lower valley is now dry. The valley of Speke’s Mill has been truncated by the sea near St. Catherine’s Tor.

There are numerous other examples. In the Old Red Sandstone coasts of eastern Scotland falls are not uncommon; that in the geo1 at Crawton is a perpendicular cascade. But perhaps the finest coastal waterfalls in Scotland are in the lava areas of the Western Isles. The northern part of Skye is almost wholly formed of lava flows and sills, except that on the east coast the lavas overlie more or less horizontal Mesozoic rocks. The streams draining the interior often tumble over the abrupt cliffs in beautiful cascades. The fall of the stream draining Loch Mealt is well known, but others of like nature occur in Loch Bracadale, Talisker Bay, and elsewhere. At Invertote and Bearreraig Bay, both on the Sound of Raasay, are two handsome falls. In the first the stream has cut through the lava, and cascades over the Mesozoic rocks; in the second the lava is still largely intact. Waterfalls of this type usually imply small streams; a major river might have cut down to sea level. Moreover, their volumes fluctuate a great deal with the rainfall. Since the streams are short and drain but small areas, they are best after heavy rains.

Another feature that has a great effect on the form of cliffs are the coastal plateaus (see here). The lower ones, and presumably the newer, are the best preserved, and often form extremely level surfaces cut across rocks of different types. The plateau at 180 feet in the Tenby peninsula is very sharply cut (see Pl. XXIII).

Whatever their origin, the cliffs cut in the lower platforms necessarily show a flat and even crest line, a feature clearly visible in parts of South Wales and Cornwall. Sometimes traces of older platforms can be found inside the one now forming the cliff top, and appear as flat-topped cliffs inside and above the modern ones. It is relevant to note here that rivers cut down through these platforms and have their mouths at sea level. Thus the great powers of marine and fluviatile erosion in comparatively recent geological times can be contemplated.

It would be possible to expand this account of cliff form almost indefinitely. Each line of cliff has its own special features, and all are worthy of study. Nevertheless the reader will easily imagine variation of form and will undoubtedly know of particular cases. The boulder clay on the chalk at Flamborough Head, the soft sandy cliffs in the Crags and Westleton Beds of the Suffolk coast, the easily eroded Tertiary rocks of Bournemouth Bay, are all steep, all soft, but all quite different. The possibilities are endless, but each in its natural condition is beautiful and interesting.

There is another way in which cliffs can be considerably modified. If circumstances are favourable landslides may take place. There are several well-known examples, but that at Dowlands, east of Axmouth, is perhaps the most impressive. In order to appreciate the causes which produced the slipping, the following table of rock succession is relevant:

The dotted lines represent unconformities. Briefly, the cause of the slips depends on the dip, the unconformity beneath the Gault, and the relation of both to sea level. If the junction plane between the Foxmould and underlying clay occurs above sea level, and if it also slopes seawards, erosion of the cliff face removes the outward support of the beds, and so the upper layers slide forward over the lower after periods of heavy rain. This is what has happened at Hooken, and between Axmouth and Lyme Regis. At Beer Head and Whitecliff the unconformity is nearly all below water, and since the cliffs above are wholly of Cretaceous rock, falls of chalk drop directly on to the shore. Slipping of this sort produces undercliffs, the form of which depends among other things on the angle of dip, and also on the nature of the rocks which have slipped. If the rocks are coherent, large unbroken masses may slide down, as at Dowlands; in clays and softer rocks a species of mud glacier may be produced.

Near Dowlands the inland cliffs are of chalk, and the undercliffs are made of much disturbed Cretaceous material. The great chasm was formed in 1839. Since the previous June there had been much rain, and several gales. Fissures and cracks began to appear on the cliff top before Christmas, 1839, and on December 23 one of the cottages began to subside. By 5 a.m. on the 25th it was settling rapidly, and other cottages soon followed suit. The great slip itself occurred on Christmas night. “During December 26 the land that had been cut off by the fissures in the cliff-top gradually subsided seawards, and by the evening had reached a position of equilibrium in the undercliff. A new inland cliff, 210 feet high in its central portion and sinking to east and west, had thus been exposed, backing a chasm into which some twenty acres of land had subsided. The length of the chasm was about half a mile, while its breadth increased from 200 feet on the west to 400 feet on the east.”1 In all, some eight million tons of earth foundered. The movement also caused a ridge of Upper Greensand (Foxmould and Cowstones) to rise in the sea near the beach; it was about three-quarters of a mile long, and reached 40 feet above sea level at high water. The beds were much broken, and the mid-part of the ridge was connected to the mainland by shingle. The reef very soon disappeared, but the main chasm remains much as it was, except that most of the large pinnacles have gone. Nearly the whole of the area is now covered with vegetation, and the relative movement of the strata has produced a very uneven terrain and varied plant habitats. In Dowland’s Chasm, ash has sown itself abundantly and in several places natural ashwoods, including quite large trees, have developed. They are fine examples of self-sown virgin woodland, a rare phenomenon in present-day Britain. There is no part of the British coast on which there exists such a great extent of varied wild vegetation—a rich field for future research.