When the sea abuts against a land mass, no matter what the form of that land may be, it begins to attack it, and naturally the weaker rocks suffer most readily. A great deal, however, depends upon the arrangement of the strata in relation to the sea, in other words, on the structure of the land mass.
In the early stages, the waves will make the coast more irregular than it was initially, since they are cutting and trimming all the more easily removed parts. In course of time the waves smooth and round off projections and with the material thus produced cut cliffs and build beaches. It is sometimes contended that beaches of various kinds are best developed in the youthful stages of shoreline evolution, and that as time goes on the coast is more and more simplified so that the original great variety of beaches gives way to simplicity or even monotony. Text-books sometimes depict the various stages of evolution as in Fig. 10a, b, c, d. This evolutionary view is broadly correct, but the diagrams are only of general appplication.
Perhaps the nearest equivalent in these islands to the type of coast illustrated in Figures 10a, b, c, d is that of south-western Ireland,1 or, on a smaller scale, the coast of Pembrokeshire.1 Both these may be regarded as in the stage of youth. Another stage intermediate between 10c and 10d is represented by the coast between Start Point and the River Dart (Fig. 11).
FIG. 10—Diagrams illustrating the theoretical evolution of a coastline of submergence. Modified from W. M. Davis (From P. Lake, Physical Geography, 1949)
The rocks are relatively soft, and smooth shingle and sand beaches occur at Bee Sands, Hallsands, Slapton, and Blackpool. The coast between Speymouth and Rosehearty on the Moray Firth is an example of one formed of hard and often metamorphic rocks, all much folded. The folds run almost at right angles to the coastline and the detail is crenulate in pattern. Along east Norfolk and Suffolk the cliffs are composed of Pliocene Crags and more recent gravelly and sandy beds which physiographically differ but little from the crag. All are soft and the sea has cut back and regularised the coast, and built beaches which may dam back the small streams or deflect the larger ones. The same type of coast occurs in Essex, but at first sight it looks very different because marine forces have so far not been able to build bars of sand or shingle across the river mouths, and the coast remains decidedly irregular. This contrast is worth bearing in mind, because useful though schematic diagrams are, they often give somewhat misleading ideas, especially if an attempt is made to fit them to an actual stretch of coast.
A distinction is usually made between submerged and emerged shorelines, and perhaps neutral shorelines may also be added as a separate class. Theoretically this is sound; but in practice it is not always so easy to apply the terms, or rather to say that a particular sequence of events is characteristic of either submergence or emergence. A land mass sinking down so that the waves come to rest against a contour line is more than likely to have an irregular shoreline, whereas a shoreline resulting from the uplift of part of a shallow sea floor will probably be fairly simple in outline, especially if deposition has had time to blot out any irregularities that may have existed.
In a general way it is true to say that throughout these islands the coastline is a submerged one. But it is not so merely as a result of one particular movement of land or sea which brought the sea against the land at the present level. Even now (Chapter 9) there is slight movement still in progress. Our shores are submerged mainly on account of the rise of sea level in post-Glacial times. There is every reason to believe that immediately after the Ice Age, sea level was about 200 feet lower than at present. But it also follows that since the Ice Age was characterised by more than one advance and retreat of the ice, the sea level before, during, and after the Glacial period must have fluctuated considerably. Hence, our coasts have been alternately uplifted and depressed relative to sea level, and it by no means follows that they are back at precisely the same height at which they stood in pre-Glacial times.
FIG. 11 The Coast between Start Point and the River Dart. (Based on Ordnance Survey)
Since effective cutting back of a coast by marine erosion implies a Fair degree of exposure to wave attack, and so to open sea, it follows that many miles of the more sheltered parts of the western coast and islands of Scotland, intricate though they may be in detail, owe little if any of their outline to wave action (see here). On the other hand, softer rocks in fairly sheltered positions yield quickly. Along our south and east coasts the main wave attack is somewhat oblique to the general trend of the coastline. It is, however, along these coasts that the effects of differences in the hardness of the rocks are well seen. The old and hard rocks of much of Cornwall and Devon yield but slowly, and the coast from Start Point right round the Lizard and Land’s End and north-eastwards to perhaps Hartland Point can be regarded, in plan, as in a young stage of development. But even in this long stretch it is interesting to note how the hard igneous rocks often form capes and headlands. The same is true, but on a smaller scale, of that part of Pembrokeshire between St. David’s and Strumble Head, where it is astonishing how intimate is the connection between the details of the coastline and rock type.
Along the Channel coast the Mesozoic and Tertiary rocks of the Hampshire Basin, Isle of Wight, and the Weald produce an irregular coast, which may run parallel with the folds as in the south of the Isle of Purbeck, or cut directly across them on the east and west of the Isle of Wight, or truncate them obliquely as in the eastern Weald. Taken broadly, the south coast shows a fairly smooth, even outline in marked contrast to the intricate pattern of Devon and Cornwall, because the sea has been able to wear back the rocks and make sweeping lines of cliffs. Beach material travels along the coast and forms smooth beaches at the foot of cliffs, deflects bigger streams by shingle bars, and blocks little brooks, and at Dungeness and other places collects in great shingle forelands, the seaward sides of which show smooth and sweeping outlines.
There is no need to discuss further examples of the evolution of the coastal plan in this chapter; they are more appropriately dealt with in other contexts.
1 See page 260.
1 Geogr. Journ., 114, 1949, 191.
1 It is not implied that, even if this is the case, the present cliffs represent actual fault scarps. They may perhaps be regarded as fault-line scarps,
2 Along much of the Durham coast the cliffs are not of modern origin in the sense of being formed wholly by the sea at its present level. Low flats occur in front of several Unes of cliff, and stacks wholly or partly above present sea level are found on them.
1 E. A. N. Arber, The Coast Scenery of North Devon, London, 1911. See also W. G. V. Balchin, Trans. Roy. Geol. Soc. Cornwall, 17, 1946, 317.
1 In northern Scotland geo, or sometimes goe, is a term applied to a deep and narrow inlet, usually with steep sides, and cut in hard rock. They are extremely well developed in the Old Red Sandstone of Kincardine, Caithness, and the Orkney Islands. The voes of Shetland are somewhat similar.