CHAPTER 2 THE PHYSIOGRAPHICAL BACKGROUND
A Résuméby J. A. STEERS
THIS BOOK is concerned with the ecology of the sea-coast and the seashore. The various types of ground that come under this broad title are subject not only to constant change but often to violent change. Even the hardest cliffs are comparatively unstable, and almost always subject to strong winds and storm-waves which may do much superficial damage, even if the actual rate at which the cliff retreats is, in terms of human life, extremely slow.
Great Britain has a remarkably long and intricate coastline and a long and varied geological history. Strata of nearly every period are well represented. These rocks, and the associated igneous rocks (also of very different ages) give the coast great interest and variety. We can observe the white and often perpendicular cliffs of the Chalk, the magnificent ranges of dark red cliffs of Old Red Sandstone in Caithness and Kincardine, the grey walls of Carboniferous Limestone with their flat grassy tops in west Pembrokeshire, the rapidly wasting cliffs of glacial deposits of north Norfolk and Holderness, the heavily glaciated cliffs of the whitish-grey Lewisian Gneiss, alternating with those of the brown and often spectacular Torridon Sandstone in north-western Scotland, and the granite cliffs of Land’s End and the Isles of Scilly. These are but a few examples from many; the point is that the rock type alone—quite apart from whether the beds are folded, broken, horizontal, or cut into diverse forms by marine or sub-aerial erosion—makes the coastline extremely interesting.
CLIFFS
In the preceding paragraphs the word cliff has been used but not defined. The Oxford English Dictionary gives the following meanings for the word:
1 A perpendicular or steep face of rock of considerable height, usually implying that the strata are broken and exposed in section; an escarpment.
2 (especially in modern use) A perpendicular face of rock on the seashore, or (less usually) overhanging a lake or river.
These definitions are descriptive, comprehensive, and do not presuppose any particular origin. Only too often it is assumed that because cliffs face the sea, they are wholly the result of marine erosion. The mere presence of plants on a cliff face is not, of course, evidence that no erosion is taking place; it may suggest that erosion is fairly slow, or that it may operate by large and infrequent slips. But a steep, grassy slope running down to near sea-level may be quite untouched by marine erosion. Some of the so-called Hog’s Back cliffs of north Devon are only affected by the sea in their lowest parts; their upper slopes have been produced in some other way (Pl. V).
Many cliffs are found at the back of a flat or platform where they are no longer washed by the waves. Along the Durham coast this is so, and still more along many miles of the coast of Scotland, especially in Galloway, Kintyre, Arran, Ayrshire, and many other districts. Sea-level, relative to the land, has altered since the platform was formed, and where the alteration is considerable, the old cliff may now be well away from the sea and its vegetation only indirectly influenced by it. The boulder-beach at the cliff-foot south of Duncansby Head in Caithness indicates a slight change of level, since the boulders are lichen-covered and are somewhat above the normal height which wave attack reaches. In the Gower peninsula, the limestone cliffs run right down to sea-level, often in unbroken slopes. But in many small inlets traces of raised beaches are found, and it follows that the outer cliffs, even if they do run down to and below sea-level, cannot wholly be the product of modern marine erosion. In parts of Scotland there are sometimes two or three old beaches found in the same place, often with a normal shingle flora. Between the mouth of the Findhorn and Burghead (Moray Firth) there is a great series of ridges, all well above the present sea-level. To-day the sea is cutting into them and making a shingle cliff fifteen or more feet high. Their natural vegetation closely resembles that of modern beaches, but has undergone changes as a result of its more stable position. In many other places around the Moray Firth, especially between Hilton of Cadboll and Tarbat Ness, and in various localities on the west coast, particularly on Islay, Colonsay, and Jura, magnificent expanses of shingle-flats and old cliffs are to be seen.
In other places the sea-level has risen relative to the land, so that the lower valleys are flooded, and what were cliffs are now submarine slopes. Much of the beauty of Pembrokeshire, Cornwall and Devon depends on the influx of the tides into long and intricate inlets like Milford Haven, Plymouth Sound, the Camel estuary and the mouth of the Dart. These were carved first by rivers and streams on the land, later drowned by a relative rise of sea-level. Only their outer cliffs have been modified by marine action; the sheltered inlets now often contain salt-marshes. The pulsation of the tides into their innermost recesses, and the intimate relation between land and sea vegetation create beauty.
In western Scotland there is another type of inlet, the sea-loch or fiord. Many of the narrow straits between islands or between the mainland and adjacent islands are similar. These lochs usually owe their present appearance to ice action which has scoured out, widened, and perhaps deepened and straightened pre-existing river valleys, which sometimes followed lines of weakness produced originally by faulting. The sides of these lochs and straits are cliffs in the sense of the dictionary definition, but are not the product of marine erosion. Up the fiords there are often salt-marshes, and at low water the expanse of golden, brown, red and varicoloured seaweeds, surrounded by the higher marsh plants, under a strong sun is unforgettable.
Cliff-form is usually closely related to structure. If the beds are more or less horizontal and thick only one may form the cliff, but more often two or more drop out. At Hunstanton the brown Carstone overlain by the Red Chalk and this again by the White Chalk makes a spectacular cliff, subject to rapid erosion because of the ease with which the sea eats into the soft Carstone, thus producing falls. Near Lyme Regis, and in Glamorgan, the nearly vertical cliffs are composed of thin beds of limestone and clays or shales arranged horizontally. Elsewhere, the beds may be steeply inclined seawards or landwards, folded or faulted, and the form of the cliff will depend much on the trend of the cliffs relative to that of the folds. The rocks of the Isle of Wight and the Isle of Purbeck are strongly folded in an east-west direction. The cliffs along their south coasts run with the beds which, seen from the sea, appear almost horizontal. But if one sails round Durlston Point to Poole Harbour, or between Ventnor and Ryde, the beds are seen on end, and the way in which the sea has cut into the softer ones is evident. Where folding is acute and intricate and the rocks are hard, the sculpture is often bizarre. A view from a boat close inshore between Boscastle and Hartland Point, or between Berwick and Cockburnspath, or around the west of Pembrokeshire reveals details of surprising interest.
Where all the rocks are soft and perhaps geologically young, changes go on at a quicker rate than where the rocks are harder and older. Changes are rapid between Flamborough Head (Pl. XXVI) and the Isle of Wight. If the Angles and Saxons could revisit the country they certainly would not recognise it; on the other hand, the Phoenicians might see little difference in the Cornish coast, except near St. Michael’s Mount. Even now there is good reason to suppose a slow sinking of south-eastern England. Since the end of the Ice Age there have been great changes in the levels of land and sea, and these have often had a more profound effect on the present appearance of the coast than have erosion and accretion.
BEACHES
If the beach along a fairly straight coast is examined, it will usually be noticed that if shingle is present it is at or near the top. Sand and finer particles occur lower down, and usually the fineness
