A pebble begins its life as a rock fragment that has become detached from the parent rock by some natural agency. The usual cause is the battering of the shore cliffs by rough seas, but the sea is not responsible for all the damage. The weather, very slowly but very surely, breaks down even the hardest rock. Sun, wind and rain gradually etch its surface. Water, freezing in the crannies of the rock, exerts a powerful leverage and, in the course of many years, splits it ever more widely and deeply, while vegetation, rooting in the split surface, contributes to the slow disintegration.
Let us briefly consider the process of rock erosion by the sea on a beach backed by cliffs. Day in and day out the waves batter the base of the cliffs, gradually and steadily undercutting them. The smallest cracks in the rock aid the destructive effect of the water, as the inrush suddenly compresses the air in the cleft and the out-rush suddenly releases it. Thus the cracks are widened and the explosive force becomes all the greater. At last the undercutting brings down part of the face of the cliff with a rush and a pile of broken rocks lies at the bottom of the cliffs. For a time this heap of debris acts as a defensive barrier against the incoming waves, but once the sea has begun to level it down, the broken rocks help in the attack. They are a supply of ammunition with which the waves can increase their battering force. The pace of erosion is quickened as the process of undercutting is resumed.
The waves have other supplies of ammunition at hand. In rough weather the beach pebbles take a share in the bombardment. Even the submerged rocks below the low-water mark can help the attack. On a shallow shore the waves break some distance out from the beach. They scour the bottom, wrench out stones embedded in the sand, wear down the rocks that outcrop from it and carry all this eroded material forward. Some of it is sharp-edged, like the fragments that have fallen from the cliff-face, and causes a particularly destructive impact. The mass of fragments that fall to the beach by wave erosion or weathering grows ever greater and the first stage in the life of the pebbles is over.
Then begins the long, slow process of converting the jagged, angular fragment of rock into the smooth and fairly round pebble of the shingle beach. We have seen how the recurring tides, day after day, year after year, and century after century, roll the stones of the beach, one against another, with endless and remorseless regularity, grinding and smoothing them and even sorting them in accordance with their size. How long does this process take? It is impossible to estimate it with any accuracy, because rocks vary so much in their degree of hardness and other circumstances, which we will consider later, have to be taken into account. If the beach cliffs consist of a very hard compact rock such as basalt, which is of volcanic origin, they will not only stand up to the onslaught of the waves more sturdily than softer rocks but also resist more stoutly the weathering action of sun, wind and rain. And when at last disintegration begins, the rock fragments withstand far more tenaciously the efforts of the waves to grind and smooth them. But if the sea cliffs are of sandstone, the process is much shorter, for only the most compact sandstone is tough enough to maintain a very long struggle against both weather and tide. Indeed, a pebble of soft sandstone would be ground down to nothing long before wave action could rub a single corner off a fragment of basalt.
Sandstone is a striking example of a life-cycle ending as it began and is, therefore, comparable to the ‘dust to dust’ cycle of man. The sand grains of which it is composed may have been laid down hundreds of millions of years ago at the bottom of some sea or lake which subsequently disappeared. The grains meanwhile had become a compact mass through being held together by a cementing material, either silica or iron oxide. Then, as we have seen, the fragment of sandstone is broken off the sandstone cliff, becomes in course of time a pebble and, as it is rubbed down by the waves, resolves itself at last into the separate grains of which it was composed. If this earth lasts for millions of years again, the cycle may repeat itself.
Perhaps the greatest of contrasts is to be found in the pebbles that are formed out of chalk cliffs; the ‘White Cliffs of Dover’ and the chalk of the Yorkshire coast. The chalk contains numerous nodules of flint. It is comparatively soft and is easily crumbled, but flint is hard and unyielding. The shingle resulting from the wearing down of the cliffs is, therefore, rich in flint pebbles, very durable and very attractive, but the chalk that had held them in its embrace for some scores of millions of years has a much shorter life once it has been torn from the cliff and become a fragment on the beach.
If you walk along any of the beaches that he below the white cliffs of chalk you will notice that many of the flint nodules have not had their corners completely rounded off. They are still to some extent angular and have yet to be shaped by wave action into the mature beach pebble. You will rightly conclude that the hard flint is strenuously resisting the efforts of the waves to grind and shape it. Unlike the sandstone and the limestone pebble, the lump of flint is not composed of tiny fragments. It is a solid mass of unyielding silica. In time, however, the rubbing and grinding process will shape it into a fairly round pebble; it will become smaller and smaller and will at last disappear. The minute fragments into which it has been filed down will become sand grains and will join the uncountable millions of grains that form the sands of the sea. These vast stretches which, it has been computed, form a border to nine-tenths of the coastline of the world, are a colossal pebble cemetery, for much of them consists of pulverized pebbles. A sand beach that has been formed by the breaking down of flint pebbles and of other stones comprised of silica, such as quartz, is said to consist of ‘sharp’ sand. It is so called because the grains are angular and some of their edges are sharp. Why is it that the pebbles of which they once formed a part were rounded, but the grains are sharp-edged and angular? That was their shape when they were broken off the parent pebble and they have kept it because an extremely thin film of sea water separates one grain from another, preventing them from rubbing one another down.
But even sand is not the ultimate stage in the lingering death of a pebble. The last is the conversion of sand into silt. The minute particles that make up a deposit of silt are very much finer than the smallest sand-grains. When the deposit is very thick the upper layers press heavily upon the lower layers and consolidate them gradually into stone. Many of the rocks of this country have originated in this way. One of the commonest is shale and we shall encounter many shale pebbles in our beach wanderings. The life-cycle of a shale pebble is mud-shale-mud, for beginning as mud, it becomes shale in the course of ages and is then slowly worn down again through the centuries into the fine particles that help to make up a deposit of mud.
So far we have been considering only those pebbles which originated as rock fragments torn from the sea cliffs, but every beach contains pebbles which have reached it by a longer route and we must now give them some attention. It would be substantially true to say that the sea cliffs or, in the absence of cliffs, the land adjacent to the shore, contributes by far the highest proportion of pebbles to all of our beaches, but the other contributory sources supply a share by no means small or unimportant. The study of beach pebbles would be very much easier and very much less fascinating, if all of them were derived from the land at the back of the beach, because all we should then have to do would be to determine the nature of the local rock and we should at once be able to name every pebble on that beach. The task of identification would not be quite so light if the local rocks were not all of the same kind. This is not at all unusual. As some rocks are harder than others, the sea makes a greater inroad upon the softer rocks and this is one of the main causes of the indentation of the coastline. Coves and bays mark the successful attacks of the sea upon the softer rocks and so we should expect to find upon the beach a higher proportion of pebbles derived from them.
Some of the pebbles that are not of local origin have not made a long journey, others have travelled some scores of miles, while there may be some that have come hundreds of miles. Let us see how they make these journeys.
Firstly, one important agency in the transportation of pebbles is river action. If a river makes its way to the sea on, or within reasonable distance of, your beach, you may be sure that pebbles drawn from the rocks through which the river has made its course will be present on the beach in fairly large numbers. A river is a very effective pebble-maker. Even a little stream is not to be despised as a pebble source, for the power of running water is astonishing. It carries away so much of the land into the sea as to make one wonder how long it will be before the world’s rivers have intermingled land with sea completely. For example, the rivers of the United States of America alone transport annually 800,000,000 tons of rocks and soil into the Atlantic and Pacific oceans. Setting aside the soluble salts that a river carries away in solution, we may say that it transports material in two ways: it carries fight and small substances such as sand grains, silt and particles of mud; and these, in a fairly swiftly flowing river, neither float nor sink but are held, so to speak, in suspension; and it also bears along with it larger and heavier material, chiefly stones, dragging them along the bottom. The first of the two is called the suspended load and the latter the traction load, because it is drawn or dragged. The suspended load of sand, silt and mud goes to form the bed of the estuary and to spread itself out on the lowest slopes of the beach, much of it descending below low-water mark. The traction load is an important contributor to the pebbles on the beach. American authorities on river transportation have calculated that the Mississippi carries annually into the Gulf of Mexico a suspended load of 136 million tons and a traction load of 40 million tons. Much of the traction load must consist of rock fragments and pebbles, so we are forced to the conclusion that the beaches of the Gulf of Mexico receive many millions of tons of pebbles every year from the Mississippi.
Our own, much smaller, British rivers make, of course, a much more modest contribution to our beach deposits, but it is a far greater one than is popularly supposed. Let us see how they contrive to be pebble-makers and how the pebbles make their journey to the beach.
A river erodes or eats away its own bed in two directions, downwards and sideways. It drags away material from its bed and from its sides; the steeper the descent of the river, the faster is its flow, and the faster the flow the more material does it hack out from its bed and sides and carry to the sea. Thus a river may be said to be a sculptor of the land surface. In our temperate climate, with its fairly heavy rainfall, the rivers of Britain, though comparatively short, are numerous and generally swift and they are consequently, except during droughts, very busy transportation agents. They are, therefore, said to possess fairly high powers of abrasion. This term, derived directly from the Latin abradere, ‘to shave off’, simply means the power of rubbing away or wearing down. The actual process of the cutting away of material by a river from its bed and sides is called corrasion, from the Latin corradere, ‘to scrape’. Experts who have devoted themselves to the study’ of erosion by rivers have computed that the corrasive power of a river varies as the square of its velocity. This formula may bewilder the reader who is not mathematically minded, but the calculation is very simple. Let us suppose that heavy rain has trebled the velocity of a river, and let us consider the effect of that increase of speed upon a rock embedded in it. The number of fragments of stone and other material that now scrape and bump against that rock will be multiplied by 3, and every one of them will strike it with threefold violence. Consequently the corrasive, or scraping, power of the river has increased to 3 x 3 = 9 times, or to the square of 3, or—to put the same thing in the way the formula has it—as the ratio of the square of the new velocity to the square of the old.
As for the transporting power (and a moment’s thought will enable you to see the difference between that and the corrasive power) the increase is even more startling. Earnest students of the subject have conducted experiments in rivers of many kinds and under diverse conditions and have reached the conclusion that the biggest object that running water can carry varies as the fourth, fifth or sixth power of its velocity. This means that, if the velocity be trebled, it can transport objects at least 3x3x3x3 =81 times as large. The trebling of the velocity of a river pouring down a steeper slope than the normal might enable it to carry away objects 3x3x3x3x3 x3 = 729 times as large.
Well, these figures may seem incredible, but we must bear in mind that, in ordinary conditions, a river can roll only small fragments along its bed and that, when the size of these is multiplied by 81 or even by 729, it is not enormous. It becomes enormous only in times of disastrous floods, when rivers burst their banks, alter the whole surface of the countryside and sweep away trees, walls and houses. Our mathematical formulae, which at the best are only very approximate, must then be abandoned, because the river is no longer confined to its bed and has become a wide flood.
And now for the birth and life of the pebble formed in a riverbed and carried to the beach. Let us suppose that the stream, by its corrasive power, has torn away from its bed or its side a fragment of stone. That fragment, as in the case of the piece removed from the sea cliff, will be rough and angular. It will at once sink to the bottom and will remain there until heavy rainfall has raised the speed of the current to the point at which it can move it, or, in more technical language, at which it is capable of bearing a traction load. The fragment bumps its way along, scraping against its fellows, and against other stones embedded below or on either side. The grinding and shaping process has already begun. It could have begun earlier if the river had its source on a mountainside and had dislodged the fragment on its way down to more level ground.
The rain ceases and settled weather sets in. Our infant pebble sinks down again and has an undisturbed life until rain again quickens the river into fresh activity. So the process continues until the pebble reaches the sea to receive its final shaping and smoothing by wave action on the beach. It may, of course, have become a mature pebble before it completed its journey. One of the many deciding factors is, naturally, the length of the river; another is the hardness of the rocks through which it flows and against which it has scraped the pebble, but, of all the agencies which convert the rock fragment into the river pebble, the most potent is the pot-hole.
Here and there in the course of a stream there are eddies in which the water has a whirling motion. The eddy sucks into its centre several jagged fragments of rock. This at once creates a crude drilling machine. The stones caught in the vortex swirl around, scraping one another and deepening the circular hollow in the bed. The pot-hole becomes the perfect pebble factory. In all probability it is the most efficient and rapid of all pebble-makers. Heavy rain will sooner or later raise the speed of the stream and carry away the rounded or partly rounded pebbles on their way to the sea, but, as the velocity decreases, other stones will take their place in the pot-hole to have their corners removed and to be ground down in their turn.
Hence it is clear that a pebble may reach the sea already shaped and smoothed and that this process and the journey downstream may take a comparatively short time. On the other hand, a rock fragment that has not been sucked into an eddy may have to wait for its final conversion into pebble form until the waves that pound it to and fro on the beach have completed their work. Other fragments, borne along by sluggish streams which pursue a meandering course, may take ages to complete their seaward j’ourneys. Some are diverted into quiet backwaters where they may lie undisturbed for countless years until a surging flood hurls them out. So many and so diverse are the circumstances to be taken into account that it is impossible to estimate the average time taken by a piece of rock to travel from the upper reaches of a river to its estuary and thence to join the other pebbles on the beach. All that we know is that under the most favourable conditions it may take a few weeks, while under the least favourable it may take many centuries.
Secondly, another and far more powerful agency in the transportation of pebbles is, or rather has been, ice action. It is necessary to speak in the past tense of the carriage of pebbles by ice because it ceased in all parts of the world thousands of years ago, except in the Arctic and Antarctic regions and among extremely high mountains.
In order to gain a clear understanding of pebble transportation by ice we must give some attention to the effect of the ice ages upon this country. You will note, in the first place, that Britain underwent several ice ages. For many years geologists keenly disputed with one another upon the number and duration of these periods of glaciation, but they have now accumulated enough evidence of the advance and retreat of the vast ice sheets that spread into Europe from the Arctic circle to agree among themselves that there were altogether four ice ages and that, from the first advance of the ice to its last retreat, something like one million years elapsed. It appears, too, that the ice made its final departure from Britain about 10,000 years ago.
We must note also that man had already arrived here, though in an early stage of his evolution, before the first advance of the ice and that he had become Homo sapiens before its fourth and last departure. To the geologist, who has a time scale of at least 2,000,000,000 years, an event occurring a mere 10,000 years ago is more recent than last month is to the historian.
Another and most extraordinary feature of the ice ages is that between the second and third there was a long and very warm period. Fossils of creatures that lived in Britain at that time reveal the amazing fact that southern mammals, the elephant, the rhinoceros and the hippopotamus, roamed the land and warm-water molluscs inhabited our seas. Altogether the glacial and the inter-glacial periods form so attractive a chapter in the geological history of this country that one is tempted to dwell upon it, but we must confine our attention to the transportation of pebbles by ice.
At the time of maximum glaciation, which was probably the second Ice Age, an enormous sheet of ice covered this country from the far north of Scotland to as far south as a line drawn from the Bristol Channel to the valley of the Thames. The line does not follow the course of the Thames strictly but turns a little northward, misses London and comes out on the East Coast near Harwich. The surface of the country north of this line contains bountiful evidence of ice action.
Many British visitors to the Continent have seen Swiss glaciers. Compared with the glaciers that crunched their way across this country they are utterly insignificant. For instance, the glaciers of Snowdonia were some 1,000 feet thick. The polar icecap had spread itself southwards in all directions. A huge sheet of ice stretched from Scandinavia across the North Sea. Like a gigantic bull-dozer the glaciers and ice-sheets tore over the land, ripping all obstacles out of their way, scraping and scouring the rocks and carrying the rock fragments for enormous distances. Many of these fragments were frozen into the body of the glacier and thus enjoyed a comparatively tranquil journey, but others became wedged into the sides and the bottom of the glacier. In this position they provided the moving ice with a set of teeth with which to tear and score the ground. The glacier became a giant file, its rock teeth rasping the surface it traversed. The small glaciers of Switzerland appear to be stationary. They move very slowly indeed, but the great ice-sheets that sculptured the surface of this country moved with some rapidity. Of an incalculable weight, and studded with rock teeth, they broadened and deepened the valleys and levelled undulations in the ground. When the great mass thundered over a surface of rock, the screeching teeth cut furrows in it. The technical term for such furrows is ‘striations’ (Latin: stria, ‘a groove’).
As the Ice Age neared its end, the melting of the vast ice-sheets began, but the earth and rocks they had scooped up on the way could not melt. They were deposited on the ground. Some had travelled short distances; others had been carried for hundreds of miles. It must, therefore, follow that, if you find a piece of rock in an inland region that possesses no rock at all of the kind you have found, you have come upon an instance of ice transportation. That piece of rock had been torn out of its natural setting by the glacier, frozen into it and carried along until rising temperature stopped the ice and melted it. Geologists call large blocks of such rocks ‘erratics’, because they have erred or wandered from the parent rock. There are innumerable erratics in Great Britain. For example, there are certain kinds of granite in Galloway and Ailsa Craig, in Scotland, that are not to be found in any rock formations in southern Britain, yet erratic blocks of this granite have travelled to Cheshire, Lancashire, North Wales and the Isle of Man. Boulders of a rock peculiar to Merionethshire have been transported to Staffordshire and Warwickshire, while blocks of a Norwegian rock, laurvikite, have travelled in and across the North Sea ice-sheets to the coast of Yorkshire.
The passengers carried by the ice were not only separate pieces of rock. The main load was a mass of debris torn up from the surface of the region traversed by the ice. This debris was a mixture of powdered rock and crumbled soil, a sticky compound, in which innumerable pebbles and boulders were embedded. When the Ice
Age ended, the melting ice-sheets left enormous deposits of this boulder clay, as it is called by geologists. It lies over most of Lowland Scotland, northern and central England and East Anglia, and in some places it is hundreds of feet thick. Consequently, many of our sea cliffs are either capped by it or are wholly made up of it, and so the beaches below them have a ready-made supply of pebbles close to hand.
Among the pebbles on the beach are many that have travelled directly thither as ice passengers. We should naturally expect to find them on the coasts north of the line defining the most southerly advance of the ice. Yet some of them are present in the shingle beds on coasts south of that line. How can this have happened? Well, clearly, a second transporting agency must have been at work. One that we have already considered, river action, could have helped in the later stages of the journey, and one to which we must now give some attention, the transporting of pebbles to, and along, the coast by the tides, could have carried them for even greater distances than the ice.
Thirdly, the movement of pebbles by sea water is the most powerful of all the transporting agencies. A pebble is of relatively lighter weight in water than in air and is more easily moved in salt water than in fresh.
Until very recent years the movement of pebbles along the coast was thought to be due to longshore currents, I.e. currents running roughly parallel to, and near the shore, but patient investigation has proved the action of waves and not of currents to be mainly responsible. Currents are capable of transporting fine sand and silt but not pebbles. On the other hand, if the current is strong enough to be classed as a tidal race, it can easily move small pebbles for long distances, but tidal races off our coasts are not numerous.
On many of our beaches we see that groynes made of stout timber or concrete have been fixed at intervals along the shore at right angles to the incoming waves. This is man’s device to maintain the stability of his beaches by hindering the drift of sand and shingle along them. One glance at the beach on either side of one of these groynes enables us to decide the direction of the drift, for the sand or shingle is very much higher on the side against which the drift moves. In the absence of groynes the general configuration of the beach should be a reliable guide, but, if we are in doubt, we must fall back upon our memory of the following little summary of the prevalent direction of drift along the coasts of England and Wales. (a) Along the south coast it is from west to east. (b) Along the east coast it is from north to south. (c) Along the west coast it is from south to north.
There are, alas, exceptions to this as to almost all rules, but they are easy to remember. The important ones are 1. A large stretch of the Norfolk coast runs E—W. Here the prevailing drift westward of Sheringham is westwards in the direction of the Wash. 2. Much of our heavily indented west coast runs in an easterly direction, e.g. the coast of North Wales. On all these stretches the prevailing drift is eastward. 3. Rule (c) does not apply to that part of the west coast which lies between Walney Island and St. Bees Head. From the latter to the former the drift is southward.
So long as we remember that the three statements (a), (b) and (c), and the three exceptions to them, are for our general guidance and that purely local conditions may make them inapplicable to certain beaches, we shall not go far wrong.
Because wind produces waves, and because waves produce the drift of shingle along the shore, we seem to be forced to the conclusion that the direction of the prevailing wind in any part of the coastline will determine the direction of the drift of beach material along it. The argument may be logically sound but is nevertheless misleading. We have to distinguish between winds that are prevalent and winds that are dominant. The latter have much more powerful influence than the former upon the drift of shingle. The prevailing wind along our coasts is from the south-west. It speeds the long Atlantic rollers that approach the Cornish coast from the south of Ireland, and, because it happens to be also the dominant wind, it drives the waves up towards the English Channel and the Irish Sea, causing the eastward drift along the south coast from Land’s End to Dover, and the northward drift along the west coast from Land’s End to the Solway Firth. But on the east coast, although the south-west wind is the prevailing one, it has little effect on wave action because it blows out to sea from the land. The dominant wind there comes from the north-east over the North Sea, causing the waves of that huge expanse of water to beat from that direction upon the east coast and to make its shingle drift southward.
The transportation of beach material for great distances along the coast is not confined to the shingle that lies between high- and low-water mark. It includes also the submerged material lying below the latter. As on land, there are many outcrops of rock on the sea bottom. The sea very gradually erodes them by scouring them with sand and by dashing fragments of rock against them. It has been proved by the experiments of students of current action that a current flowing in the sea at a higher speed than 2 ½ feet per second can move fairly large pebbles along the bottom if the bottom is a stretch of sand. We have already noted, however, that fast currents are not very common, so the amount of material transported by their agency is only a fraction of the vast amount moved by the action of waves on the beach. That fraction, however, is far from insignificant. The English Channel, for instance, holds the waste material of the land that once, and not so long ago in geological time, joined this country to the Continent. That huge deposit is an endless source of supply to the shingle beaches of the Channel coast.
Seaweed also makes a small contribution! It attaches itself to under-water pebbles very firmly, grows towards the surface of the water and is helped by the air-vessels it contains, to float. A very familiar sight on most beaches, roughly between high and low-water mark, is dark brown seaweed bearing bladder-like growths.
One of the childish delights of a seaside holiday is to produce little explosions by banging these small bladders between two flat stones. The commoner kinds are called bladder wrack and knotted wrack. In the former the bladders grow in pairs; in the latter the bladders are larger and grow singly. The bladders give great buoyancy to these weeds in the water. The buoyancy and their tenacious hold upon the pebbles to which they have attached themselves help them to drag large stones for long distances.
And now we come to the last of the transporting agencies: man himself. Let us suppose that a ship bound for a British port from Australia takes in ballast before the voyage and that this ballast, as is often the case, consists of shingle. When nearing the home port she is driven shorewards by a gale and breaks up on submerged rocks some little way out to sea. In the course of time she disintegrates. Then, over a much longer period, the shingle is very gradually borne ashore to be distributed over a long stretch of coast. The result is that pebbles from the other side of the globe have made their way to a British beach. If the parent rock from which they were formed in Australia differs in its composition from rock in this country, they will present a problem of no little magnitude to the geologist and the geographer, to say nothing of the amateur pebble-hunter.
Then, again, man builds sea-defences such as breakwaters and sea-walls. He also builds moles, jetties and promenades. If the local stone does not lend itself to the construction of these works because it cannot easily be shaped into rectangular blocks, he transports stone from a distant quarry. Some years pass and then pebbles of that stone are found on the beach. It is, naturally, much easier to determine their origin than to identify the fragment of Australian ballast, but, like all other strangers to the beach, they give the inquiring searcher enough food for thought to make his quest interesting.
Thus we see that the journeys of a pebble through time and space can vary enormously; from a few yards to thousands of miles and from a few months to measureless ages. At the one extreme we have the pebble of soft sandstone, coming into existence as a fragment of the sandstone cliff and being rapidly ground into sand-grains on its own beach by the waves. At the other we have the piece of Norwegian rock borne by ice to our shores many thousands of years ago and still lying there ten thousand years after the melting of the ice.
The cycle goes endlessly and steadily on. The finest grains become compacted into solid rock. Millions of years later the encroaching sea, aided by sun, wind and rain, breaks up the rock. A pebble is born. The waves roll it along the beaches from Cornwall to Sussex. It is resolved into sand-grains and then the whole process starts again and another cycle of millions of years begins once more.