How it was formed and why it is constantly changing. Nature’s method of smoothing, shaping and grading the Pebbles.
Like all other familiar objects known to man it is easily recognized but not easily defined. The usual dictionary definition, ‘a small stone rounded by the action of water’, has the merit of conciseness and is fairly accurate, but not every pebble has been rounded by water action and there is some disagreement among the authorities upon the degree of smallness which a rounded stone must attain before it can be classed as a pebble. Some maintain that a pebble can vary in size from a maximum of six inches in its longest diameter to a minimum of £ inch, the breadth of a pea, while others would accept a much larger range: from a rounded boulder to a minute stone little larger than a grain of sand.
Then there is the question of the stage of rounding which a stone must have reached before it can be regarded as a pebble. That question will always be unanswerable because there is an infinite number of gradations between the jagged fragment of rock and the smoothly rounded pebble and it is impossible to fix upon one point in the long process of shaping and smoothing as a clear division between a rock fragment and a pebble.
So we must content ourselves with the short and simple dictionary definition and assume that every one of the stones on a shingle beach is a pebble, complete or in the making.
The shingle beach is a familiar sight to all who dwell upon or visit tne coast of this island. There are few stretches of our coast without one. Indeed, we find shingle everywhere except on the mud flats of expansive estuaries, on beaches consisting wholly of sand, and in places where the sea, even at low water, does not recede from the cliffs.
To understand how pebbles come into existence in such vast numbers, by what means they are shaped and smoothed and how they come to be graded in orderly fashion, from large to small, on the beach, we must consider in some detail the formation of beds of shingle.
Firstly we must always bear in mind that a pebble is a transient thing. It is in the half-way stage of a long existence. Beginning as a fragment of rock, which itself is millions of years old, it ends its existence by being pounded into minute particles or grains. Similarly, the shingle beach which consists entirely of pebbles, is also transient, for it is constantly being moved along the shore by the action of waves. There are beaches, of course, which have large beds of shingle above high-water mark that appear to be permanent enough to harbour vegetation in the narrow spaces among the pebbles. Such shingle, however, must have been hurled upon the beach in gales of exceptional severity. It only awaits another severe gale to be dispersed in its turn.
Secondly, the shingle bed is fairly shallow. It lies on a base far more permanent than itself, usually a shelf or platform of solid rock. The sea cut out, or wore down, this platform when, long ago, it eroded the land on which the beach now stands: the resulting debris then formed the bed of shingle. The sea continued its attack upon the retreating cliffs, from which additional layers of debris came and still ceaselessly come.
Thirdly, with these two points in mind, we may assume that much of the shingle on a normal beach consists of fragments of the local stone torn originally from the cliffs that have long disappeared, and later from the cliffs that are still being eroded. But it would be rash to assume that this is true of all beaches. The effect of incoming and outgoing waves is to move pebbles along the shore at a rate which naturally varies with the strength of the wind, the force of the breakers and the slope of the beach. The rate, even in comparatively calm weather, is far from negligible. Consequently there is a steady drift of shingle along the shore. A beach in a deeply-cut cove enjoys a large measure of immunity from drift, while one on a coast not heavily indented suffers considerable drift. A headland jutting far into the sea on that side of the beach from which drift usually progresses acts as a barrier against it. It will hold up the migration of the shingle for a very long period, but eventually a storm will force some shingle round the corner and bring to the sheltered bay its quota of pebbles from distant beaches. The shape of the coastline will, therefore, be some guide to us in deciding whether the bulk of the shingle on a beach comes from the land that it fringes.
And now to consider in more detail the effects of wave action. Learned men have spent years in observing the behaviour of waves and have written abstruse mathematical treatises about them. These need not concern us, but we must understand the rudiments of wave action in order to appreciate its effects upon the movement, the shaping and the smoothing of pebbles.
Wind produces waves. It beats up the little undulations on the surface of calm water into waves. They then travel forward in the direction in which the wind is blowing. This appears to be so obvious as to be hardly worth mentioning, but it is not so simple as that. Although it is true that wind-driven waves travel forward, it is only the shape of the wave that travels in deep water; its substance remains almost stationary. The explanation of this is that every drop of water in a wave not near the beach revolves in a vertical circle, the drops on the crest of the wave moving forwards and those in the trough of the wave moving backwards. The water immediately in front then takes the shape of the wave, and so on, until the wave breaks near the beach. The breaking wave has travelled far, but the water of which it is composed has scarcely travelled at all.
If you desire proof of this you have only to throw out a small piece of wood from the beach beyond the breaking waves and you will at once observe that the crest of the wave carries it forward and the trough draws it back. Or, stand near a field of grown wheat on a windy day. Waves appear to pass right across the field, yet the corn remains rooted. The wave-motion in the cornfield closely resembles that of the sea.
Remembering this, we can now consider the effect upon the beach material of the incoming and breaking waves. From the very moment when the wave breaks it ceases, of course, to be a moving ‘shape’ and the mass of water rushes up the beach upon which, as we shall see later, it acts either constructively or destructively.
It will be helpful at this stage to grasp and to remember the meaning of three technical terms in use by those who study wave movements. They are ‘swash’, ‘backwash’ and ‘fetch’. The swash is the rushing water driven up the beach when the wave breaks. The backwash is precisely the opposite and, as its name suggests, is the water returning down the beach after the swash has spent its force. Just as the swash decreases in speed and force as it makes its way up the slope of the beach, so the backwash increases in speed and force as it rushes down. It naturally follows that if the beach has a gentle slope the swash will travel farther and act more powerfully, but if the slope is steep the backwash will be swifter and stronger. Therefore the swash will tend to push pebbles up a gently sloping beach, making it gradually steeper until at last it becomes steep enough for the backwash to become the more energetic and to drag the material down again, eventually making the slope gentle once more. The process can continue indefinitely.
Fetch is the length of the stretch of wind-swept water over which waves travel. The longer the fetch the larger the waves. Such waves, therefore, carry more material up and down the slope of the beach, and so the parts of our coast that he open to extensive seas are subject to fiercer and more sustained attacks than those which look out upon comparatively narrow stretches of water. For example, the south-west coast bears the brunt of the long Atlantic rollers which have a fetch extending far beyond the south of Ireland, and our East Coast is washed by the North Sea waves with a fetch as far away as the coast of Norway. The beaches of Kent and those of south-west Scotland, on the other hand, enjoy some immunity from far-fetched waves, the Continent being so near to the former and Ireland to the latter. Even where stronger winds blow over those narrow waters they produce smaller waves than moderate winds raise over an expansive fetch. The comparison breaks down, of course, when the narrow seas are lashed by local storms, but in all other conditions it holds good. If you stand on an open beach—that is, one not enclosed in a bay—and observe the incoming waves on any day when there is neither a fiat calm nor a raging storm, you will almost certainly notice that the line of the waves is not entirely parallel with that of the coast. In other words, the waves approach the shore obliquely. They do so because the wind that predominates over their fetch rarely blows at right angles to the shore, and so the waves come in at an acuter angle than one of ninety degrees.
As the waves break, their swash reaches up the beach carrying shingle and other material before them. But, because the swash comes in obliquely it will run obliquely up the beach and so will the shingle that it carries. After the swash has reached its limit, the backwash takes command and rushes down the slope of the beach, also bearing shingle with it. The backwash, however, returns by the shortest possible route, that is, straight down the beach. With almost every rising tide this process goes on. The steady inrush and outrush of water along the lines of these right-angled triangles transports the shingle, not merely up and down the beach, but along it. It is for this reason that we have described the shingle beach as a transient thing, for some portions of it are being moved along with almost every tide and nearly always in the same direction. ‘Longshore drifting’ is the name usually given to this shifting of beach material resulting from the oblique approach of waves to the shore. It is fairly regular and rhythmical, but the up-and-down movement of shingle can sometimes be spasmodic and violent. Storm waves, for instance, can reduce the gradient of a steep beach in a few hours. One terrific gale on the Dorset coast in 1852 altered the gradient of the famous Chesil Beach from about 1 in 4 to about 1 in 9 and shifted a huge mass of pebbles up the beach to form a conspicuous ridge, traces of which are still discernible after more than a century. But storm waves are not the only kind that alter the contours of a beach. Long observation of wave behaviour has shown that in fair weather most waves tend to build up the beach while others tend to drag it back. The former are, therefore, termed constructive waves and the latter destructive. The factor which determines whether a wave is constructive or destructive is the rapidity of wave-succession. If they break at the rate of about six to eight per minute with fair regularity, there is time for the backwash of one wave to recede before the swash of its successor rushes forward. These are the constructive waves. They push more shingle up the beach than they drag back down it. But if the frequency is greater, the breaking wave, receiving the thrust of its predecessor’s backwash, plunges down more vertically. Its swash is consequently more feeble and its backwash more powerful so it pulls more material back than it pushes forward. It is, therefore, called a destructive wave.
Here and there we must have noticed that the shingle bed has a scalloped edge along the margin nearest to the sea.
The curved ridges of pebbles are highest on the landward side and taper down towards the sea. The little bays usually vary more in depth than in width. The most feasible explanation is that cross-winds have been at work, producing the crossing of waves near the beach. The resulting swash does not flow in the usual long line up the beach but comes up in protruding tongues, making a series of indentations among the bed of small pebbles and the little bays broaden and deepen as the process goes on.
We must now examine the shingle itself and see how the sea has graded the pebbles. Many people are astonished to find that on most beaches they visit the biggest pebbles are at the top of the beach, the smallest near the bottom and, in between, those of intermediate grade. The explanation is simple. Firstly, the bigger stones offer a greater purchase to the incoming waves which can thus push them farther up the beach. Secondly, unless the beach has a steep slope, the swash is more powerful than the backwash. Before the backwash can exert a powerful pull, much of its volume of water will have sunk down into the bed of shingle over which the swash has flowed. The swash started with the impetus of the breaking wave, but the backwash begins the return journey down the beach with no impetus at all. The result is a grading of the pebbles by the tides, from the large stones approaching the size of boulders at the top of the beach, down through all the intermediate stages to the very little pebbles that appear to constitute a bed of gravel at the bottom of the beach. Below them again the sand stretches out beneath the water.
One heavy storm, of course, can upset this symmetrical array. It can flatten the slope of the beach, disperse the large pebbles at the top and drag them back in the powerful backwash of its waves. A large storm wave loses a smaller proportion of its volume of water in the sand and shingle when its backwash rushes down the beach and so it has more dragging power than a normal wave.
Even in long periods of tranquillity the pattern of the beach can vary, often from day to day, as the result of longshore drifting, a change in wind direction, ground swell or exceptionally high tide. Thus, for example, the belt of pebbles of intermediate grade may shrink or expand in width over a week-end.
But enough of the beach in general. Let us look more closely at the pebbles themselves.
If one were to devote a whole lifetime to the scrutinizingof pebbles one would be very lucky indeed to find two of them exactly alike, for, if they were identical in shape, they would almost certainly differ in texture, colour, surface pattern, degree of hardness or the nature of the rocks from which they originated. Various causes combine to give them their form: the nature of the parent rock, the original shape of the rock fragment from which the pebble has been shaped and ground down, the degree of hardness of that rock, the nature of the minerals it contains, its veining and jointing, the amount of material in it that is subject to decomposition or solubility, the position that the pebble occupied on the beach when it first began to be rubbed down by the scraping of other pebbles against it and the means by which it was transported (waves, currents, rivers, glaciers, etc.) to its present position.
As to their shape, pebbles fall into three groups, but between the first and the second, and between the second and the third, there is an infinite number of gradations: 1. SPHERES. In this class we include all those that are only approximately spherical, for a pebble that is a perfect sphere must be a rare find indeed. This is the smallest of the three classes, as the chances of a pebble being scraped and rubbed down uniformly on every part of its surface are slender and, even if the chances were greater, it would have to consist throughout of entirely homogeneous material with no joints, veins or fissures before it could attain this shape. A pebble consisting wholly of fine, hard grains, all of which can put up the same measure of resistance to scrapes, rubs and bumps coming from all directions, is likely to come within this class. Spherical pebbles are more common on beaches that are frequently swept by rough seas than in sheltered coves. Good examples are stones of homogeneous material that have been whirled continuously around, one against another, in river pot-holes. 2. OVOIDS. This is a much larger class. The word ‘ovoid’ means egg-shaped. As conditions are generally against the attainment of spherical form, it is but natural that the daily rolling to and fro of a beach pebble should make it egg-shaped. We may include in this group those of a much rarer shape—the cylinders. They are much longer than they are broad and have rounded ends. Probably they originated as fragments of a rock that tended to splinter upon being broken. 3. FLATTENED OVOIDS, or oval discs. Again a large class. It is fairly safe to assume that nearly all pebbles of this shape come from laminated rocks—that is to say, rocks made up of thin plates or layers. These plates all run in one direction and the stone can be cleft in that direction only. A common example of a laminated rock is slate. When a slate pebble is worn down by rubbing, it naturally assumes a flattened form. The rubbing also rounds off its sides. The outcome is a flattened ovoid. The spheres and ovoids are rolled along the beach by the tides, but the flattened ovoids are pushed to and fro. Spherical pebbles on sheltered beaches are comparatively few because the backwash rolls pebbles of that shape more easily than it can drag back flattened ovoids. The former are rolled back into deeper water; the latter are pushed up and down the beach.
From the above we can arrive at one important conclusion: the determining factor in the shaping of most pebbles by wave action is their texture—that is, the disposition of their constituent parts and the material of which they are made. The waves do the rolling, pushing, grinding, rounding and smoothing, but the material and texture of the pebble govern its form.
There are occasional exceptions. For instance, a pebble wedged in the underside of a glacier can have its protruding portion completely filed away. Glaciers have carried many pebbles to our beaches. Then, again, sand constantly passing over a pebble that has become firmly fixed in a river-bed can wear its upper surface down flat. Driven sand has a filing power that is astonishing. A not uncommon sight on our beaches is a line of sand-dunes at the back of the bed of shingle. Sand-dunes consist of extremely fine grains which, in strong winds, produce a natural sand-blast, and this, in course of time, can file the pebbles down and give them curiously contorted shapes.
Though shingle is present on most of our beaches, there are some shingle beds that are specially noteworthy and that provide exceptionally good opportunities for the study of beach formation and movement.
Foremost of these is Chesil Beach, once described as the ‘most extensive and most extraordinary accumulation of shingle in the world’. It may not be the most extensive in the world but in other ways it is probably unique. Civil engineers, geographers, geologists and geomorphologists have contributed articles to learned journals about it, while many non-technical writers have described it in rapturous and romantic terms.
The beach runs south-east along the Dorset coast from Bridport to the Isle of Portland, an uninterrupted stretch of 18 miles. The Isle of Portland, jutting 2 ½ miles out to sea, acts as a natural groyne and impedes the shifting of the shingle to the east. Chesil Beach is not a beach in the usual sense of that term because, in its last 10 miles, from near Abbotsbury to the Isle of Portland, it is a ridge of shingle with water on both sides, the sea on the southwest side and a lagoon called the Fleet on the north-east for most of the way. Beyond the lagoon the shingle ridge proceeds seawards and links up the Isle of Portland with the mainland. Both its width and its height increase in its last 10 miles between Abbots-bury and the Isle. At the former it is 170 yards wide and 23 feet high and at the latter 200 yards wide and 43 feet high.
Chesil Beach possesses several striking and fascinating features: 1. Its pebbles, particularly those on its seaward side, are graded with an accuracy that is astonishing. They increase steadily and uniformly in size all the way from Bridport to the Isle of Portland, ranging from tiny pellets the size of a pea at the former to stones of 5 or 6 inches in diameter at the latter. It has been stated that, if a blind man who had known the beach from boyhood were taken in a boat to any part of it, he would only have to pick up one pebble to know precisely on what part of the beach he stood. Experiments with broken bricks have proved how inevitably and accurately the process of sorting goes on. Pieces of brick were chosen because they are easily distinguishable from the shingle. They were placed on the beach at various points between high-and low-water mark. Every one of them in due course made its way to its appropriate place on the ridge and lay side by side with pebbles of the same magnitude.
The inference to be drawn from these experiments is that longshore drifting by wave action is responsible. But if that is so, why does not longshore drifting produce equally precise sorting on other long stretches of shingle? So far there has been no complete answer to that question. Further prolonged and detailed study of wave action may provide one. 2. Equally extraordinary, if not more so, is the behaviour of the pebbles below low-water mark on the seaward side of the beach. As they are always submerged they are almost hidden from view, but it is apparent from all the observations made of them and their movements, that they are also graded, but in the opposite direction! Evidence of the truth of this is not entirely complete. A long series of diving operations would be necessary to confirm it. Various theories have been advanced in explanation of the phenomenon, but a completely convincing one has yet to be put forward. 3. Nine-tenths of all the pebbles are flint. This in itself is not surprising, as flint-bearing cliffs of chalk lie to the west at intervals between Seaton and Sidmouth, but their contribution to Chesil Beach seems to be out of all proportion to their extent. It is possible that those cliffs are the remnants of a huge mass of chalk that was eroded by the sea ages ago and now lies beneath its surface and that the flints it bore have been subsequently dragged by tidal action eastwards to form the beach. Pebbles of limestone and chert, a stone that resembles flint, form the bulk of the deposit at the Isle of Portland end. The rest of the shingle consists of beautiful quartzite pebbles from Budleigh Salterton; jasper, carried to the sea by the river Otter; granite from Cornwall and some pebbles that were originally fragments of rock entirely foreign to the south-west coast.
Another impressive stretch of shingle, one of the finest in Britain, is on the Suffolk coast. It extends for 11 miles from the pleasant seaside resort, Aldeburgh, in the north, to the appropriately named Shingle Street, in the south. All this long beach consists of shingle only. Sand is barely visible. One of its curious features is that it presents an insurmountable barrier to the river Aide in its efforts to reach the sea. The river comes within about 50 yards of the sea south of Aldeburgh but is held back by the bank of shingle and runs behind it until the shingle peters out at North Weir Point, 11 miles to the south-west. There the frustrated river at last flows out to sea. Nearly half-way between Aldeburgh and North Weir Point the coast rather sharply alters its southerly course and turns to the south-west. The corner or foreland thus formed is shown on maps as Orford Ness, but the Ness is really the whole 11-mile stretch of shingle. On this coast longshore drifting proceeds southwards, as the waves are driven in by the dominant winds from the north-east. The shingle ridge has consequently extended itself gradually in that direction. There is evidence that it grew in the 700 years ending in 1897 about 5^ miles, or 13-14 yards every year. It suffered a setback in 1897, when a violent storm shortened it by a mile, but it has since renewed its advance.
Orford Ness displays another interesting feature, which is to be seen in a few other large deposits of shingle. At the foreland the shingle is deeply furrowed, giving the impression that a gigantic plough has been at work upon it. The ridges and hollows are roughly parallel with one another, the former being known as ‘fulls’ and the latter as ‘swales’. South of the foreland they run along the shingle bank for several miles. Each of the fulls probably represents a stage in the growth of the shingle bank, or ‘spit’, as it is technically termed. The popular conception of a spit is a narrow ridge of sand projecting seawards or running parallel with the beach. There can be spits, however, consisting entirely of shingle and, of these, Orford Ness is the outstanding example.
Large stretches of furrowed shingle can be seen on two other parts of the coast. East of Eastbourne, in Sussex, is the extensive pebble beach known as the Crumbles. At one part it is furrowed into 60 fulls, all running parallel with the sea, some of them being more than a mile long. Walking north-eastwards along the beach from Eastbourne towards Pevensey, one becomes aware that there has been some grading of the pebbles, for they tend to become smaller as one proceeds.
Going much farther east, we come to Dungeness, lying between
Rye and Hythe on the Cinque Ports coastline. On that coast are spread out the greatest stretches of shingle in this country. The term ‘ness’ is derived from an Anglo-Saxon word meaning nose. Dungeness, one mass of shingle, is a nose pointing out into the sea. This famous foreland has been the subject of a considerable literature and numerous experts have tried to elucidate its complex structure. One coast of the ness, or nose, faces south, the other east. Waves driven up the Channel from the west meet the former obhquely and cause longshore drifting towards the tip of the nose. The shingle which is eventually driven around that point is thrown up on the east coast of the ness by waves blown thither by north-east winds. The fact that the tip of the nose continues sharp is thought to be due to the short distance between it and the French coast, towards which it directly points, as waves travelling straight across from France to the nose have insufficient space in which to become destructively large.
The furrows of the Dungeness shingle span its whole breadth. These appear to be innumerable fulls and swales lying parallel to its east coast. Those running into the centre of the ness and on to its southern shore are arranged in a more complicated pattern. The probability is that every one of them marks the boundary between sea and shore at one stage or another of the building up of the ness.
Almost all its pebbles are of flint. The few interlopers are of sandstone and quartzite, some of which have travelled far.
Two kinds of shingle beach that are not sea-washed at all deserve our scrutiny. They are raised beaches and lake shores. 1. RAISED BEACHES. Around the coastline of the British Isles, but elevated some distance above it, are beaches that the sea has deserted long ago. Some of them have not felt its impact for centuries, some for thousands and some for millions of years. How has this come about? Well, clearly, either the land must have risen or the level of the sea must have declined since the formation of the beach. Both these changes and their opposites have occurred, not once but many times. Here and there along our coast traces of tree-stumps near low-water mark prove that either the water level has risen or the land has suffered depression, while the existence of raised beaches demonstrates the reverse of both these alternatives. There is geological evidence that, on the whole, it has been the variation in the sea level rather than that of the land which has raised the beaches and submerged the forests. The process is still going on, though almost imperceptibly. For example, part of the Swedish coast, on the Gulf of Bothnia, rose 19£ inches during the last century and is still rising. Our own south-east coast, on the other hand, is losing height relative to that of the sea, but so gradually that the rate of dechne cannot be measured with complete accuracy; it is estimated at less than -J-inch per annum.
Raised beaches and submerged forests can be found on the same piece of coast, the raising and the lowering having occurred at different times. In most cases the submergence took place after the elevation, so the original elevation must have been that much higher.
The major cause of the relative changes in the height of land and sea was the enormous expansion of the polar ice-cap about one million years ago and the consequent spreading of huge sheets of ice over large areas of Europe, North America and much of the intervening seas. (a) To produce these thousands of square miles of thick ice an incalculable amount of water was necessary. Where did it all come from? Obviously, nearly all of it must have been drawn from the sea by evaporation. The result of this loss of water was a drop of some hundreds of feet in sea level all over the globe.
Beaches previously formed along the coastline were left high and dry. (b) Then the terrific weight of the massive sheets upon so large a part of the earth’s surface would subject the outer crust to a severe strain and depress the surface. The depression would not be uniform. It would be much more severe in some areas than in others, but the general effect would be to restore to some extent the difference in the relative levels of land and sea caused by (a). (c) Finally, with the melting of all the ice comes a universal rise in sea level, followed more slowly by a rise in the depressed land level.
In our part of the world there have been no violent fluctuations in the relative levels of sea and land since the effects of the final melting of the ice wore away, and that was about 10,000 years ago. The sea then began to build new beaches on our coasts at a level not greatly different from the present one.
It has for long been customary to classify the raised beaches of Great Britain into three grades to which the terms ‘25-foot beach’, ‘50-foot beach’, and ‘100-foot beach’, are applied, but this is an over-simplification, as in all three classes there is considerable variation in height. In searching for these deposits we must reahze that, in the ages that have elapsed since the sea built them up, vegetation has overgrown and now completely obscured many of them. The towns of Dundee and Greenock have been built upon such beaches.
Here are a few which still preserve the character of beds of shingle: Brighton provides an excellent example of one, 30 feet above the sea-washed shingle; at Portland Bill there is a notable one of 65 feet in height; one of 20 feet is at Hope’s Nose near Torquay; at St. Ives in Cornwall, Barnstaple Bay in Devonshire, near Weston-super-Mare, at St. Helen’s in the Isle of Wight, and on the Gower Peninsula in South Wales are other noteworthy examples. But Scotland has the most numerous and most impressive raised beaches, some of them consisting of large spreads of shingle. They range in height above present sea level from 10 to about 100 feet. The greater concentration of massive glaciers there in the Ice Age, and the heavier strain they exerted upon the land surface, probably account for the number and height of the Scottish raised beaches. 2. LAKE SHORES. Lakes, even small ones, have their beaches. Winds disturb their surface and produce little waves which roll small stones up and down the lake’s marginal strip of sloping land. But a lake is landlocked and has no tides, so the rolling of pebbles to and fro is insignificant in comparison with the effects of sea-wave action. The great grinding mill to be seen at work on the sea beach is scarcely noticeable at the lake-side. There are, of course, enormous lakes in other parts of the world which have so large a ‘fetch’ that their pebbles are transported along their shores by wave-driven winds and are, in course of time, shaped and smoothed. Lake Michigan, for instance, shows ample evidence of the capacity of a landlocked and non-tidal expanse of water to produce longshore drifting of beach material.
It follows from this that the waves on the surface of a British lake take a much longer time to convert a rock fragment into a pebble. Yet one finds on the shores of many of our small lakes well-fashioned and beautifully-smoothed pebbles. The lake, however, is not entirely responsible for the production of all of them. One of several other agencies than lake waves could have rubbed them into pebble form. Here are two possible ones:. (a) Giant glaciers in the Ice Age scooped up the hollows in the gravel which later became filled with water and which now form the basins of many of our lakes. The pebbles, gathered up by the glacier in its journey, and already fully shaped and smoothed, fell into the lake basin when the ice melted. (b) We usually find that a lake is fed by a river or by mountain streams. They could have carried small stones over a long distance. On the journey those stones, bumping and rubbing against others in the river-bed, would have assumed the form of pebbles before they reached the lake. The process of rubbing and shaping would then slow down very considerably but it would go on, the little wind-waves gently moving the pebbles, perhaps no more than a fraction of an inch in the course of a day.
Naturally, we expect to find that nearly all the pebbles on a lake beach are composed of the same material as the local rock. Those that are not must have been carried to the lake by the action of glaciers or rivers; if by the latter they will probably be made of one or other of the rocks of the region through which the river has flowed; if by the former they may originally have been pieces of a rock hundreds or even thousands of miles away.
Many people are bewildered when they come upon pebbles at the top of a hill far from the coast. Can it be, they ask, that the sea once covered the land to such a depth? That possibility must not be ruled out altogether, because there have been some astonishing vertical movements of the earth’s surface in the course of geological time. The British Isles lay under the water in the Cambrian Age, between four and five hundred million years ago, and did not completely emerge from the sea until that immense span of time was nearing its end. Nevertheless, so much of the surface has been subsequently worn away by weathering action that the chances of our finding any traces of a sea beach on a hilltop far from the coast are remote.
River pebbles still abound on high ground above the river that shaped them, because the river has cut deeper and deeper into its valley and has greatly lowered its bed. The Thames, for instance, eroded its valley at one period to a depth of 80 feet and river pebbles, rounded by the Thames thousands of years ago, have been found on the 100-foot contour line. Yet that height is insignificant in comparison with the distances to which glaciers have raised pebbles. One extraordinary example of glacier-borne pebbles transported inland to a great height is to be seen on the slopes of Moel Tryfan, a mountain in Snowdonia. There the pebbles He 1,400 feet above sea level. The glacier which bore them had scraped them up from the floor of the Irish Sea into the heart of Snowdonia and deposited them there when it melted. So it is not very surprising after all to discover pebbles on a hilltop. If there are traces of glacial action in the vicinity you may be fairly certain that ice was the agent that brought the pebbles up the hill and left them there. One of the delights of the pebble-seeker is to discover the objects of his search in the least likely places and to determine whence and by what means they came to be there.