Their varieties of composition, texture, structure, colour and hardness. How to distinguish the commoner kinds of pebble one from another.
We now come to the heart of the matter: the identification of pebbles. All who have read the previous
Textbooks on the subject tend to be forbidding, as they are written in a scientific jargon comprised of long words derived from Greek, Latin and German. Doubtless, this terminology is essential to the professional geologist and the serious student of the subject, but it is baffling to the layman. In recent years some short and comparatively simple books for the general reader have appeared.
There are three classes of rock: igneous, sedimentary and metamorphic. Every rock in the world must be one of the three. 1. IGNEOUS ROCKS, as their name (Latin: ignis, ‘fire’) suggests, are those which were formed under conditions of intense heat by the solidifying of molten material. This material has either been erupted by volcanic action out of the earth’s crust and then has quickly cooled or been forced up towards the surface, beneath which it has slowly cooled. Thus, we see that the igneous rocks are of two kinds: the volcanic, or extrusive, which are erupted in liquid form out of vents, such as the craters of volcanoes, and cool quickly; and the plutonic (so called after Pluto, the god of the underworld), or intrusive, which have been forced up, again in liquid form, towards, but not through, the earth’s surface and in that position have cooled very slowly. Millions of years later, when that surface has been worn away by weather, the plutonic rocks are exposed.
Now let us glance briefly at each of these two kinds of igneous rock and select a few of their common varieties. (a) Volcanic. We have no active volcanoes in Great Britain to-day, but this country was once the scene of widespread volcanic activity. Many of our mountains and hills are composed of hard rock, which was once hot molten matter that was erupted vol-canically. Some of the mountains of North Wales and the Lake District are of volcanic origin, but we must not think of them as extinct volcanoes. They are the remnants of huge eruptions of lava and cinders ejected by volcanoes of which there is now no trace. The greatest spread of lava in the British Isles is in Antrim, in Northern Ireland. It covers 1,600 square miles, but even this is insignificant when compared with the vast lava flows in Asia and America. Those in the Deccan, in India, for instance, cover 200,000 square miles. The term ‘flows’ in this connection may be misleading. They were flows at one time and then only for a very short time, the period of rapid cooling. They have been solid and very hard for many millions of years.
Now, what are the main rocks that were formed in these volcanic flows? Leaving aside such eruptive products as volcanic ash (which, when compressed into a rock, is called ‘tuff’) and pumice, which, strangely enough, is a kind of glass, its sponge-like structure being due to the presence of steam and gas in the molten lava, we can select one rock as the main product of volcanic action. It is basalt.
Basalt is solidified lava. As the lava cooled too quickly for it to crystallize thoroughly, the tiny crystals in basalt cannot easily be detected without a microscope. It is dark, very hard and very compact. When rubbed down, it is very smooth to the touch. It has a habit, when cooling, of solidifying into hexagonal columns. Fingal’s Cave, in the Isle of Staffa, provides a striking display of such columns. Another curious characteristic of basalt is its unwillingness to be ground down from a boulder to a pebble. Its hardness and compactness help it to withstand for a very long time the rubs and bumps of the tide and river, but it has a tendency to break up into fragments before it can be reduced to pebble size. There are basalt pebbles, of course, but they are few in proportion to the prevalence of the rock. (b) Plutonic. Having cooled very slowly below ground, these rocks had ample time to crystallize naturally, so we have one certain means of distinguishing them from the volcanic rocks. We can call the plutonic rocks crystalline, and the volcanic rocks, so far as the naked eye is concerned, non-crystalline. Another distinguishing feature between the two kinds, also the result of the difference in their rates of cooling, is that the volcanic rocks are fine-grained and the plutonic rocks are coarse-grained. These contrasts in appearance and texture will be helpful to us when we come to examine pebbles more closely.
The most common and most familiar of all the plutonic rocks is granite. Great masses of it outcrop over considerable parts of the earth’s surface, particularly among mountain ranges and on high ground which has been severely denuded by the weather of countless centuries. Dartmoor is one mass of it, extending for 200 square miles. It abounds in the Cornish peninsula, Cumberland, Westmorland, the Scottish Highlands, and the Mourne Mountains and Wicklow Hills in Ireland.
Let us look at a lump of Dartmoor or Cornish granite. With the naked eye we can see the three minerals of which it consists. One of them looks exactly like glass. This is quartz, one of the commonest minerals on earth. It consists of silica (oxide of silicon) and it constitutes about one-third of the lump. The large white crystals are those of another mineral, felspar (sometimes spelt ‘feldspar’), also very common. It makes up the greater proportion of the rock. Thirdly, the little glistening black flakes are mica. There are several varieties of felspar and mica, so the names are those of groups or families of minerals and not of a specific variety. They vary very much in colour. The mica in common commercial use is white, yet in our piece of granite it is black. There is also a brown variety. Felspar in our specimen is a dull white. It can be pale pink and sometimes almost red. Then it imparts to the granite a slightly reddish hue. The rock’s usual colour, however, is grey, like that of our Dartmoor specimen, because the dull white of the felspar, the colourlessness of the quartz and the black of the mica flakes combine to produce a general greyishness.
Of the other varieties of plutonic rock we can briefly consider two. They are much less abundant than granite, because they do not form such gigantic masses. They are dolerite and gabbro. Dolerite is a rock which has intruded itself when in a molten state between other rocks. When occupying a vertical space between those other rocks, the dolerite which filled it took the shape of a wall or, to give it its geological name, a dyke. When forcing its way between horizontal beds, the dolerite formed itself into a flat slab or, in geological language, a sill. So dolerite is usually found in either dykes or sills. As it cooled more rapidly than granite, its crystals are smaller, but they are easily discernible by the naked eye. The white ones are felspar. The other mineral in dolerite is augite, which gives the rock its dark colour, a deep brown that seems almost black. Sometimes the augite has a greenish tinge. The rock is both hard and heavy and is consequently in frequent use as road metal.
Gabbro. Unlike dolerite, this rock is not found in the form of dykes or sills but in large bosses or protruding lumps; but, like dolerite, it is very dark, very heavy and consists mainly of the same two minerals, felspar and augite. The difference in appearance and texture of the two rocks is simply this, that gabbro is much more coarsely grained than dolerite. 2. SEDIMENTARY ROCKS. This name is self-explanatory, for such a rock consists of sediments laid down in water, salt or fresh, and sometimes on dry land. The sediments have come from various sources. The main sources were: the breaking down of rocks exposed on the surface into small fragments by the action of sun, wind, rain and frost, the fragments being washed down into seas or lakes to form deposits; and the sinking down of organic remains into water.
In the former case the destruction of old rocks has led. to the formation of new rocks; in the latter case the death of innumerable creatures that lived in water has produced a deposit of the hard portions of their bodies and this deposit has ultimately consolidated into rock.
As some of the sedimentary rocks have come into existence as the result of the destruction of old rocks, it necessarily follows that some sedimentary rocks are derived from the breaking down of igneous rocks.
Sedimentary rocks make up the largest of the three classes of rock, because they cover the greater part of the earth’s surface. Here, again, in order to distinguish between the processes by which the rocks came into existence, we must employ a subdivision into groups. (a) The Limestones. The sediments that compose these very common rocks were laid down, grain by grain, in clear water. As the deposit thickened, so it was compressed by its own weight and hardened into rock, a process accelerated by a self-cementing quality possessed by all the little particles. Much of our limestone is formed from the skeletons or the fragments of skeletons of sea creatures: corals, sea-lilies (crinoids), sea-urchins, shells and so forth. All these skeletons contained carbonate of lime. If the limestone has undergone a high degree of crystallization we cannot detect the fossils of these little creatures in the stone, but when there has been little crystallization, the fossils can be detected by the thousand. Sometimes the rock consists entirely of the remains of one kind of creature. For example, some of the chalk, a very familiar kind of limestone, white, friable and rather soft, consists of minute sea organisms, called foraminifera, too small to be visible except through a microscope. Another limestone, called oolite, resembles the roe of a fish, being apparently comprised of millions of tiny eggs. These were originally sand grains, shell fragments or other tiny particles which received a coating of calcium carbonate from the water in which they were deposited and became compactly cemented together. Then there are limestones which consist entirely of the remains of corals, and others which are a mass of fossilized sea-lilies or crinoids. The latter is not uncommon and is called crinoidal limestone. Crinoids and large shells are sometimes found in chalk.
The massive limestone of our hilly districts is the hard, crystalline kind. Great, irregular blocks of it are quarried for building purposes. Hard though it looks, however, it dissolves, as all other limestones do, in weak acid and can be penetrated to great depths by water which has become slightly acid through the absorption of carbon dioxide from the air. Consequently, it is in the regions abounding in limestone that we are most likely to come upon caverns and gorges. Of all the common rocks limestone is the only one that can be easily eroded by very weak acid. (b) The Sandstones and Grits. Its name is simple and accurate, because the rock consists wholly of sand-grains. These grains could have been deposited in the sea, in fresh water or on dry land. If on land they must have been laid down when this country was undergoing desert conditions. There are very massive formations of sandstone in Britain which were deposited under those conditions. It is called the New Red Sandstone. The term ‘new’ may seem to be a misnomer when applied to rocks which came into existence at least 190,000,000 years ago, but it is used in comparison with the Old Red Sandstone, which is some 90,000,000 years older still. The latter may also have been laid down on dry land or, possibly, in shallow water. It is important to remember that these two sandstones are not the only ones.
There were other deposits in the long course of geological time, some of which are yellowish, some brownish and some greyish.
The nature of the grains is a guide to the manner in which the rock was formed. Desert sand-grains are very small and well-rounded. Sand-grains laid down in the sea are a little angular, while the grains in river sands are more angular. Nearly all the grains consist of quartz, but other minerals, sometimes a host of them, reveal themselves in very small proportions on analysis of the sandstone. The stone varies greatly in its hardness, its colour and the size and angularity of its grains, and it is not easy to decide precisely at what point a hard and tightly compressed sandstone becomes a grit. Obviously, a stone formed of well-rounded, almost spherical grains is more porous and therefore more easily crumbled than one formed of angular, flattish grains. The former is certainly a sandstone and the latter a grit, but the dividing line is blurred. Moreover, there is a sandstone of great hardness and toughness, consisting almost entirely of quartz grains closely held together by a silica cement. This is called quartzite. It is a fascinating rock and will repay closer study when we come to examine the pebbles that are made of it.
Grits are coarse-grained sandstones, the grains being more angular. The best known is Millstone Grit, so called because it provided excellent material for millstones and grindstones. There are huge deposits of the rock, some of which are thousands of feet thick, in Derbyshire and the Pennines.
There is a fairly common rock which is neither sandstone nor grit, but is more appropriately classified with them than in any other category of sedimentary rocks. It is Conglomerate, or pudding-stone. Instead of sand-grains it consists of pebbles held together by some cementing material. The pebbles themselves may be compounded of fragments of rock of many kinds. Conglomerate rocks are usually the surviving parts of ancient beaches, but the deposits which form them could have been laid down in the sea or on river-beds. (c) Shales and Slates. The original material of these rocks was clay, great beds of which were laid down in the shallow waters where rivers join the sea. Shale is clay, consolidated and compressed. It is a laminated rock, made up of very thin layers, each of which marks a stage in the thickening of the original clay bed. The pressures it underwent have helped to make it fissile—that is, to split into these thin layers when a sharp instrument is forced in between any two of them. There are very extensive and very thick deposits of the rock in Britain and some of them form part of our cliff scenery, notably at Whitby, on the Yorkshire coast. The rock is dark grey, very fine-grained and of smooth texture, as one would expect a consolidated clay to be.
Shale is often present in beds of great thickness in coal-mining districts, the coal-seams running through the beds. This shale contains the remnants of plants, pressed and fossilized, which grew in the age when coal was being formed from vegetable deposits.
Slate was originally shale or clay. It has suffered terrific pressure from movements of the earth’s crust in those remote ages when the surface buckled and mountain-chains were raised. So the natural setting of slate is in mountainous or very hilly districts. The great pressures it has endured have crushed out of existence the laminations or bedding layers that marked its growth as a shale and have substituted cleavage layers at an angle that bears no relation to the bedding. These cleavage planes appear to be at right angles to the direction in which the pressure came. The slates cleave easily along these planes into very thin slices. This property, together with their durability and imperviousness to water, gives them their great value as roofing material. Slate is more lustrous than shale, because the greater pressure that it has undergone has made it slightly crystalline. Very minute flakes of mica and other minerals give slate its sheen. The rock has a slightly purple tinge, but there are variations in colour from greyish-green to reddish-purple.
We must not leave the sedimentary rocks without reference to two very common rocks from which multitudes of pebbles are formed but which are not sedimentary in the strictest sense of having been deposited as sediments. We can class them with this group because they always form part of sedimentary formations. They are flint and chert.
Flint, like quartz, is solid silica; but quartz is crystalline, and flint is non-crystalline; and it is easy to distinguish one from the other. The usual setting of flint is inside a mass of chalk, either in the form of continuous bands or layers or of nodules or lumps. If we look up from the beach to a white cliff of chalk we see the thousands of dark flints in strong contrast with the whiteness of the chalk in which they are embedded. How did the intruders get there? Geologists are not completely sure, but the generally accepted view is that the flints were not deposited while the chalk was being laid down. The silica which is slightly soluble in water flowed into the joints and bedding-planes of the chalk at a later stage and then solidified. Much of it entered into the spaces occupied by fossil sponges and sea-urchins. They abounded in the chalk sea and their skeletons were deposited by the million in the chalk bed. The liquid silica absorbed the skeletons completely and their outlines can still be traced very clearly in the flint nodules.
Sometimes we find flint in beds of brown clay some little distance from chalk. The clay is the product left after the more soluble parts of the chalk have been dissolved in water. The water has washed out this residue, carrying away the insoluble flints with it. The result is a bed of material known as clay-with-flints. Where we find such a bed we can assume that near to it there was once a thick bed of chalk.
The influence of flint upon the history of early man was very great. As soon as he had discovered that a broken flint has a sharp edge of considerable hardness and keenness, he began to fashion flint implements of many kinds and went on doing so for untold thousands of years. This was one of the reasons why there were large settlements of people in our chalk-lands for many years after the dawning of the Bronze Age.
Chert differs only slightly from flint. It originated in much the same manner, usually occurring in limestone in the same way as flint occurs in chalk. In many of the cherts found in limestone, the silica of the chert has replaced the calcium carbonate of the limestone, but traces of the original structure of the limestone are still preserved. Chert is usually grey, brown or black in colour, flint varies from dark grey, through brown, to nearly black. 3. METAMORPHIC ROCKS. Once more the name indicates the nature of the rock. Metamorphosis means change of form (Greek: meta, ‘change’; morphe, ‘form’), but there is more than change of form in these rocks. There is often a considerable change of character. Originally, they were all very old rocks and every one of them must have been either igneous or sedimentary before the change. Great heat or great pressure, or both, were the causes of the change. If that is so, you might well ask, why is not slate a metamorphic rock? Well, to some extent it has experienced change, but not so much as to alter its form and nature out of recognition. It still possesses some of the characteristics of a sedimentary rock.
There are various kinds of metamorphism, but we can confine our attention to the two main ones: local and regional.
Local metamorphism comes about when a mass of plutonic material pushes its way up among other rocks and slowly cools, giving off, in that process, heat of hundreds of degrees centigrade. The effect upon the surrounding rocks is severe. They melt, absorb new minerals from the plutonic intruder and then re-crystallize. As heat rather than pressure is the agent in such cases, the change can also be called thermal metamorphism.
Regional metamorphism is far more important for the simple reason that its effect is not merely local but widespread, sometimes covering thousands of square miles. A combination of heat and pressure produces it. Imagine the creation of a huge mass of sedimentary rock at the time when this country lay beneath the sea. The depositing of sediments through millions of years, and to a depth of hundreds of feet, buries the older rocks and exerts upon them an enormous pressure. This in turn produces great heat and a disturbance of the earth’s surface in that region. The buried rocks are crushed, sheared and melted. Millions of years elapse, the land rises and the thick sedimentary cover wears away under the very gradual denuding action of sun, wind, frost and rain. The original surface rocks are exposed again, but they are unrecognizable. In solidifying they have changed their mineral content and have re-crystallized ; and, under the crushing weight of the mass that lay above them, they have suffered a change of form. The convulsive earth movements have also had a shattering effect. Such is regional metamorphism, also termed dynamic metamorphism because movement and pressure, as well as heat, have produced it. Almost the whole of the northern half of Scotland consists of metamorphic rocks. They are fairly extensive in Anglesey, but in England the outcrops are few and small. Cornwall, Devon, Cumberland, Westmorland and the Malvern Hills contain some well-known examples.
The three most familiar of the metamorphic rocks are gneiss, schist and marble.
Gneiss (pronounced ‘nice’) is a word of German origin, derived from an Old High German verb gneistan, ‘to sparkle’. In sunshine, especially after rain, it certainly does sparkle, as it is a highly crystalline rock. Its outstanding characteristic is that it is banded or streaked. Gneiss is usually metamorphosed granite and, if not granite, a plutonic rock of similar composition. But, whereas in granite the mineral crystals of quartz, felspar and mica are intermingled, in gneiss they are arranged in bands or streaks. Such an arrangement is called foliation. The coarse-grained layers or foliae of crystals give the rock a very attractive appearance.
Schist (pronounced ‘shist’) is derived from a Greek word schistos, meaning ‘easily split’. This is an accurate description. Like slate, it has innumerable planes of cleavage and very thin layers can be split away from it. Its foliations are much narrower than those of gneiss and it is also more finely grained. The shimmering lustre of schist in the sunlight is fascinating. Mica and quartz are the two minerals of which the rock is usually composed. Their glittering qualities impart the sheen to it. Another property possessed by schist that helps us to distinguish it from gneiss and other metamorphic rocks is its flakiness.
Marble is metamorphosed limestone. The carbonate of lime (or calcium carbonate) of which limestone is mainly composed is re-crystallized into a granular mosaic of a very pleasing texture and appearance. It can have any one of a large range of colours and is consequently in great demand for the ornate pillars and interior decoration of spacious and impressive buildings.
Now, although we have affirmed that every rock must belong to one of the three classes—igneous, sedimentary and metamor-phic—we must realize that the boundaries separating the groups are not sharply defined. Slate, for instance, a partly metamorphosed clay or shale, comes under the heading of sedimentary. We have also included quartzite in that class, because the change in the structure of many quartzite rocks from the original sandstone has been only partial, but there are quartzites which display an almost total change. The re-crystallizing of the quartz grains and their close interlocking has made the rock more compact and more lustrous. The grains show no traces of their sedimentary origin. Thus it is possible for a partly metamorphosed quartzite to be classed as sedimentary and one that is almost completely changed as metamorphic. This classification is not made to bewilder the student by creating exceptions to the rule. It is, after all, just common sense to allot a rock to the class to which it is most applicable.
This ends the lesson in geology. It is most desirable, indeed essential, to keep it in mind, for we depend upon it to aid us in the task of recognizing the pebbles on the beach.
And now to the beach, to pick up the pebbles that attract our eye, to determine their origin and to decide what kind of journeys they have made.
First of all, is any special equipment necessary? Yes, if you intend to be a serious and earnest beachcomber (in the better sense of that usually opprobrious term). You will find the following of great service: 1. A geological map of the district . This will show you the geology of the whole region behind the coast. You will then know at once the nature of the cliffs or soil above the beach. This is knowledge of the utmost value, for, as you have already seen, the local rock is one of the major contributory sources of the beach pebbles. 2. A knife that has at least one strong blade. You will have frequent recourse to this for the scraping and scratching of the pebbles: scraping, in order to remove part of the skin they have acquired in their wanderings, and scratching, to test the hardness of the stone—a very necessary aid to the detection of its nature. 3. A small hammer with a fairly heavy head. Scraping with the knife does not always reveal the surface of the pebble clearly. The only way to see the texture and structure of the pebble in all its clarity is to make a fresh fracture—that is, to break off a piece of it. If you crash one stone against another you risk the loss of an eye. That would not only bring your seaside holiday abruptly to an end but rob you of your zest for pebble-hunting once and for all. The easiest and safest way to break a corner off a pebble is to place your foot firmly on the stone, leaving a portion exposed to the aim of the hammer. 4. A pocket lens. This is indispensable to your scrutiny of the structure and texture of the pebbles. However good your sight may be, it cannot reveal the small details that a lens exposes to view. 5. A small piece of flint with a sharp edge. This will test the hardness of rocks which the steel blade of your knife cannot scratch.
It may well be, however, that you prefer to conduct your pebble quest from the depths of a deck-chair, making an occasional effort to stretch out a hand towards a pebble that attracts you. Even in this posture of luxurious sloth you can enjoy the hunt, but you cannot pursue your inquiries to a satisfactory conclusion unless you take home your selection of pebbles and then subject them to knife, hammer and lens. The nature of some of them may be obvious at a glance—for instance, flint and quartzite. Of those not so obvious, be sure to take home a pair of each, because you may have to maltreat one of the pair with knife and hammer.
Here we must sound a note of warning and possibly of discouragement. Although the reading of this article aided by your own patience, persistence and zeal, should enable you in time to identify most of the pebbles that were originally fragments of the commoner rocks, you will find many that baffle you. But this need not dishearten you, for some of them also baffle the professional geologist, equipped though he may be with a sound knowledge of crystallography, petrology and mineralogy. In order to ascertain with precision the contents of a pebble of peculiar structure, he has to take it to his laboratory, cut off an extremely thin shce of it with a rock-slicing apparatus and examine the slice through a petrological microscope fitted with polarizing and analysing prisms, a rotating stage and other refinements. He can thus determine, in their correct proportions, the minerals of which the pebble consists, their mutual relations and the order of their crystallization.
But all this is beyond our ken. We must be content to plod along the beach and to get all the help we can from our more homely appliances.
Let us imagine ourselves on any pebble beach on the coastline of England and Wales. Having studied the geological map of the district, we have already made a good start in the search, because we know what the local rocks are and that they will be well represented among the pebbles. We have also noted whether there are any rivers in the neighbourhood and, if so, what rocks they flow through. The shingle will contain a fair contribution from those rocks also. The uninitiated pebble-seeker, perhaps attracted by the fresh, clean look of the belt of shingle from which the tide has recently receded, feels tempted to confine his search to that strip of the beach. This is not often the happiest of hunting-grounds. The upper belts of shingle that lie below high-water mark have also been washed by the sea not many hours before and will probably yield a richer store. This is especially true of beaches of the lonelier kind.
On picking up a pebble, particularly a well-travelled one, and asking yourself which of the three classes of rock it belongs to, you may be at a total loss, because it is completely covered by the skin or coating it has acquired on its journeys. The skin is often a thin crust of carbonate of lime. It obscures the true surface entirely and you may at once leap to the conclusion that you are holding a pebble of chalk or of whitish limestone. The old adage about appearances being deceitful applies with especial force to pebbles. That is why a strong knife is an essential item in your equipment. With it you can lay bare part of the surface of the pebble. You may even find that the pebble has grown two coats of differing colours. Scrape hard until you have scoured these integuments away from a square inch or so of the surface. Then you may begin to find an answer to your question.
The colour of the outer skin can be very deceptive to people who are reluctant to make thorough investigation. Long exposure to the weather has bleached many pebbles a dirty white or grey.
Some that have lain for long in pools that contained iron in solution have a thin reddish coating of oxide of iron, others may have a greenish tinge given them by carbonate of copper, or a staining of black by oxide of manganese.
What has your scraping revealed? Of one thing we can be certain. The pebble will show truly the original bedding and structure of the rock of which it once formed a part. Your first task is to decide whether it was igneous, sedimentary or metamorphic. Of the three^ the one that reveals its nature most clearly is the sedimentary, simply because you can see the sediments themselves: the round grains of the sandstone, the more angular grains of the grit, the comparatively soft and powdery white grains of chalk and its minute marine organisms, the pudding-like make-up of the conglomerate, the fish-roe appearance of oolitic limestone, the fossilized remains in the crinoidal, coral or other fossiliferous limestone or the very compact and slightly crystalline grains of the massive limestone that bears no fossils. Here, again, the knife will help to narrow down the search, as it will easily scratch chalk and any kind of limestone.
A pebble of flint, once you have exposed part of its original surface to view, is self-revealing, despite its lack of sediments, by its hardness, the very sharp edge of any fragment you break away from it, its homy look and its habit of appearing to be translucent or not entirely opaque. If the pebble looks and feels like flint, but is much lighter in colour, you have good reason to think it is chert, yet some chert is almost black.
Slate should also make itself known to you by its slightly purple colour and by its readiness to split when you press the knife-blade down among its cleavage-planes. A pebble of shale, however, is less easily recognizable. It is helpful to remember that shale is consolidated clay and looks just what you would expect such clay to look like. It is also less compact than slate and, on being scraped by the knife, powders rather easily. Breathe on it and you may detect a slightly earthy smell.
Now, let us suppose that after a very careful scrutiny you have decided that the pebble is not of sedimentary rock. Then it must be igneous or metamorphic. Well, if it is metamorphic, it will have one of the following qualities: the banding of gneisses, the finer foliations and flakiness of the schists, the compact and glittering crystallization of the quartzites or the crystal mosaic of marble. Only large pebbles, naturally, can display the broad banding of the gneisses.
Let us now assume that the pebble possesses none of these characteristics. Then you are forced to the conclusion that it is formed of igneous rock. Which of them? Take first the plutonic rocks, which occupy a much larger area of this country than the volcanic. The qualities that distinguish them are their very marked degree of crystallization resulting from the medley of minerals that compose them, their lack of foliation and lamination and their pronounced hardness, toughness and weight. Enough has been said already of the texture and structure of the more familiar ones: granite, dolerite and gabbro, to guide you some little way towards their identification. If the pebble is of granite and you have made a fresh fracture in it, you will not need the pocket lens to see the three minerals that compose it. Nor will you need it to differentiate the coarse-grained gabbro from the finer-grained dolerite. In both of them the white or greyish crystals of felspar are visible to the naked eye.
Our one representative of the volcanic rocks, the solidified lava, basalt, is more difficult to identify. Its hardness, heaviness, smoothness, very fine-grained nature and iron-black colouring should help you to distinguish it from all other members of the igneous family.
By a process of exhaustion, such as the one above, you can narrow down the field of inquiry to a small group of rocks and, with widening experience and extending study, often to one rock. But it cannot be emphasized too strongly that the procedure outlined above is a simplification, probably an over-simplification, of the lines of inquiry to be followed in the attempt to identify the rock of which a pebble is made. In order to avoid complexity of treatment we have selected only a few of the common rocks as examples. Not only are there many others, but there are other varieties of those we have mentioned. Still, that need not daunt you, for once your interest in, and curiosity about, pebbles have been aroused you will take pleasure in the reading of books on geology that are not too highly technical. Your surest aid to familiarity with rocks is to make regular visits to a geological museum. London readers will find all, and more than all, they want in the Geological Museum and in the Natural History Museum (Department of Mineralogy), both of which are in South Kensington.
After that little digression we can return to our beach. There are one or two other properties possessed by certain pebbles which can help us towards correct classification. One of them is their shape. There are three usual shapes: spheres, ovoids and flattened ovoids (or discs); and one less usual: cylinders with rounded ends. Roughly—indeed, very roughly—speaking, the igneous rocks tend to become spherical pebbles, as they have the same hard crystalline structure throughout and become evenly rubbed down or worn away. If not spherical they develop an ovoid form resembling that of a broad egg, the longer diameter not greatly exceeding the shorter. Of the sedimentary rocks, flint resists the rubbing and bumping on the beach for a very long time, so a flint pebble is more likely than other stones to display a few corners that have yet to be rubbed off. In time, of course, all nodules of flint lose their angularity and generally become thick ovoids. The very hard, compact sandstone and quartzites, like the igneous rocks, withstand wear fairly equally from all directions and take for the most part a shape between the spherical and the ovoid. The harder limestones are mostly ovoid and the softer ones, together with the softer sandstones and the shales and slates, flattened ovoids. There is much variation in the shape of the meta-morphic rocks. In spite of the foliated nature of the gneisses, their great hardness and their crystalline structure help them to resist erosion and so their shape is usually between spherical and ovoid, but the finer laminations of schist and its extreme fiakiness make it less resistant. Schist pebbles therefore vary in shape between ovoid and flattened ovoid. Marble, which is crystalline throughout and is neither foliated nor flaky, strenuously resists the rubbing down on the beach and so tends to become a more spherical than ovoid pebble. We have already noted that when a fragment, much longer than it is broad, is torn from a rock it tends to take the form of a narrow cylinder. The majority of pebbles of this rather unusual shape are schist.
We occasionally find pebbles that are piriform (pear-shaped). The material that once surrounded the narrow end must, evidently, have been softer than the rest of the pebble. On the south coast of Devonshire there is an extraordinary lagoon, Slapton Ley, nearly two miles long, cut off from the sea by a bar of shingle. Here are to be found among the flints many small piriform pebbles of white quartz.
There are pebbles which disclose their identity to you when you feel them, provided always that you are feeling their true surface. The best-known example of them is serpentine, so called because its colouring suggests the appearance of a snake-skin. Serpentine is an igneous (plutonic) rock which has been partly metamorphosed by contact with water. The mineral, olivine, of which it was mainly composed in its original state, has been changed and softened to serpentine. It is a highly attractive rock, usually of a very pleasing green. The most prominent mass of it in Britain is the Lizard headland, in Cornwall. It forms nearly all the headland. Much of the Lizard serpentine is green, but some of it is a dark red. Serpentine pebbles are usually ovoids and they possess a waxlike lustre. To touch them after the removal of their coating is to become aware of a sensation that can best be described as caressing, but some unimaginative people have been content to call it soapy. That adjective is best applied to another rock called steatite, popularly known as soapstone. It is a compact variety of talc, the softest of all minerals, easily scratched by the finger-nail.
Some pebbles have been called firestones because they produce sparks when struck, or violently rubbed, together. Everybody knows that flint behaves in this way, but not so many people are aware that pebbles of quartz and of metamorphic quartzite, when so treated, produce bigger and better sparks. Take two quartzite pebbles, or break one of them into two pieces, and strike one against the other in darkness. An orange-coloured flash results. There is also a smell, not unpleasant, but very difficult to describe. It is still more difficult to account for the fact that, if you strike the stones together under water, they will then emit an exactly similar flash and smell. So, if you are doubtful whether a pebble consists of quartzite or not, you have only to conduct this simple experiment.
If your beach lies north of the line marking the farthest advance of the ice-sheets, you may find among the pebbles some of the long-distance travellers, including even some formed from the rocks of Scandinavia. But you must not assume that beaches south of the line will be entirely free from such pebbles. A river, as we have already noted, could have carried them over the last stage of the journey. They could also have arrived on a southern beach as passengers on, or in, a floating block of ice. The melting of the sheets at the end of the Ice Age must have dislodged portions of them from the main body and these blocks or bergs, floating into slightly warmer waters, must have released, as they melted, the pebbles embedded in them.
Of course, if your beach is bounded by land containing layers of boulder-clay, you can expect to find a bountiful assortment of ice-borne pebbles gathered from the rocks of regions near and far. Such a beach is always interesting, as its contents are usually very varied and they present the pebble-hunter with more than enough problems to hold his interest.
Pebbles that have been ice-borne during any part of their journey from the parent rock to the beach on which you find them can disclose that fact to you by their appearance, but they can do so only if the glacier scraped them against other rocks, either in the act of scooping them up or in carrying them along beneath it, frozen into its underside. Then the glacial pebbles may bear unmistakable marks of their rough journey. If so, they will be grooved, scratched or faceted—that is, one part of the pebble will appear to have been planed down flat. The grooves in a glacial pebble, as we have already seen, are called striations. If you cannot see them on the surface of a pebble, you must not assume that it is not glacial. It could have been scooped up with a mass of other small rocks by the glacier, thus completing its journey unmarked. On the other hand, many pebbles that were formerly striated have been worn down and have lost their striations.
To many pebble-seekers the striated glacial pebble, bearing the marks that tell of its long and hazardous journey, is the most fascinating of all, but to others, who look for colour, glitter and lustre in their pebbles, it neither arouses their interest nor evokes their imagination. They search for the gleaming quartzite, the crystalline gneiss, the lustrous, apple-green schist, the bright red, yellow or green jasper, the conglomerate with its medley of patterns and colours, the marble with its crystal mosaic, or the waxlike serpentine. Perhaps they are attracted most of all by the pebbles of porphyritic texture. This is to be seen fairly often in many pebbles of igneous origin, e.g. granite. The crystals of felspar and quartz, instead of having their usual appearance of being closely intermingled, are conspicuously visible, because they are surrounded and outlined by fine-grained material. Against this darker background they seem to stand out in crystalline beauty. A porphyritic pebble, cut and polished, is a delight to the eye. The word ‘porphyry’ comes from the Greek and means ‘like purple’. The Greeks and Romans imported from a quarry in Egypt a porphyritic stone of purple hue and used it for decorative purposes in their buildings. There is no purple tinge in any of our porphyritic rocks, as the word has lost much of its original meaning.
Other pebble enthusiasts find joy in the quest for fossil-bearing pebbles. Their texture, when cut and polished, is also delightful.
As there are fossils innumerable, we must limit ourselves to a few of the common ones. Readers who wish to study fossils in detail should consult one of the many textbooks on palaeontology.
Fossils are the remains of animals or plants preserved in rocks. The study of them is a highly important branch of geology, because they tell the story of the history of much of the earth’s surface and of the creatures that lived on it through the ages. The first man to discover that each stratum, or layer, of rock is characterized by the organic remains it contains was the great William Smith (1769-1839), justly called-’the father of English Geology’. From this beginning he was able to build up a table of the comparative ages of the strata and thus enabled geologists to compile a life-history of the earth. This history is still incomplete, for, though there are millions of fossils in the rocks, they portray only a fraction of all the life that was going on at the time when the rocks they now occupy were being laid down. Smith had next to no education and wrote very little, but in 1815 he published the first geological map of England and Wales. It embodied the results of all his researches. As the work of a man who was the product of a village school it was a rare achievement.
Almost all fossils occupy sedimentary rocks. Why? They would have been crushed to fragments by the pressure exerted upon metamorphic rocks, by the violent convulsions of the earth’s surface, and their skeletons would never have survived the high temperatures that accompanied those movements. The igneous rocks of the plutonic kind pushed their way up from below the crust where no life went on at all; those of the volcanic kind were erupted from the boiling mass below, where life was impossible. Yet it is possible for rocks formed of volcanic ash to contain a few fossils. Obviously, the organisms must have entered the volcanic ash after it had cooled and before it was compressed into a rock. The sedimentary rocks which are richest in fossils are limestone, chalk and shale. This is not surprising, as all of them were slowly deposited under water, and these are ideal conditions for the preservation of organic remains. The vast majority of the fossils in these rocks are consequently those of marine creatures. In the chalk, which was laid down in clear water, we must expect to find the remnants of organisms which inhabited such water, but in the shale, which is consolidated clay, we look for the fossils of those creatures whose habitat was muddy water.
With a hammer and a small chisel we can easily extract the fossils from a cliff of soft limestone or shale, while from a cliff of chalk we can remove them with a knife-blade. Flint will not yield its fossils at all.
Although we have defined fossils as the remains of animals or plants preserved in rocks, we must not be misled into thinking that every fossil is a remnant of an actual animal or plant. It is very often the cast image or mould of the organism. Long after the creature, bone or leaf became entombed in the hardening rock, it may have decayed and finally left in the rock an exact mould of itself. Silica, carbonate of lime, or other material which is carried in solution, then gradually filled up the space and formed a hard replica of the departed organism. For example, a shell-fish dies and is washed into a slowly forming bed of sediment. The creature itself disappears in time and the space it occupied is filled with mineral matter brought by water through the porous rock. That matter solidifies and forms a perfect mould of the shell-fish. The shell, however, is preserved and may still be there when you dig the fossil out. But if the shell has also decayed, or has not been replaced by sediment, the Httle ridges and cavities will be those of the inside of the shell, giving you the impression that the creature had worn its shell inside out. If the original shell is still in existence or has been replaced when you find the fossil, its markings will be, of course, the natural ones, those on the outside of the shell.
Very often the teeth and bone of a fossilized creature and the harder seeds and roots of the fossilized plant have been preserved intact in the encasing rock, their hardness having saved them from disintegration. The minerals that have replaced the softer parts make so close a copy of them that it is not always easy to distinguish those parts of the fossil which have been preserved and those which have been replaced by a mineral mould. Even the slightest traces of the past life of a creature or plant rank as fossils for geologists regard as fossils the impressions of organisms. Thus the tracks and burrowings of worms and the footprints of birds, reptiles and mammals all rank as fossils. So also do the droppings of reptiles and fishes. These droppings are called coprolites (Greek: Kopros, ‘dung’). They have provided the most valuable evidence to geologists of the creatures that existed during the ages when the strata that now enclose them were in process of formation.
And now let us glance at some of the more familiar fossils.
Graptolites (Greek: graptos = engraved or painted; lithos = stone), long ago extinct, were little marine creatures of low organization. Thin horny rods, some of which resemble tuning-forks in shape, supported them, each individual creature occupying one small cup-like cavity. The cavities were linked together by a common canal or channel of living matter. The fossilized remains of the rods look like fretsaw blades, the teeth having been formed by one side of the outer edge of each cavity. The fossils abound in the darker shales and slates. Many of the rods are single, some are double and have a forked shape, and others have several branches.
Trilobites, meaning creatures with a three-lobed body, have also been extinct for hundreds of millions of years, though they lasted longer than the graptolites and were of a much higher organization. They were the earliest of the Crustacea, the group of creatures represented to-day by lobsters, crabs, crayfish, prawns and shrimps.
Brachiopods (Greek: brachion = an arm; pous, podos = a foot) were shell-fish. Their popular name is lamp-shells, as some of them resemble the oil-lamp of ancient Rome. As the soft body is contained within a double, hinged shell, the brachiopod is a member of that large class of shell-fish called bivalves. Each of the two valves (or shells) is symmetrical about a line drawn through its centre. In this respect it differs from several other bivalves. The shell of the brachiopod varies so much in its shape and pattern that a picture of a typical brachiopod is not easy to draw. Sometimes the shell is thin and glassy, at other times it is more solid and less lustrous, but in nearly all instances it is very attractive. Limestones and shales contain vast numbers of the fossil. Note in the three drawings the concentric lines running roughly parallel to the outer edge of the shell. A similar ornamentation is to be seen on the shells of certain other bivalves, such as cockles. Crinoids, or sea-lilies. Both the name and the appearance of these fossils are misleading. They suggest a plant, but the crinoid was an animal. A long flexible stem attached it to the sea-floor. Almost all its fossilized remains are fragments of the stem, which was much harder than the calyx at its top end. The calyx was almost the whole of the animal. The crinoid of later ages had no stem, but, like its predecessor, had arms that closely resembled the stem. Millions of these fragmental stems and arms occupy limestone cliffs and headlands. Indeed, there are some massive limestone formations that consist almost wholly of them; the stone is consequently called crinoidal limestone.
Ammonites, so called because the creature lived in a spirally-curved shell resembling the ram’s horn on the statue of Jupiter Ammon. The fossil is popularly known as a snake-stone, because its spirals suggest a coiled-up snake. It has some thousands of varieties and is one of the commonest of fossils abounding in limestone and shale. The fossils can be seen to great advantage in the East Cliff at Whitby, on the Yorkshire coast. Probably the name of snake-stone originated from a Whitby legend, to which Sir Walter Scott refers in his poem, ‘Marmion’:
Thus the nuns of Whitby told, How of thousand sfiakes, each one Was changed into a coil of stone, When holy Hilda prayed: Themselves within their sacred bound Their stony folds had often found.
We have wandered some distance from the subject of pebbles, but the excursion has not been purposeless. Many long stretches of our coast are backed by cliffs of limestone, chalk, shale and sandstone; and the pebbles in the beaches below those cliffs are rich in fossils. It is, therefore, most desirable that you should be able to recognize the common ones. In so doing you will assuredly feel an urge to make a wider and deeper study of fossils. Moreover, the zealous pebble-collector is always eager to have the choicest of his finds cut and polished so that the delicate tracery of the intermingled crystals, the porphyritic structure, the marble mosaic or the outline and detail of the embedded fossil stand out in fascinating clarity. And his interest is all the keener if he is not only able to name the rock of which the pebble consists but also to identify the kind of fossil it contains.
To the casual beholder a lump of flint may appear to be nothing more than a piece of hard, homogeneous silica, but to the eye of one who knows how the flint nodules came to be inside the bed of chalk it can reveal much more. He looks for, and very often finds, in the flint traces of the skeleton of the organism around which the liquid silica solidified. Very frequently this is a fossil sponge, which betrays its presence in a complicated and very pleasing pattern of little spikes. These needle-like rods are the petrified remains of the framework of the sponge. They are called spicula (the plural of the Latin word: spiculum = a small sharp point). They do not readily reveal themselves to the naked eye, because the surface of the translucent flint has been bleached and coated. Break off a flake of the flint, hold it up to the light and you will then know whether the pebble is worthy of further scrutiny. In all likelihood you will see some small, opaque spots in the horny, translucent flint, perhaps a minute fragment of a shell or the impression of a shell, possibly some tiny corals or the scales of primitive fish. If you see any spicula you have good reason to think that the pebble contains some part, at least, of a fossil sponge. The spicula display themselves to the best advantage if the pebble is broken in half and one of the two interior surfaces is cut and polished by a lapidary. Then the colour, delicacy and complexity of the spicula make up a most pleasing intaglio.
When Brighton became a fashionable resort in Regency days a craze for pebble-collecting and polishing began and it continued well into the reign of Queen Victoria. Lapidaries were busy, cutting and polishing flint pebbles that contained fossil sponges and other marine organisms. Good specimens were highly prized and commanded astonishingly high prices. The cult spread quickly to other resorts which possessed good and varied shingle beaches. Fossiliferous flint pebbles were not the only objects of search. There was frantic rummaging for semi-precious stones.
We sometimes find flint pebbles with hollows in them. It is possible that the hollow in the pebble was caused by some irregularity in the deposition of the silica which composed the flint. Small pebbles, washed into the hollow by the tides and eddying round within it, deepened and smoothed it, and now, after the lapse of ages, the finished product of Nature’s own grinding and smoothing mill lies on the beach. The better specimens make excellent inkpots and are an ornament to the writing-desk. The Sussex beaches below the chalk cliffs, especially those near Beachy Head, are happy hunting-grounds for the seekers of the deeply hollowed flint pebbles.
We end this article with some words of warning against the artificial intruders on the beach. They can masquerade as pebbles and very successfully lead the unsuspecting searcher to think he has made a rare find. The sea washes up on the beaches a medley of flotsam and jetsam, wreckage, fragments of brick and concrete from quays and jetties and pieces of man-made articles borne by rivers into their estuaries. Man himself is no tidier on the beach than he is on Hampstead Heath on a bank holiday. His contribution is also miscellaneous: broken milk bottles, ginger-beer bottles and earthenware galore. The tides treat all these intruders to the beach as they treat the pebbles themselves: they grind them down, knock their corners off, smooth them and eventually bleach them and give them a coating. If the rounding and smoothing has gone on long enough, the deception can be very baffling. It may be less baffling if you keep the following in mind: 1. The pebbles have had a very much longer start in the grinding, rounding and smoothing race, so they all, including even the tough flints, are much more likely to have had their corners rubbed away than the artificial fragments which have been washed up by sea or dropped by man on the beach. 2. You have been advised to use your knife to scrape away the coating from a pebble in order to have a better chance of identifying the rock. The same treatment will enable you to see the true surface of the false pebble. It should quickly unmask any fragment of brick, china or earthenware. As for the two last-mentioned there is no rock in this part of the world that closely resembles them. Some types of earthenware may bear some slight resemblance to very fine sandstone, grit, shale or hard, fine-grained limestone, but once you have become familiar with the texture of those rocks you are unlikely to mistake a piece of earthenware for a pebble of any one of them. Brick should also be self-revealing. If it is the red variety you recognize it at once. The only rock at all like it in colour is the red sandstone, but, in that rock, the sand-grains are unmistakable. If the brick is the yellow kind, the only rock that bears the slightest resemblance to it is the yellow sandstone and, here again, the sand-grains will emphasize the difference.
Pieces of concrete are much less easily dismissed, because the sand in them may convince you that you have found a pebble of sedimentary rock. If the concrete consists of the usual mixture of cement, sand and gravel you can tell it at once for what it is, but if the gravel is missing from the mixture, as it often is, you may find it harder to distinguish the piece of mortar from a pebble of sandstone or grit. Scratch it vigorously with the point of your knife-blade and examine the powdery stuff that runs into your hand. You are aware that the grains of sandstone and grit are held firmly together by silica and the sand-grains in concrete by cement. The latter was a fine powder before it was mixed up with the sand to form concrete. The knife-scratching will disclose, in each case, the sand-grains, but if the pebble is of concrete, the accompanying powder will be whitish and fine; if it is of sandstone or grit it will be coarser, tougher and more angular. Even so, you cannot be sure. The grains of some sandstones are held together by calcareous cement, which makes them difficult to distinguish from mortar. 3. Glass. A fragment of glass dropped among the shingle and carried up and down by the beach undergoes a curious change. It loses its glassiness or transparency by acquiring a crystalline surface like frosted glass. A piece of an old lemonade bottle of greenish glass that has endured the rolling of thousands of tides takes on a very complete disguise. It has lost its sharp edges and has become a pebble of crystalline beauty. Its greenish tinge adds to its charm and it may beguile its finder into the belief that he has found a semi-precious stone of some rarity, perhaps a pebble of chrysoberyl or chrysoprase, greenish stones with delectable names. But these are vain hopes. The chances of finding a pebble of either of these stones on a British beach are almost precisely nil. He would not be the first to imagine that a jewel lies at his feet among the shingle. The chrysoprase was a favourite stone in this country in the early nineteenth century for setting in rings and brooches. The lapidaries in Brighton and other fashionable seaside resorts at that time made a brisk sale at high prices of green, crystalline stones that were merely beach-worn fragments of old bottles.
Ordinary colourless glass also has its fascination as a pebble. The quickest and most efficient means of coming to a decision about it is to break off a small piece with your hammer. If it is of glass, its obvious glassiness will be at once apparent. Now there are two rocks of complete, or almost complete, transparency, and you have to decide whether the pebble can be one of those. They are obsidian and pure quartz. Obsidian is a volcanic rock that cooled so quickly after being erupted that it had no chance to crystallize and solidified into glass, but, though there are some glassy rocks in Scotland, they have not the clarity of obsidian and, accordingly, the possibility of finding an obsidian pebble on any of our beaches can be almost entirely discounted. Pure quartz has been known to find its way to a shingle beach, where, in due course, it acquires a frosted appearance. It then looks exactly like a piece of glass that has received the same treatment. You break off a piece. If the quartz is absolutely pure and is a flawless rock crystal (apart from its frosted-looking surface) it will look just like colourless glass, but it will survive a test that glass cannot. Take your sharp piece of flint from your pocket and scratch the glassy surface. If the pebble is quartz, the flint will slide over it and leave it unscratched, because quartz is high up in the scale of hardness. If the flint scratches the pebble, it is glass. There is also another simple test. Strike the pebble hard with the blade of your knife in a dark room. If it emits a spark, accompanied by a slight smell of burning vegetable matter, it is quartz.
Impure quartz, white, opaque and crystalline, appears in pebble form on many of our beaches, but it is easily distinguishable from pebbles of any other rock. Like the common flint, it consists almost wholly of silica. There is a strange fascination about silica. It is to be seen in many guises.