Книга - Britain’s Structure and Scenery
Britain’s Structure and Scenery
L. Dudley Stamp
Britains Structure and Scenery deals with the physical background, the stage on which the drama of life is played and which provides the fundamental environment for plants, newnaturalists.comIt would be difficult to find an area of comparable size anywhere in the world with such a variety of physical conditions, scenery and consequently of plant and animal life as the British Isles. Our homeland is indeed a geological museum, epitomising in miniature the geological history of the globe. Each hill and valley, each plateau and plain reflects the underlying geological structure or build; this volume attempts not only to describe the surface features, but also to sketch the long and complex series of events which have given the land its present form - the building of the British Isles. It thus deals with the physical background, the stage on which the drama of life is played and which provides the fundamental environment for plants, animals and man.
Collins New Naturalist Library
4
Britain’s Structure and Scenery
L. Dudley Stamp
Editors: (#ulink_3dd7c1d8-b5c0-5111-b433-b6937222db23)
JAMES FISHER M.A.
JOHN GILMOUR M.A.
JULIAN HUXLEY M.A. D.Sc. F.R.S.
L. DUDLEY STAMP C.B.E., B.A. D.Sc.
PHOTOGRAPHIC EDITOR:
ERIC HOSKING F.R.P.S.
The aim of this series is to interest the general reader in the wild life of Britain by recapturing the inquiring spirit of the old naturalist. The Editors believe that the natural pride of the British public in the native fauna and flora, to which must be added concern for their conservation, are best fostered by maintaining a high standard of accuracy combined with clarity of exposition in presenting the results of modern scientific research. The plants and animals are described in relation to their homes and habitats and are portrayed in the full beauty of their natural colours, by the latest methods of colour photography and reproduction.
TO
A TRUE LOVER OF THE COUNTRYSIDE
THE RT. HON. LORD JUSTICE SCOTT, P.C.
TO SERVE UNDER WHOM FOR A YEAR
IS A LIBERAL EDUCATION
Table of Contents
Cover Page (#u5aeb306f-d797-5d49-a7ee-b746def6eaaa)
Title Page (#u921bfea5-ee85-5f17-b389-727486ecea2a)
Editors: (#u0a7ed49c-2c80-5b81-b824-5eee0e34140e)
Editors’ Preface (#u552308c9-702f-5352-bab1-08507d155e9c)
Introduction (#uce819408-661e-5a67-ab84-fca06fb6e83a)
CHAPTER 1 (#ub9b561ff-6b56-5a74-8681-eaf1bc70ae68) HIGHLAND BRITAIN AND LOWLAND BRITAIN
CHAPTER 2 (#udd76c093-8932-5238-b163-0a43bbe52a5a) READING THE ROCKS
CHAPTER 3 (#ue947d751-e5aa-5342-bcc6-076dcc6aef33) EARTH HISTORY—TIME AND LIFE
CHAPTER 4 (#uac0feacc-292c-5029-ac2b-bd912660cc70) THE GEOLOGICAL MAP
CHAPTER 5 (#u151a84d2-825e-5355-b131-96f2731e07b7) LAND FORMS AND SCENERY THE WORK OF RIVERS
CHAPTER 6 (#u0f6d4b27-1ef7-579d-aff3-6e13b6e2a699) THE WORK OF THE SEA
CHAPTER 7 (#u732e3e53-af81-5d49-baa8-3a7ea69e6c8a) THE SCENERY OF THE SEDIMENTARY ROCKS
CHAPTER 8 (#litres_trial_promo) THE SCENERY OF LIMESTONE COUNTRY
CHAPTER 9 (#litres_trial_promo) THE LAND FORMS OF VOLCANIC COUNTRY
CHAPTER 10 (#litres_trial_promo) THE SCENERY OF GLACIATION
CHAPTER 11 (#litres_trial_promo) SOILS
CHAPTER 12 (#litres_trial_promo) GEOGRAPHICAL EVOLUTION
CHAPTER 13 (#litres_trial_promo) THE PLIOCENE PERIOD
CHAPTER 14 (#litres_trial_promo) THE GREAT ICE AGE AND AFTER
CHAPTER 15 (#litres_trial_promo) THE REGIONS OF BRITAIN
CHAPTER 16 (#litres_trial_promo) THE WEALD
CHAPTER 17 (#litres_trial_promo) EAST ANGLIA AND THE FENS
CHAPTER 18 (#litres_trial_promo) THE ENGLISH SCARPLANDS
CHAPTER 19 (#litres_trial_promo) THE ENGLISH MIDLANDS
CHAPTER 20 (#litres_trial_promo) THE SOUTH-WEST
CHAPTER 21 (#litres_trial_promo) THE WELSH MASSIF
CHAPTER 22 (#litres_trial_promo) THE NORTH OF ENGLAND—THE LAKES AND THE PENNINES
CHAPTER 23 (#litres_trial_promo) SCOTLAND
CHAPTER 24 (#litres_trial_promo) IRELAND
Annotated Bibliography (#litres_trial_promo)
Index (#litres_trial_promo)
Photographic Insert (#litres_trial_promo)
Acknowledgments (#litres_trial_promo)
Copyright (#litres_trial_promo)
About the Publisher (#litres_trial_promo)
EDITORS’ PREFACE (#ulink_5b761b46-a9fe-523d-a396-29ec4a33bd8a)
IT IS ONE of the principal objects of the NEW NATURALIST series to present in simple language to the lay reader the results of recent scientific work in the many fields covered by the general term “Natural History.” Another is to take the results of laboratory research into the realm of field studies and particularly to recapture the spirit of the old naturalists whose keen delight was in the study of animals and plants in their native haunts.
The present volume may be regarded in many respects as a background volume to the whole series in that it attempts to trace the evolution, through the many millions of years of geological time, of the geography of the British Isles and so to present a general view of the stage and setting of Britain’s Natural History.
The task has been rendered especially difficult for several reasons. In the first place it has been necessary to compress a large section of the science of geology into a very small space; in the second place it has been necessary to eliminate a whole scientific terminology which to the geologist makes for brevity and precision but which would be unfamiliar to the non-geologist. In addition, any attempt to reconstruct the geography of past ages is beset with pitfalls, so that the generalisations here presented may appear to have a definiteness which is not warranted by the facts. They must be regarded as liable to constant revision and even now, as the results of the borings undertaken in the intensive war-time search for oil are studied, they may be greatly modified.
THE EDITORS
INTRODUCTION (#ulink_ab29d044-8f47-5980-9c40-c26be2c499ad)
THE WEALTH of a country’s fauna and flora is not to be measured by numbers of species alone. Its wealth lies rather in variety, and to a naturalist in the British Isles the fascination of the native fauna and flora is in the great variety to be found in a small space. Gilbert White’s immortal Natural History of Selborne is, in essence, the natural history of a single parish of a few square miles. Yet like many another English parish Selborne, at the western end of the Weald in Hampshire near the borders of Surrey and Sussex, embraces within its limited area many distinctly different habitats or environments, each with its characteristic and often contrasted plants and animals. On the one side lie the open, wind-swept chalk downs with their calcareous soils and lime-loving plants, on the other the coarse sands of the Lower Greensand formation with sterile, acid, hungry soils—too “hungry” to attract the farmer and so given over to heathland and woodland of oak, birch and pine—whilst between the two are the Gault vale with its heavy clay soils and the magnificent “foxmould” developed on the Upper Greensand and accounted one of the finest agricultural soils in the whole of Britain. Such contrasts within a single parish or group of parishes are by no means unusual—indeed parish boundaries were often drawn originally so as to include as great a variety as possible of types of land—and they are reflected in the relief or form of the ground, in soils, in the natural vegetation cover and its associated animal life as well as in the way man, though kept within certain limits, has adapted the natural environment to his own ends. Small differences of elevation, slope, aspect and shelter cause purely local variations in the climate giving rise to different “microclimates” in the area, but they are variations sufficient to spell success or failure in many a farming enterprise, just as they permit or prevent the survival of a given species of the wild flora or fauna.
Those who are accustomed to larger spheres are apt to be obsessed with the discovery that it is by no means difficult to travel by road or rail from coast to coast of Britain, from east to west or even from north to south, in a single day. Yet in 25 miles of such a journey may be found a variety of scenery only to be equalled in a journey of ten times that distance in other lands. The kaleidoscopic rapidity with which the British scene changes is well illustrated from our coastline. It has recently been calculated that the coastline of England and Wales alone—the deeply indented and island-fringed coastline of Scotland is much longer in proportion—is some 2751 miles in length. Within that length may be found mud-flats, sand-dunes, shingle beaches, raised beaches and drowned valleys, sheltered bays and stormy headlands, together with cliffs of the most varied types. The cliffs alone range from the crumbling or slippery boulder clay slopes but a few feet high along parts of the east coast to giants rising almost sheer for a thousand feet from the sea below ; in colour and material they range from the dazzling white chalk of the south and east, or the brilliant red of the New Red Sandstone of Torbay, to the majestic greys and ochres of the granite coasts or the forbidding grey and black of some of the slate cliffs. Inland the story is the same. In the Scottish Highlands it is easy to find a dozen square miles not only without a human habitation or track, but also where the foot of man rarely treads and which even scarcely knows the foot of one of his domestic animals. Yet another dozen square miles of a part of the English lowland may be almost as densely peopled as any similar area on the earth’s surface and one where wild nature seems almost to have been eliminated.
England, Wales and Scotland are divided into 85 counties or shires (England 40, Wales 12, Scotland 33) most of which have persisted with few changes of boundary for more than a thousand years. These years have seen vast changes and some of the counties appear to-day anomalous in a modern world—small, sparsely populated, and poor—but, notwithstanding, intensely proud of their history and tradition and jealous of their ancient rights and privileges. Others have become, with vastly increased populations, too unwieldy and have been divided, so that there are now in England 50 Administrative Counties, making a total for Britain, without the Isle of Man, of 95 Administrative Counties, not including the large towns and cities which are County Boroughs having the status of counties. It would be interesting to know how many of the 45,000,000 people of Britain can lay claim to have set foot in each of the 95 counties. There are whole counties so far off the beaten track that not a man in a thousand has visited them or knows anything of the conditions of life in them, often so greatly different from his home area. How many, for example, know the Shetlands where the midwinter sun at noon does not rise more than six degrees above the horizon but where it is possible to read at midnight in summer without artificial light? If one liked to make the test more severe and ask how many of the 45,000,000 have visited every one of the inhabited islands which make up the British Isles it is most probable that the answer would be—none.
All this is intended to suggest the remoteness, the inaccessibility and the generally unknown character of so many parts of what, if one is content to think merely in terms of square miles and average density of population, is a small and very densely peopled country. So much of it is, indeed, a terra incognita even to the well-travelled minority. To the new naturalist there is much to be explored. It may be that to find a new species of plant or animal not yet described would be an event of unlikely occurrence, but there are many areas where the vegetation has never been mapped or described, where the changing balance of plant and animal life is waiting to be observed and recorded for the first time and where the explanation of observed changes is still a matter of guesswork. Some of the unexplained features are matters of the highest economic importance—the changing character of hill pastures, the new plant relationships created by afforestation and the introduction of foreign trees are some that spring to mind—and all are pregnant with possible scientific results.
Such are the opportunities awaiting the field observer. He has in his homeland what is in many respects a museum model illustrating the evolution of the world as a whole. For the great variety of environments is the outward and visible reflection of a long and complex geological history. Each of the great ages in the earth’s evolution has left its mark on these islands; rocks laid down in all the great periods of geological time are to be found represented in the British Isles.
It thus becomes the purpose of this book to trace the geological evolution of our homeland—to trace, step by step, the building of the British Isles. By this means we are able to understand the structure or the build of its contrasted regions. We are, in fact, attempting to understand the structure and the development of the stage upon which the drama of British natural history is played. In studying the broader aspects we need to consider the British Isles as a whole but, since so many of the books in this series consider primarily the island of Britain—that is England, Wales and Scotland—with its associated smaller islands, we shall consider in more detail and draw most of our examples from it. In tracing the evolution of the structure of the country and of its physical features we are, in fact, attempting to visualise the long history which lies behind the basal elements in its scenery—mountain and plain, hill and dale—recognising at the same time that the intimate details of that scenery are the work of man and lie outside the scope of this volume.
CHAPTER 1 (#ulink_ba92461a-1623-5276-b462-737a1b12c378)
HIGHLAND BRITAIN AND LOWLAND BRITAIN
SIR HALFORD MACKINDER in his now classic book, Britain and the British Seas, published in 1902, made a simple yet fundamental distinction between two roughly equal halves of the island of Britain. If one draws a line approximately from the mouth of the Tees to the mouth of the Exe it will be found that all the main hill masses and mountains lie to the north and west, the major stretches of plain and lowland to the south and east. To the north and west lies Highland Britain, to the south and east lies Lowland Britain. There is rarely, in nature, a sharp line between two such regions but rather does the one fade gradually into the other. This is true in Britain; nevertheless it is possible to draw a line with some accuracy and this has been done in Fig. 1.
In Highland Britain the dominant character of the country is upland. There are large and continuous stretches more than a thousand feet above sea level; plains and valleys occur but they are of limited extent and tend to form interruptions in the general upland character of the country as a whole. In some places are rugged mountains and even at lower levels crags of rock may appear at the surface. Even where the rocks do not thus appear at the surface itself they are often but thinly covered by poor stony soils, whilst the many steep slopes as well as the broken character of the relief may make farming difficult. On the whole man has sought out for himself the more sheltered situations for his farms, his villages and his towns. They nestle in valleys ; they flourish and spread only where the larger tracts of flat land or more fertile soil occur or where man has been attracted to otherwise inhospitable surroundings by stores of mineral wealth. The higher, poorer, wetter or less accessible parts of Highland Britain have been left largely to nature. There are vast stretches of moorland, some of it covering land from which the original forest cover has been removed, as well as mosses and bogs, scrub woodland and forests. We may summarise the position by saying that in Highland Britain human settlement is essentially discontinuous: the cultivated areas occupy valleys and plains separated by large expanses of uncultivated hill lands. This is well shown in the aerial view of a Lakeland valley in Plate I.
Lowland Britain offers a striking contrast in many ways. Though so much less rugged, there are few parts where level land is uninterrupted by hills and such true plains as do exist are to a considerable degree the result of man’s handiwork—as the large stretch of the drained fenlands of eastern England bears witness. Lowland Britain is best described as an undulating lowland where lines of low hills are separated by broad open valleys and where “islands” of upland break the monotony of the more level areas.
FIG. 1.—Highland Britain and Lowland Britain
Even the highest of the hills scarcely ever exceed a thousand feet above sea level, though many of the ridges reach 600 or 700 feet. The environment is kinder ; the soils tend to be deeper and richer, there are few steep slopes to interrupt cultivation and plough lands are to be found right to the tops of the hills. There is little to hinder man’s use of the whole: human settlement is essentially continuous and the cultivated land of one parish merges into that of the next. Villages and towns are closely and evenly scattered ; their siting has sometimes been dictated by convenience of a water supply, sometimes by situation on a natural routeway, sometimes just to maintain an even spacing of settlements. It follows that the greater part of Lowland Britain is occupied by farmland—by “cultivated” or “improved” land, which includes both plough and grass land—and that such moorlands, heaths, “wastes” and other unimproved lands as occur, do so as islands interrupting the otherwise continuous farmland and coinciding with patches of poorer soils.
It so happens that the rain-bearing winds in Britain blow from the west, especially from the south-west, and that the hill masses of Britain are in the west of the country. As a result Highland Britain as a whole gets a heavy rainfall, for the hills stand in the path of the rain-bearing winds, whilst most of the Lowland Britain is relatively dry. Taking a very general figure, most of Highland Britain gets more than 30 inches of rain a year while most of Lowland Britain gets less. Since the trouble with our climate is that we tend to get too much rain and too little sun it is clear that differences in climate reinforce those due to other features between Highland and Lowland Britain. In the far north-west, for example, where lowland does exist it is of comparatively little value because heavy rainfall and constant cloud result in water-logged conditions and almost useless bogs occur where otherwise good farmland might be found. The contrast in rainfall is shown in the accompanying map, while the effect of the combination of high ground and high rainfall is well brought out in Fig. 3 which shows the general distribution of moorland.
Although the chief visible contrasts between Highland and Lowland Britain are thus to be found in elevation, relief of the ground and soils, these are, in fact, the results of the underlying geological structure. The rocks which underly the hills are for the most part old even in the geological sense and in the course of ages have tended to become indurated and hard or at least resistant to weathering and hence tend to form hill masses rising above the general level. The rocks which underly the lowlands on the other hand are younger in the geological sense, though often very ancient if measured in years. They are softer or less resistant to weathering—there is evidence of this in the muddy streams to be seen after heavy rain, showing that soil or mud or silt is being swept off the land and carried seawards.
It happens that Highland Britain is made up mainly of rocks which are older than the Coal Measures (which contain the bulk of workable coal seams in this country) whereas Lowland Britain is made up mainly of rocks which are younger than the Coal Measures. It is for this reason that we find most of the coalfields of Britain on the margins of the highlands and the lowlands.
FIG. 2.—The Mean Annual Rainfall of the British Isles, showing the contrast between the wetter west and the drier east
The greatest of them all, the Yorkshire, Nottinghamshire and Derbyshire Coalfield, stretches up on to the Pennines and down on to the lowlands ; so do the coalfields of Lancashire and of Northumberland and Durham. The great South Wales field lies just north of the fertile Vale of Glamorgan and amongst the moorland heights.
FIG. 3.—The Chief Moorland Areas of Great Britain, showing the close correspondence with areas of heavy rainfall (Fig. 2) and their association with Highland Britain
As a prelude to understanding the different regions of Britain and consequently the varied habitats in which our plants and animals live it is thus essential to understand. the geology and structure of the ground and to appreciate something of the long and complicated geological history of the country.
CHAPTER 2 (#ulink_71c373bb-98ed-5360-aefc-152a1a0a537a)
READING THE ROCKS
IT HAS long been known that the solid rocks which build up the earth’s crust sometimes contain remains of animals and plants. A slab of shale from the tip-heap of a coal mine may be split open to reveal a beautifully preserved and delicate leaf which, whilst bearing a superficial resemblance to some ferns, on closer examination is found to be different from anything now living. In one of the literary classics which geology has given the English language the young Scottish stone-mason, Hugh Miller, has described the thrill of the chase when his hammer was used to make the Old Red Sandstone rocks give up entombed fragments—so clearly of fish yet so utterly different from their counterparts of the present day. Sometimes it is merely the footprint of some long-extinct reptile, sometimes the actual bones or teeth ; at other times it may be the remains of some minute creatures only revealed by the microscope which excite interest and inquiry into the origin of fossils, as all these remains of the animals and plants of the past are called.
Fossils were once hailed as incontrovertible evidence of the reality of Noah’s flood. Since it was soon made clear that they were to be found in different rocks and at different levels there arose the idea of several successive creations each in its turn overwhelmed by a great deluge. Forming as it were a mantle over many parts of the country are superficial deposits of clay, sand and gravel, to which reference will be made later. These “drift” deposits not infrequently contain shells and other fossils and the inference that these deposits were laid down by the latest flood was so obvious that they were called by the eighteenth century geologists “diluvium” (Latin, diluvium, a deluge or flood) or “diluvial deposits.”
A great advance was made when William Smith (1769 – 1839), who has very rightly been called the Father of English Geology, showed that the same fossils (i.e. different specimens of the same species) were to be found in different parts of the country, sometimes in the same type of rock but sometimes in rocks of different types. If several different species were associated together in one area then the same ones would be associated together in another. So he introduced the Law of Strata identified by fossils, and was able to produce the first geological map of England and Wales (published 1815). Two limestones from different parts of the country might appear to be very similar but if they contained different sets of fossils the inference was that they were of different ages. If on the other hand a sandstone from one region contained fossils identical with those from a shale in another the inference was that the two rocks were being formed at the same time—that they were of the same geological age or “synchronous” and could be “correlated.” It was found that when a species died out or “became extinct” it did not reappear in later rocks. Of course this was quite consistent with the idea of a succession of separate creations and it was not till much later that the theory of evolution permitted the tracing of the relationship between the fossils of one set of rocks and those of another.
The determination of the geological age of rocks by the fossils they contain is one of the two fundamental principles underlying the whole study of historical geology. The other is the Law of Superposition. Where one bed of rock rests upon another it is presumed that the upper bed was laid down after the lower and hence that the upper bed is the younger. A large proportion of the fossil-bearing rocks are sedimentary rocks (i.e. they were laid down as sediments under water—in the sea or in fresh-water)—and this law is true for nearly all such rocks. It is also true for streams of lava poured out from volcanoes or associated beds of ashes. Pompeii was there before the ashes by which the city was buried. But the Law of Superposition only remains true so long as the original order of the rocks has remained unchanged. With earthquakes and mountain-building movements the original order may be changed—the rocks may be folded, or even bent right over so that the original order is reversed. But reversal of the order in this way is the exception, not the rule, and can be detected by detailed survey. No one who has spent a holiday on the magnificent coast of north Cornwall can fail to have noticed how folded and broken are the rocks exposed along the sea cliffs. Examples are shown in Plates III and IV.
Presuming the original order to have been maintained, if the upper bed in one locality be traced laterally it may be found to pass under still higher beds in another so that the higher beds in the latter area are still newer. In this way a whole succession of strata may be built up—from the very oldest at the bottom to those still being formed at the top. Such a succession has, in fact, been constructed as a result of patient research and forms what is sometimes called the geological column. It must be realised that the rocks of the geological column are not to be found complete in any one area.
It must not be thought that if a hole were bored in the earth’s surface it would pass through all the rocks shown in the geological column. At the present day deposits of sand, silt and mud are being formed in the shallow waters near the estuaries or deltas of the great rivers of the world while other deposits, some of them consisting mainly of the hard parts of organisms living in the water, are being formed over the floors of most of the seas and oceans. In the lakes of the world other deposits are being laid down and even on parts of the land surface deep layers of sand and dust brought by wind are being spread over the older rocks. This is quite clearly seen where, as in the Culbin Sands of Morayshire shown on Plate XII, sand dunes are burying growing vegetation. These are all areas where deposition is taking place and where new strata are being formed. But over most of the land the rocks are being worn away by the combined action of rain, wind, sun, frost, running water and moving ice and its surface slowly but inevitably lowered. These are areas of denudation (Latin, denudo, I lay bare) and to them may be added the margins of the oceans where waves beat against the shores and wear them away. There will be no deposits in such areas to mark the present day and it was the same in the past. Thus beds present in one locality may be absent in another so that in the latter place there is a gap in the succession. Such a gap may indicate that the region concerned formed part of a land mass at the time in question or came otherwise under the influence of denudation. When the region again sank below sea-level and strata were again deposited it was perhaps after a lapse of many millions of years. Here is a “stratigraphical break” between the older and younger rocks. The younger rocks are said to rest “unconformably” on the older in those cases where the older had been folded and denuded in the meantime. A typical unconformity is shown in Plate IVB.
There is another difficulty in reconstructing the stratigraphical column. When one bed of rock or stratum is traced laterally it may change its character and unless the whole change can be traced and a limestone, for example, found to pass laterally into a shale or sandstone, it may be difficult to say that the limestone in the one place is of the same age as the sandstone in another. Of course if both types of rock contain the same fossils the answer is easy, but just as different habitats at the present day—muddy waters and clear lime-rich waters—may not have even a single species in common, so it happened in the past, and the fauna of a limestone may be completely different from the fauna of a bed of shale of the same age. To take a specific example, the beds known collectively as the Old Red Sandstone were laid down in freshwater lakes at the same time as the marine beds of the Devonian were being deposited elsewhere. In such cases the rare instances where the faunas are mixed, or there are “marine bands” representing incursions of the sea in the midst of a fresh-water succession, are invaluable in establishing the essential correlation. Thus the evidence which the geologist has to piece together is at the best fragmentary: it is rarely too that the rocks he wishes to study are “exposed” over large areas. In a country such as Britain the surface is hidden by soil and vegetation and only in some of the higher mountainous areas or along sea cliffs do the bare rocks outcrop at the surface. Elsewhere the geologist has to seek his evidence in quarries, mines, railway cuttings, well-borings, casual excavations for drains and sewers and even in some cases may be faced with the necessity of opening up a special pit in a crucial spot.
The rocks which are seen in the Stratigraphical Column were deposited over an immense period of time. Time is continuous, but there are certain natural phenomena which serve to divide it into definite units. The phenomenon of day and night serves to define one unit of time—the day ; the movement of the earth on its orbit round the sun defines another—the year. Larger units than the year are difficult to define but just as the astronomer uses a “light-year”—to define an enormous distance, so the geologist needs a larger unit than the year. The historian frequently takes the time between two important events to define a period ; thus when we talk of Tudor Times we mean the period when the Tudor kings were on the English throne, though we are able to define this period accurately in years—from the accession of Henry VII in 1485 to the death of Queen Elizabeth in 1603. The prehistorian is no longer able to measure his periods so accurately: he is obliged to define them in terms of the works of man in the periods concerned. The geologist, in his turn, has to deal with the vast periods of time which elapsed before the appearance of man on the surface of the earth ; for the definition or delimitation of such periods the year is an inadequate unit. No one would hand a traveller going on a long sea voyage a six-inch ruler and ask him to measure thereby the distances between the ports en route. Yet the voyager, by careful observation of time and direction, might be able to give a very fair account of the relative positions of the points touched, a close estimate of the distances between them and a good general account of their chief features. It would depend on his power of accurate observations and of using all the available evidence in its appropriate place. Thus the geologist has built up a good general picture of the evolution of the earth’s surface, a picture which is continually gaining in accuracy, and the geological time-scale is divided into a few great eras and a number of periods. The smallest unit of geological time is the hemera, usually named after a dominant animal or plant which was living at the time. A difficulty is that the animal or plant may have been local in its distribution, so that its absence from the sequence in a given locality is scarcely sufficient evidence that no deposition of beds was going on there at that time. A somewhat larger unit is the zone which, though named after a characteristic fossil, is usually to be defined by a characteristic associated series of fossils. A number of zones normally comprise a formation of rocks, while several formations make up a system of rocks. Thus we talk about the Chalk and the Lower Greensand as two of the formations in the Cretaceous System of rocks. But the word system refers to the rocks in the geological column: the rocks in a system were laid down in the period of time known as a geological period so that the measure of time concerned in this case is the Cretaceous Period. A number of periods are included in each of the four great eras into which geological time has been divided since the general appearance of life on the surface of the globe—i.e. since the deposition of the rocks which contain the earliest recognisable fossils. Even before that the earth had a long and complicated history which is gradually being unravelled and lowly forms of life doubtless existed but have left little or no trace.
On the general divisions of the geological time-scale and on the sequence of the periods all geologists are agreed though there is constant discussion regarding the exact definitions of the periods and whether a given bed of rock was laid down at the end of one period or the beginning of the next.
The layman is constantly demanding to be told the age of a given bed in years and amongst geologists themselves the age of the earth has always been a fascinating subject.
FIG. 4.—The Geological Column with the names of the geological periods and an approximate time scale in years.
One ingenious calculation worked out the amount of dissolved salts carried down to the sea every year by the rivers of the world and consequently how long it would have taken the ocean, presuming the water of the ocean to have been fresh originally, to have reached its present degree of salinity. A rough method at best, it breaks down as there is no evidence that the waters of the world ocean were originally fresh. In recent years a method of estimating geological time in years has been devised and used with considerable success. There are certain elements—the radioactive elements—which undergo disintegration at a constant but very slow rate which can be and has been measured. When a minute crystal of a radioactive mineral is enclosed in a larger crystal of certain other minerals—such as the dark mica, biotite—the emanations from the radioactive mineral cause a visible change in the surrounding mineral. When studied in section under the microscope the size of the zone of alteration, or “pleochroic halo,” affords a means of measurement of the time which has elapsed since the original formation of the rock. Another method is by the very accurate chemical analysis of the unaltered radioactive substance proportionate to the amount of the final end-products of its disintegration.
Piecing together the evidence, the geological column and the approximate duration of each period are in Fig. 4.
The names of the periods are reminders both of the richness of the British Isles in its varied geology where all the great systems are represented and also of the pioneer part played by British scientists in the geological field. The Cambrian takes its name from Cambria or Wales; the Ordovician and Silurian from two tribes of ancient Britons who lived on the Welsh borderland where these rocks are well developed and where they were first described. The name Devonian is from the county of Devon. Permian is a name which honours the pioneer studies of the British geologist Murchison in the province of Perm at the request of the Russians. Carboniferous (carbon- or coal-bearing) and Cretaceous (chalky) are descriptive of British conditions whilst the names of the divisions of the Tertiary are reminders of the Greek scholarship of Charles Lyell and his followers. The Jurassic (Jura Mountains) combines the older English Lias and Oolites; the Trias takes its name from the three-fold division typical of that system in Germany. Only the Rhaetic (Rhaetic Alps) is poorly represented in Britain. The Quaternary is not really comparable in duration or importance with the other great eras.
The coal-bearing strata of the Coal Measures form the upper division of the Carboniferous.
* Often written Cainozoic or Kainozoic.
CHAPTER 3 (#ulink_f0082ec1-70c7-5f18-82e7-33abfe68c37d)
EARTH HISTORY—TIME AND LIFE
WE ARE fortunately living in one of the quiet periods of the earth’s history, though not at a time when it has been quiet for so long that the surface has been worn down to a dull and monotonous level. Although the accurate instrumental observation of earthquake phenomena demonstrates that the earth’s crust is rarely absolutely still, the occasional earthquakes which do occur, however severe they may be, are in reality but very gentle reminders of those periods in the history of the earth when the whole surface must have been torn by the most violent and long-continued cataclysms which bent and folded and broke the hardest rocks; which caused whole blocks of the earth’s crust to be thrust many scores of miles over other blocks; which caused continents to sink below the waves of the sea, or which caused vast tracts of the ocean bed to be raised to form the world’s highest mountains. The earthquakes of to-day are like the final murmurs of a great storm which has passed. Even they tend to occur in certain defined earthquake belts and are reminders that the earth’s crust has certain lines and zones of weakness whereas other parts are relatively stable.
If we exclude the great mountain-building movements which took place in the dim early days of the earth’s history—in the Pre-Cambrian—there have been three great periods of mountain building or “orogenesis” (Greek, oros mountain; genesis origin, creation) as far as Europe is concerned. Each of these has played a major part in determining the present-day features of Britain. These three great mountain-building periods were:
(a) at the end of the Silurian and beginning of the Devonian periods—the Caledonian earth-movements, so called because they built up the great mountains which have since been worn down to form the Highlands of Scotland (Caledonia);
(b) at the end of the Carboniferous and beginning of the Permian periods—the Armorican or Hercynian earth-movements, so called because they caused the great folding of the rocks seen in Brittany (Armorica) and the Hartz Mountains of Germany. In Britain they caused the uplift of the Pennines, the Malvern and Mendip Hills and folded the Coal Measures into basins.
(c) in the middle of the Tertiary era—the Alpine earth-movements, so called because they were responsible for the rise of the Alps as well as of many of the great mountain chains of the world to-day but which affected Britain much less than the previous movements.
Of the still earlier earth-movements at least one left very important marks in Britain—the Lewisian, which caused the folds in the rocks in the extreme north-west of Scotland and which may have been contemporary with the folding of the ancient rocks which peep from beneath a cover of later strata in the Charnwood Forest of Leicestershire.
In each case the mountain-building movements gathered strength slowly as with a developing storm and gradually reached a peak when the whole earth must have experienced a constant succession of gigantic earthquakes. Then gradually they must have died away again, the whole cycle stretching over an immense period of time. The result of these earth-building or orogenic movements was to form a series of gigantic wrinkles in the crust of the earth—these are the main mountain chains—between which are broad areas but little disturbed—the “tectonic” basins (Greek: tektonikos, related to building—i.e. not formed by later excavation. Sometimes these basins were below sea-level and became the areas of sedimentation in the succeeding periods, while the surrounding mountains as soon as formed were attacked by the forces of denudation which started to wear them down. So we get the idea of the geological history of the earth moving in great cycles. The first is what may be called the major cycle of denudation. This may be considered to begin when earth movements have caused land to rise above the level of the waters in the surrounding ocean. No sooner does this happen than the forces of sub-aerial denudation get to work. The heat of the sun heats the rocks and the different minerals of which they are composed have differential rates of expansion so that, especially with nightly cooling, the rocks are disrupted and a peeling or exfoliation (Latin: folium, a leaf) by successive layers takes place. This is sometimes called onion weathering and is well seen in hot dry countries at the present day. The direct action of the sun is called insolation.
FIG. 5.—Diagrams showing the mechanism of exfoliation or onion weathering of rocks under the sun’s heat
Falling rain has a direct mechanical effect in washing away the finer particles, a less direct effect by dissolving some of the less stable minerals and an indirect effect by soaking into crevices. There it may be frozen and the water in changing to ice expands so that the crack is widened. This is the basis of frost action, through which great blocks may be split off from mountains and fall to lower levels as screes. Wind, too, plays its part by blowing away the finer dust and sand whilst strong wind armed with sharp sand particles is a powerful abrading agent. In newly formed mountain areas gravity itself plays a large part—for example in the formation of screes. Both in mountain areas and at lower levels landslides are by no means unknown. Gravity also causes the well-known phenomenon of soil creep, whereby soil gradually slides downhill. The process is seen at work in Plate 9B. Rain collects together to form mountain torrents which in turn unite to form swift rivers sweeping masses of debris always from higher to lower levels, from the land towards the sea. The eroding and transporting action of running water is paralleled in colder climates by the action of moving ice—glaciers which move slowly but inexorably down valleys or great icesheets which ride over the whole surface of the land, scooping out hollows where the rocks are soft, smoothing and polishing them where they are hard. In tundra lands the sub-soil remains permanently frozen whilst the surface thaws in summer and, where there are steep slopes, masses of sludge slide downhill, the whole process being called solifluction. On the margins of the seas and oceans wave action is a powerful force in wearing away the newly formed lands.
Whilst the major surface features of Britain owe their origin to the mountain-building movements of the past and to the character of the rocks which make up the land masses, many of the most striking scenic details are the result essentially of the different processes of weathering on varied rocks. In high mountain areas frost plays a large part and accounts for the angular rock surfaces such as those seen on Striding Edge (Plate XVIB) or in Snowdonia (Plate 8A) or on Cader Idris (Plate XXIX). Sometimes the sculpturing action of frost produces fantastic results, as in the well-known Sphinx Rock on Great Gable in Lakeland. Screes of fallen angular blocks and fragments of rock, most of them broken off by frost action, are a well-known feature in all mountain areas and sometimes dominate the landscape. Plate 30B shows the famous screes on the south side of Wastwater. Blocks of rock dislodged by the undercutting action of the sea and the action of rain form screes along many sea cliffs; a typical example from Cornwall has been shown in Plate 8B to illustrate the angle of rest assumed by loose rock of average character. The angle is much lower where rocks such as clay-shales become slippery when wet, and is lowest where the actual rock may “flow” when wet, which is the case with clay.
Onion weathering under the influence of the sun leaves hard, rounded cores of rock. In tropical countries, these may be almost true spheres; in this country such “cores” scattered over the country are familiar in many granite areas. A good example may be seen on Crousa Common (The Lizard, Cornwall), whilst the interesting weathering of granite, seen in such “tors” as those of Dartmoor (Plate XXVII) is to be ascribed mainly to the same action.
The most interesting results are seen where the original rock varies in hardness. A sandstone, for example, may be indurated along certain lines and the denuding agent whether wind, rain, running water or the sea finds out the pockets of softer sand and scoops them out. The interestingly fretted rock shown in Plate VI is actually the result of the action of the sea, but a very similar appearance might be due to wind action. Where a rock is fractured rain washes out the loose, crushed rock and produces striking cliffs such as those shown in Plate IB. Even in Lowland Britain the “High Rocks” of Tunbridge Wells are simple examples of differential weathering.
Immediately after a great earth-building movement the deposits which fill the hollows—the tectonic valleys and basins—are coarse and often consist of angular blocks which are actually screes and may become consolidated to form a “breccia.” Beds of roughly rounded boulders and large pebbles may be deposited by swift streams to become consolidated later as conglomerates and pebble beds. Plate VIIIB shows an example from the Lake District of such boulders being swept down by a stream in flood. As time goes on the mountains are worn down, yield less material and the beds laid down in the basins and seas become finer grained in character—sands and silts and muds, which may become consolidated respectively into sandstones, siltstones and shales. In the later stages of the cycle muds and clays will definitely predominate and when the lands have been worn down almost to plains (called “peneplanes” or “peneplains”—Latin: pene, almost) they will yield so little sediment that the waters of the surrounding seas may become quite clear. These conditions of clear tranquil water are those under which corals flourish and also other organisms which build up their hard parts of calcium carbonate; thus the deposits then formed are often limestones. The cycle of denudation on the land and of sedimentation in the water is brought to a close by earth movements, it may be slight at first, which herald the oncoming of a new storm. More often the major cycle of events is varied by minor earth movements—it may be the so-called “eustatic” movements, not of folding of the earth’s crust, but of the gentle elevation or depression of blocks of it relative to the level of the waters—so that minor cycles of sedimentation occur within the major. This is well illustrated in the geographical evolution of the British Isles.
So far nothing has been said regarding what is now known of the structure of the earth as a whole. It cannot be too forcibly stated that the old concept of a solid crust, rather like the skin of an apple, covering a molten interior, is entirely wrong and that the simple deduction that the whole was cooling and contracting so that wrinkles—which were the mountain ranges—were being formed just as when an apple dries is equally false. We now know that there is a central sphere, solid and very heavy and probably consisting of an alloy of iron and nickel—thus agreeing in composition with some of the meteorites which from time to time fall on the earth’s surface. This iron-nickel core accounts for the magnetic phenomena of the earth. Enveloping this is the crust, in all about 700 miles thick—a figure which may be compared with a height of 5 miles for the highest mountain and a depth of 6 miles for the deepest ocean. It is well known that there is a rapid increase in temperature as one goes downwards in the crust so that even in a deep mine it is almost unbearably hot.
FIG. 6.—Diagram of the Fault shown in Plate IA. This is a typical example of a very small normal fault. The fault plane separates the downthrow side on the right from the upthrow side on the left. The angle which the fault plane makes with the vertical is the hade; the vertical displacement (here only a few inches, though in big faults it may be thousands of feet) is the throw. Normal faults occur under tension whereas thrust faults and structures such as are shown in Fig. 72 occur under extreme compression.
It does not necessarily follow that the solid core of the earth is extremely hot, since it is now known that heat accumulates in the lower layers of the crust through radioactivity. What is important is not the temperature of the central core but of the crust. At no great depth the temperature must be such that all rocks would be molten were they not kept in a solid or more probably a plastic condition by the pressure of the solid rocks above. Towards the end of a major cycle of denudation, however, so much material has been removed from one part of the surface of the crust to another that the pressure is lessened over the land. Some of the underlying heated layer becomes actually molten and seeks to find weak spots or lines in the crust through which it can escape. It may reach the surface and be poured out through the craters of volcanoes (volcanic eruptions) or through cracks in the surface (fissure eruptions) as lava. Some of the molten rock does not reach the surface but forces its way into cracks and there consolidates as wall-like masses or dykes; or it may force its way parallel to the bedding planes of sediments to form sills. A striking example of an old volcano with associated sill is found in Arthur’s Seat, Edinburgh, shown in Plate XVIA. In all these cases the molten rock bakes and hardens the rocks through which it passes—it changes their form by its contact (Greek: meta- change,
morphe form, hence the process is called contact metamorphism).
FIG. 7.—Diagrammatic Section of an Unconformity A—B is the plane of the unconformity. After the deposition of the group of beds marked C they were gently folded by earth-building movements and were subjected to denudation. Gentle subsidence followed so that the group of beds marked D were deposited gradually over a larger and larger area—they rest unconformably on the older series and at the same time overstep them. In the centre of the basin fine-grained shales were deposited and the diagram suggests that sedimentation was almost continuous. Towards the margins of the basin the fine-grained deposits pass laterally into sands and other coarser sediments and to shore deposits.
This section represents diagrammatically the relationship between the Silurian and the underlying Ordovician in the Welsh Borderlands. See also Plate IV B.
Thus clay and shale are baked into hard slatey rocks, limestone is changed into marble. Some of the molten rock is very fluid when it is first poured out and spreads evenly over a wide surface as did the basalts of Northern Ireland; sometimes it formed hexagonal columns on cooling as at Giant’s Causeway and the Island of Staffa (Plate XVA). In other cases the molten rock was very sticky and consolidated almost on the spot—the famous conical “spire” of Mont Pele of Martinique in the West Indies is the best modern example of this (it was formed during the disastrous eruption in 1902) but there are many examples from earlier periods in the British Isles, such as Ailsa Craig in the Firth of Clyde. Even more important, though it cannot be observed at the surface, is the underground movement of great masses of molten “magma.” At the height of great earth-building movements the magma is squeezed into the core of mountain ranges so that millions of years afterwards, when the mountains have been denuded down to their roots, this core is exposed. A typical rock so formed is granite. The metamorphism caused by a huge granite mass taking eons to cool can perhaps be imagined rather than described and the “metamorphic aureole” is often very extensive; it is economically important because of the valuable metallic minerals which are associated with the gases and heated liquids given off by the magma. These latter often find their way into cracks or veins and there the minerals are deposited—hence the association of ores of tin and copper with the metamorphic aureoles of the granites of Devon and Cornwall. These Devon and Cornish granite masses are the roots or cores of giant mountains formed by the Armorican earth movements but long since worn down almost to a level surface. It is clear that there is a definite cycle of igneous activity associated with a cycle of earth movements—volcanic activity heralding the oncoming storm; intrusion and movement of huge underground masses at the height of the storm; and finally renewed volcanic activity when the storm is dying away. The few volcanoes on the surface to-day which are active may be regarded as the last remnants of the once wide-spread activity at the end of the Alpine earth movements. Many of those are at the end of their lives—dormant or even extinct or merely giving off vapours (“sol-fataric stage”). Hot springs and geysers are indications of a nearly dead volcanic area.
One of the still unsolved puzzles of earth history is whether or not there have been true climatic cycles in the past. There is no doubt that at several periods there have been ice ages, though perhaps nothing as severe as that which overwhelmed the northern hemisphere so recently in geological time that man was already well established and hence known simply as “the Ice Age.” We know definitely that some of the red rocks found in the British Isles—such as the Permian or New Red Sandstone—were laid down under desert conditions and there must have been other times when what is now our country must have been wet and hot. Many of the phenomena, however, may be explained by a different distribution of land and water in the past or at most by a shifting of the earth’s axis.
FIG. 8.—Major episodes in Earth History
FIG. 9.—Some characteristic Fossils.
Those numbered I are Palaeozoic; those numbered II are Mesozoic and III Cenozoic.
Graptolites:—Ia Monograptus (Silurian); Ib Diplograplus (Ordovician); Ic Didymo-graptus extensus (Ordovician); Id D. murchisoni (Ordovician). Trilobites:—Ie Phacops caudatus (Silurian); If Calymene blumenbachi (Silurian); Ig Ogygia buchi (Ordovician). Primitive fish:—Ih Pterichthys milleri and Ii Cephalaspis lyelli (Old Red Sandstone). Brachiopods:—Ik Spirifer verneuilli (Devonian); IIb Spiriferina walcotti,; 11c Rhynchonella rimosa and 11d Waldheimia numismalis (all Lias). Ammonites:—11a Dactylioceras commune (Lias); 11e Pcrisphinctes biplex (Jurassic, Kimerid-gian); 11f Hoplites splendens (Cretaceous: Gault); 11g Hamites attenuatus (Crctaceous: Gault): 11h Hoplites auritus (Cretaceous: Greensand). Nummulites: 111a Nummulites laevigatus (Eocene).
The existence of a plastic or semi-molten layer under the solid part of the earth’s crust has already been argued and there is nothing inherently impossible in the idea that the continents consist of relatively light rocks and form masses as it were floating on a plastic layer. If this is so, it is but one step towards the idea that the continental masses may drift away from or towards one another—hence the Theory of Continental Drift associated with the name of the German geologist Wegener. But the attempt to secure observational confirmation of drift, however slight, has been disappointing.
The similarity in the rocks which make up such widely separated lands as Africa, Peninsular India and western Australia is so striking, however, that this absence of direct observational proof of drift is inconclusive. It may be that drift only takes place at certain periods when the underlying rocks are in a particular condition of plasticity.
Amidst all the changing scenes which geological time has witnessed—for we may say that geology is really geographical evolution—there has gone on the evolution of living organisms. Just as in times of war things happen and life is speeded up so there is some evidence to show that the rapidly changing environmental conditions which must have characterised the great periods of earth movement induced a rapidity of organic evolution. Whether that be so or not the fact remains that before the Caledonian movements the world was populated almost exclusively by lowly plants which have left few traces and by invertebrate animals. After the movements was the age of fishes—the many weird forms of the Old Red Sandstone—and a rapid evolution of fern-like plants. Before the Armorican movements there were some amphibians but it is after the movements that we have the great age of reptiles and the seas became populated by the well-known coiled ammonites and innumerable brachiopods. The curious graptolites which scarcely survived the Caledonian movements had completely gone. The doom of the heavily armoured and small-brained Jurassic and early Cretaceous reptiles was sealed well before the earliest inklings of the Alpine movements. The victory of the mammals, with man himself to follow later, was assured long before the time the Alpine storm really broke.
FIG. 10.—Diagram illustrating the distribution in time and space of a typical fossil. ZI, ZII, ZIII, ZIV, ZV are zones
The species 1 is found at all five localities A, B, C, D and E and is restricted in its vertical distribution to Zone III. Only in one locality, C, is it found slightly below and slightly above the limits of the zone. It is therefore a good “zonal index.” Species 2, on the other hand, is only found in three of the five localities; it has a wide range in time and is found at a much lower horizon in locality C than in locality E. It is therefore of no use as a zonal index but such a distribution is characteristic of a “facies” fossil—a species seeking some special habitat conditions such as shallow water near a shoreline.
We are now in a position to apply the general principles already enunciated and to see how far they explain the building up of the present-day structure of the British Isles and to see what evidence the rocks of this country contain of the evolution of the great world groups of animals and plants and, in the later stages, of our own particular native fauna and flora.
CHAPTER 4 (#ulink_110ad142-6a89-57ad-af6b-cc01951ef576)
THE GEOLOGICAL MAP
DURING the war an important series of maps, designed to supply information basic to any work of national planning, was initiated by the Research Division of the Ministry of Town and Country Planning and published by the Ordnance Survey. The main series is on the scale of 1: 625,000 or approximately 10 miles to the inch and the series to be issued on that scale, covering the island of Britain in two large sheets, includes many of the maps which were proposed in the scheme for a National Atlas drawn up by the National Atlas Committee of the British Association for the Advancement of Science. Amongst the maps actually prepared and available, those of the Relief of the Land, Land Utilisation, Types of Vegetation, Types of Farming and Land Classification are very relative to the study of the natural history of this country, but fundamental to any study are the geological maps. Following the practice of the Geological Survey in some of their detailed maps, there are to be two maps—one to show the history of this country, while fundamental to any study are the geological maps. Following the practice of the Geological Survey in some of their detailed maps, there are to be two maps—one to show the “solid” geology as it would appear if superficial deposits such as boulder clay, glacial sands and gravels and clay-with-flints were removed and the other to show the “drift” geology with all those surface deposits indicated.
(#litres_trial_promo) It has already been pointed out that many geologists are interested primarily in the older rocks and in the structure of the earth’s crust and the superficial deposits are to them simply a nuisance; from the economic point of view in the investigation of mineral resources the same is true and so the drift deposits and their mapping have been relatively neglected—notwithstanding their supreme importance in agriculture.
The Geological Survey came into existence in 1835 as an offshoot of the Ordnance Survey and its immediate task was the mapping of the geology of the country on the scale of one inch to one mile. This work was vigorously prosecuted, especially under the energetic guidance of Sir Roderick Impey Murchison who became Director-General in 1855, and the results were published in the form of hand-coloured one-inch maps, the base maps used being the original one-inch sheets of the Ordnance Survey. These original one-inch maps were “solid” maps though indications of the presence of drift is conveyed by words printed across the map in appropriate places. Its preliminary work finished, the Geological Survey then set to work to carry out a detailed revision or second survey. The field work was done on the six-inch or even, in cases, a larger scale. Attention was concentrated on the coalfields and on areas around populous centres, together with selected tracts in different parts of the country. For purposes of publication the “small-sheet series” of one-inch Ordnance Survey maps was chosen and these have been kept up to date for use as Geological Survey base maps long after they have been superseded by other editions for ordinary uses. They cover an area of 18 miles from east to west and 12 miles from north to south, and have the advantage of showing contours in black as part of the base map. Some of the small sheets series are hand coloured but the technical advances in colour printing has meant that all the later maps have been printed in colours. Each sheet is accompanied normally by a detailed explanatory memoir. In those areas where drift deposits are widespread or important it is usual to publish two editions of the map, the Solid and the Drift Editions, which are in fact two distinct maps. After more than half a century of work only about a third of the country has been re-surveyed and the maps published, so that for the rest reliance still has to be placed on the original survey and hand-coloured maps of a century ago. There is thus an obvious difficulty in issuing generalised maps of the whole country in that the detail available is so varied from one part to another, and it explains why, in a country where superficial deposits are of such tremendous importance in the study of soils, vegetation and agriculture, there is no generalised map to show their distribution. Pending the publication of the maps on the 10-mile scale just mentioned there is a useful map of the solid geology of the British Isles on the scale of 25 miles to one inch, published by the Ordnance Survey as one sheet at the modest price of two shillings.
This map is of the greatest value in giving a general picture of the distribution in Britain of the rocks of each of the systems. In the third edition, which is dated 1939, very considerable revisions and additions were made as a result of incorporating recent work.
Speaking very generally,. in geological terms the oldest part of Britain is the north-west and on the whole the rocks become steadily younger in geological age as one goes towards the south and east so that the major stretches of the Tertiary rocks are to be found in the London and Hampshire Basins.
The great mass of the Highlands of Scotland is made up of a complex of ancient metamorphic rocks with numerous large intrusions of granite. As described more fully in Chapter 23, one must picture the whole as the worn-down stumps of the great Caledonian fold mountains and there is little to-day in the relatively tame, rolling relief of much of the Scottish moorland to suggest the wild contortions exhibited by the underlying rocks—structures the interpretation of which has long baffled and continues to baffle the most expert of geologists. The oldest rocks of all are probably the Lewisian gneisses in the Outer Hebrides but the relative ages of the different pre-Cambrian or Archaean rocks is still uncertain. Along the coastlands of the North-West Highlands is a considerable stretch of Torridonian Sandstones—still pre-Cambrian but unmetamorphosed sediments, obviously much younger than the main bulk of the Highland rocks (see Plate IB). There is a narrow belt of Cambrian also in the north-west of the Highlands but the main masses of Old Red Sandstone—reminders of the great lake basins created by the Caledonian upheaval—lie on the east. Tiny patches of Jurassic rocks both in the west and along the east coast in the far north are reminders that Jurassic seas must have stretched far north but have left only small traces, and there is little evidence of the detailed geological history of Scotland over vast periods of time. Though the Alpine earth movements failed to fold the rigid old mass of the Highlands there is abundant evidence of the way in which great cracks were formed through which poured masses of molten lava. These make the great red splashes on the map in Skye and Mull and many of the smaller Hebridean islands.
A glance at the distribution of the deep purple colour used to indicate the Cambrian and the mauves used for the Ordovician and Silurian serves to demonstrate that it is the Older Palaeozoic rocks which make up the Caledonian mountain ranges of the Southern Uplands of Scotland, the English Lake District and the Isle of Man, as well as the whole of north and central Wales. That the Old Red Sandstone occupied basins is not only clear from the Highlands of Scotland: there are broad belts in the great tectonic depression of the Central Lowlands of Scotland between the Highlands and the Southern Uplands. In the south the main stretch of the Old Red Sandstone is in the Welsh Borderland whilst marine Devonian rocks of the same age cover Exmoor and much of South Devon and Cornwall.
Three colours are used for the rocks of the Carboniferous system—blue for the Carboniferous Limestone and the Scottish rocks (with sandstones, shales and coals) of the same age; sage green for the Millstone Grit and barren Culm Measures of Devon and Cornwall, and slate-grey for the Coal Measures. In passing it must be noted that the outcrops of Coal Measures shown on the geological map are not co-extensive with the coalfields because much of the most valuable parts of the coalfields are hidden beneath younger rocks. The Carboniferous rocks, with many areas of lava and other reminders of volcanic activity, fill in the remainder of the central lowland of Scotland whilst in England one is struck at once by the great north-south stretch of Carboniferous rocks which makes up the Pennines—the so-called backbone of England. This north-south alignment is a new one: it is a reminder that the rocks were deposited long after the Caledonian folding and that after their deposition were subject to the Armorican movements. These created north-south folds such as the Pennines and Malverns, folded as it were against the older blocks to the north and west, as well as the more common east-west folds so well seen in the alignments of the beds in South Wales and in the South-western Peninsula. Where the north-south and the east-west folds crossed, the creation of basins and dome-shaped uplifts is obvious and can be clearly seen from the map. Thus the South Wales Coalfield and the Forest of Dean are examples of basins and the Mendips are an example of the uplifts.
The way in which the bright blue streak of the Magnesian Limestone (Permian) cuts across different beds of the Carboniferous shows that the latter had already been folded and denuded before the deposition of the Permian.
The remainder of the map relates to Lowland Britain. The Midlands of England show clearly the stretch of the red Triassic deposits and the islands of older rocks which appear from beneath this cover.
FIG. 11.—A Simplified Geological Map of the British Isles
Then follow the successive belts of the Scarplands (see Chapter 18 (#litres_trial_promo)), sweeping across England from north-east to south-west—brown for the Liassic clays, yellow for the Oolitic sequence, dark green for the lower Cretaceous rocks, and light blue-green for the Chalk follow one another in sequence of ever decreasing age as one goes towards the south-east. It is in the south-east that the orderly sequence is interrupted and this is a reminder that the south and south-east of Britain lay on the fringes of the great Alpine storm and that some of the rocks there were folded by the Alpine movements. The uplift of the Weald, roughly east and west in its main axis, separates the two main Tertiary basins of London and Hampshire ; the sharpest folds of Alpine date are those in the extreme south—across the Isle of Wight and the Isle of Purbeck. The location of the main stretch of Pliocene rocks in the coastal parts of Norfolk and Suffolk is suggestive, and rightly so, that by that period the geography of Britain had acquired something of its present form—only certain parts, on the whole near the present coasts, came under Pliocene seas.
There the geological story shown by the general map we have been discussing ceases. It tells us nothing of the stupendous events of the Great Ice Age. For that we must turn to the detailed drift maps and from them try to piece together what is one of the most fascinating and important, and yet, despite its recent occurrence in terms of geological time, one of the most difficult episodes to reconstruct in the geological story of the building of Britain. Before we deal with this, however, it is essential to consider how many of the major surface features of Britain have evolved.
CHAPTER 5 (#ulink_253e3f23-40f4-561d-9be9-bb691091c476)
LAND FORMS AND SCENERY THE WORK OF RIVERS
ALTHOUGH it is clear enough that the form of the surface—the relief of the land—is in the main determined by the underlying geological structure, the relationship between land forms and structures is by no means a simple one. It is only within the past few decades that the specialist study of land forms, the science of geomorphology, by developing its own technique, has demonstrated that it is possible to reconstruct a long and often complex history by detailed investigation of the form of the ground and that the details of land relief may bear surprisingly little relationship to the structural geology. For the most part geologists have paid little attention to this natural development of their studies. Apart from a few outstanding works by geologists such as J. E. Marr’s Scientific Study of Scenery first published in 1920 and more general works such as Lord Avebury’s Scenery of England and Wales, the foundation of detailed work was laid by such physical geographers as the American W. M. Davis, whose famous studies of the evolution of rivers was nevertheless carried out in our own Wealden country, and the Frenchman Emmanuel de Martonne whose Traite de Géographie Physique contains many British examples. Much recent work has emanated from America and other detailed work from Germany. In this country some of the younger geographers, headed by J. A. Steers and W. V. Lewis, have concerned themselves especially with coastal phenomena, others such as Professor D. L. Linton and Professor A. A. Miller with river evolution whilst a leader amongst those devoted to general geomorphological studies is Professor S. W. Wooldridge, whose Physical Basis of Geography: an Outline of Geomorphology, was first published in 1937.
Briefly, it may be said that land-forms depend first on the nature of the rocks and their disposition (that is, in other words, on lithology and structure), secondly on the climatic conditions, with resulting soil mantle and vegetation cover, under which the sculpturing of the land surface has been and is taking place, and thirdly on the phase or stage within the erosion cycle.
However erroneous, it is common to find references to “hard” rocks and “soft” rocks which are regarded as respectively resistant to and less resistant to weathering. Since most of the older rocks are “hard” in this sense the common distinction is drawn between the old hard rocks and the young soft rocks characteristic respectively of Highland and Lowland Britain. Although in any given area it is broadly true that the positive features of the relief, the mountains, hills and plateaus, are coincident with the outcrop of resistant rocks and the negative features, the valleys and plains, to that of “weak” rocks, resistance to weathering is not a matter of actual hardness. Chalk could not be described as a hard rock, yet it gives rise to the main hill ridges of south-eastern England. Under certain circumstances even a bed of gravel is sufficiently “hard” to form a capping and preserve a hill from denudation as in the case of Shooter’s Hill to the south-east of London. Both with chalk and gravel this is largely due to the fact that rain water soaks into the rock so readily that it does not have time to collect in rivulets on the surface and wash away the surface soil. When reached in deep excavations such as wells even clay is quite hard but when at the surface it has absorbed a certain amount of water it is impervious to more. When rain falls on the surface it is then easily eroded—as muddy streams bear witness—and so outcrops of clay are marked by valleys and lowlands.
In the British Isles we are concerned with the land-forms which develop in a moist, temperate climate. We are not, for example, directly concerned with land-forms which develop in hot deserts or in the rainy tropics except in so far as such conditions once prevailed in distant geological epochs and have bequeathed to us fragments of “fossil” landscapes in the sun-shattered rocks which peep from beneath a cover of later strata in the Wrekin or the ridges of Charnwood Forest to remind us of the deserts of Triassic days. We are, however, concerned with land forms which develop under conditions of extreme cold under great ice-sheets or valley glaciers or on the margins of ice-covered seas, for much of the surface of this country was profoundly modified during the Great Ice Age. This is geologically so recent that not a few of our lakes and swamps are the last remains of those left behind by the retreating ice.
Over large parts of this country the relief seems to be completely unrelated to the underlying structure. Plains are developed quite independently of either the hardness or dip of the underlying rocks: rivers seem to go out of their way (as does the Bristol Avon) to pass through the highest hill ranges they can find instead of following an easy passage on low ground and it is here that we realise the importance of the erosion cycle. It is in the interpretation of such apparent anomalies that the geomorphologist has made his major contribution. In the following pages we shall examine in detail a number of examples from Britain.
THE WORK OF RIVERS
The principal agent in the sculpturing of the land surface in a rainy temperate climate such as that of Britain is undoubtedly running water. No sooner does rain fall than some of it collects to form tiny temporary rivulets which soon join small permanent brooklets and rills. These, reinforced by springs which represent the reappearance at the surface of that portion of the rainwater which had soaked into the ground, unite in due course to form the river system of the country. Except in certain limestone districts where much of the drainage is underground the whole country is covered with a complex surface drainage pattern of rivers and streams.
It is a common generalisation in most books on physical geography that the course of a river may be divided into three parts. The upper or mountain course is that in which swiftly flowing water, especially after rain, is able to move stones of considerable size, to roll them along, to rub them one against the other and so to reduce angular fragments such as those broken off the mountains by frost action into rounded pebbles. The work of such a mountain torrent is well seen in Plate 2. At the same time the river deepens and widens its own valley so that its valley has a typical V-section with unstable banks. The middle course of the river is that over the foothill belt where it has lost some of its velocity but is still moving rapidly enough to carry sand, silt and mud in suspension and to roll pebbles along its bed. Its main work there is transportation ; its valley has a broad open section and has stable sides so that the erosive power of the river is strictly limited. The lower course is that in which the river meanders lazily across a plain ; though sweeping much mud out to sea or into the lake into which it empties it has lost much of its velocity and so much of its power of transportation. It lays down part of its load as shingle beaches or sandbanks but especially builds up large flat plains of deposition by spreading alluvium over a wide flood plain or a delta. Thus its work is largely deposition.
Quite obviously not all rivers conform to such a generalised pattern. In mountainous regions they may tumble direct from the mountains to the sea (as shown on Plate XXVIII)—they are young rivers associated with an early stage in the cycle of erosion. Others, including such large rivers of this country as the Thames, have no mountain course—they are relatively mature and associated with a late stage in the cycle of erosion.
It is clear that there is a very close relationship between the character of a river and the phase of the erosion cycle.
FIG. 12.—Diagram of a Meandering River This diagram shows how the water of even a slowly-moving river in its lower course swings from side to side resulting in erosion on the concave side and deposition of sandbanks on the convex side.
It is the inexorable law of nature that as soon as mountain building movements have erected a land mass and endowed it with mountains, hills and valleys, the forces of sub-aerial denudation combine to reduce that land to…the question is to what? What is the final form of the land if the forces of denudation are allowed to continue unchecked? The answer is not a flat plain but a peneplain or peneplane which, whichever way it is spelt, means almost a flat surface. The whole process is referred to as “sub-aerial peneplanation.” Although sub-aerial peneplanation is in progress all over the world and although large areas may thus have been so reduced that they have reached “base level”, below which removal of material will not take place, it is rarely if ever in nature that the process is allowed to continue to its logical conclusion. Differential uplift of the land, up or down movements relative to sea level (eustatic movements), or even a slight folding movement will upset the equilibrium which has been reached and will cause “rejuvenation” of the river systems. Before considering these complications it will be well to note the various ways in which river systems may develop and thereby to explain many of the features which are associated with British rivers. In passing we may recall that peneplanation of lands in the British Isles has probably been almost reached in past geological epochs.
FIG. 13.—Diagrammatic Section through the Deposits of a Delta and a Lake or the Sea When the river enters a lake or the sea, the velocity of the water is immediately checked and any coarse material is at once dropped, only the finer mud being carried on. In this way a delta is built up and as the flat land of the delta itself is formed, the velocity of the river is checked before reaching the sea and the finer deposits are spread as a surafce layer of alluvium over the flood plain.
The formation of shallow water limestones which requires clear water is taken as evidence that the surrounding lands had been almost reduced to base level and consequently yielded very little sediment.
We notice that there are thus two types of plains—plains of deposition and plains of denudation and that the former tend to be flatter and are more truly plains.
Let us take the simple case of the floor of the sea which is raised up (not folded) by earth movements so that it becomes land. It will be a flattish surface with a gentle slope seawards and rain falling will collect together into streams, roughly parallel, finding the shortest route seawards. These streams are consequent on the slope and hence are known as consequent streams. Tributary streams, arranged somewhat irregularly, will drain into these main ones and the pattern of drainage developed is that known as dendritic.
FIG. 14
FIG. 15
FIG. 16
Figs. 14 to 16 illustrate three stages in the development of the drainage of the Weald. In each the line of dots represents the main axis of the Wealden uplift. When the dome was first uplifted (Fig. 14) chalk covered the whole and water drained naturally and consequently down the northern and southern slopes forming consequent streams. In the next stage (Fig. 15) the chalk has been removed by denudation over the central area and some streams have become stronger than others and subsequent streams, running in strike valleys, have developed. Fig. 16 shows the developments at the present day. The three divisions shown are the Tertiary, the Chalk and the pre-Chalk beds. It is clear that such a river as the Darent has had its headwaters captured by the Medway.
Perhaps even more common in nature is the initiation of a drainage system by uplift accompanied by folding. If the rocks are raised up to form a broad arch or anticline, consequent streams flow down either side following the general dip of the rocks. Thus consequent streams follow the dip of the rocks. Very soon two things will happen. Some streams will become stronger than others—it may be through some slight differences in the relative softness or hardness of the beds over which they are flowing. They deepen their beds more rapidly than their neighbours, they cut back at their heads (headward erosion) more rapidly. Water which might have gone into neighbouring streams drains into them by laterals which, because they thus develop subsequently to the main consequent are known as subsequents. The subsequents flow at right angles to the dip of the rocks—that is along the strike—and so are flowing in strike valleys. In due course some of the more vigorous subsequents capture the waters and drain the valleys of the weaker consequents and so, by this process of river capture, a complex system develops. It may even happen that the flow of water in part of a former consequent valley is reversed so that it becomes an obsequent stream feeding the conquering subsequent.
Such a river system as that just described may cut down on to older rocks which lie underneath the sheets of strata which gave it its birth. Indeed all traces of those later rocks may be completely removed. On to the older rocks there is implanted a river system which seems to have simply no relationship to the structure. This is a common feature in many parts of the British Isles and gives us the reason for the passage of the Bristol Avon through Clifton Gorge when there apparently were so many other easier courses. Such a system is called a superimposed drainage. But in its further development the members of the system will find out the weaker rocks, the lines of faulting and crushing, and it is along such lines that the major excavation will take place.
What happens when earthquakes and folding movements take place in an area with a well developed river system? There may be reversals of drainage and many examples of river capture can only be explained by postulating differential earth movements. But if folding movements take place slowly existing rivers in their downcutting may keep pace with the growth of folds and one gets thus examples of antecedent drainage which may be defined as drainage developed in its early stages before the present surface features. For the supreme examples of this one must look to the mighty rivers of the Himalayas which cut right through the greatest chain of mountains on the face of the earth.
The sequence of development of consequents, subsequents and obsequents was worked out by W. M. Davis in the river systems of the Weald which is thus classical ground. One may picture the Wealden dome, in structure resembling an overturned boat, rising by slow stages from the latter part of the Cretaceous period downwards. At first the uplift formed a low dome scarcely above sea level but sufficiently near the sea surface for wave action to get to work wearing away the chalk and rolling the angular flint nodules into pebbles. By Middle Eocene times the ridge was sufficiently high to be partly covered by shingle beds (Blackheath Pebble Beds) and the crest probably formed an island. As soon as an island appeared above the surface consequent streams flowing from the east-west crest to the north and to the south developed. By a combination of marine erosion and then of sub-aerial denudation the chalk was entirely eroded from the central area and revealed below the varied succession of beds which make up the lower Cretaceous. Some of the beds are weak and easily eroded, others are relatively resistant. Subsequent streams found out the weaker rocks and eroded valleys at right angles to the consequent streams, some of which cut down through the chalk and to-day are seen flowing in steep-sided valleys through the chalk rim of the North Downs and the South Downs. The weaker consequents were beheaded by the capture of their head streams and many failed to cut down through the chalk. The Weald thus illustrates extremely well the association of subsequent streams with valleys in the weaker rocks which are parallel to the strike of the rocks (strike valleys) whereas the consequents have valleys parallel to the dip of the rocks (dip valleys). The later history of the Weald has been complicated by the submersion of much of the area under the Pliocene sea, then its subjection to tundra conditions during the Great Ice Age and by the complications caused by the breaching of the eastern end of the fold when Britain became separated from the continent, but the main pattern of the drainage has remained as it was developed by the gradual uprise of the Weald. The phenomena of subsequent streams occupying well-defined strike valleys is repeated all over the lowland of Britain.
The form of a river valley is able to yield much information both with regard to the age of the valley itself and the history of the river system.
Mountain torrents stand rather by themselves: they cut deep notches in the mountain sides (an example is given in Plate 2), usually finding some line of weakness, as for example along a fault where the rocks have been crushed, and the material which is dislodged is swept to lower levels both by the power of the water and by the force of gravity. If dislodged blocks fall by gravity alone they form screes with an angle of rest of about 40°—the angle of the scree shown on Plate 8B is exactly 38°. If the fall is aided by running water the debris is fanned out and has a lower angle of rest—forming what is termed an alluvial fan or alluvial cone (such as the ones shown on Plate 30B) though the word “alluvial” is apt to cause confusion with the much finer material associated with deltas and with the flood plains of the lower courses of rivers.
Where initial slopes are not quite so steep the mountain stream carves out a narrow steep-sided V-shaped valley. Even at this early stage the valley is not straight: the stream swings from side to side so that “interlocking spurs” develop between the meanders and obstruct the view upstream. Nature has provided the swiftly flowing stream with a remarkable mechanism for drilling holes in its bed. A few stones are caught in a whirl of water and swing round and round to drill out the well-known “pot-holes.” This is an active force in deepening the bed of the river and so of its valley. An excellent and large example is shown in Plate VIIIA. Widening of the valley comes gradually with the action of gravity—lateral slipping aided by tributary streams so that, broadly speaking, the older or more mature the valley the wider it is. In these early stages the form of the valley, especially its long section (i.e. the section drawn down the valley—the longitudinal profile for which the not very appropriate German word talweg is often used), is closely related to the character of the rocks over which it passes. Hard bands cause rapids or waterfalls and between these the river may assume the characteristics of maturity. In cross-section the valley sides may exhibit ledges due to the outcrops of hard bands whilst dipping strata may cause a valley with an asymmetric cross section. Even more common is the varying width of the valley—broad and open where it traverses soft rocks, narrow and even gorge-like where it passes through a belt of hard rocks or limestone. Even an old river like the Thames has these features—the beautiful narrow valley at Goring is where it passes through the chalk ridge.
Gradually, however, a river tends to reach a state of equilibrium and its longitudinal profile will form a smooth curve from source to mouth. When it reaches this stage a river is said to be graded and the land around has reached the stage of sub-aerial peneplanation. To achieve the graded curve, which will first be reached near the river’s mouth, the stream must necessarily cut back into the hills from which it takes its source and this involves headward erosion. It is found that
FIG. 17.—Diagrammatic Sections along a Talweg The upper diagram is a longitudinal section following the course of a relatively young river from its source to its mouth. Bands of hard rock cause waterfalls and rapids between which the river tends to assume a graded curve. Diagram II is the graded curve of a more mature river: the whole longitudinal section is evenly graded from source to mouth independently of any hard beds. Diagram III illustrates what happens if a fully graded mature river, such as that shown in II, is subjected to rejuvenation by a general uplift of the land surface relative to sea level. A knickpoint is formed independently of the character of the rocks and gradually works back, i.e. up the course of the river.
many mature rivers rise in a sort of amphitheatre, steep-sided but not nearly so steep-sided as the cirques from which glaciers have their origin.
Over the middle and lower courses of mature rivers, or rivers which have almost reached base-level, there are several characteristic features. The water swings from side to side and long winding meanders are the result (Fig. 12). Once a meander has been initiated there is a natural tendency for the swing of the water to make the curves ever more acute till at last the water breaks through the neck and the cut-off portion forms a stagnant “cut-off” or “ox-bow” lake. This will be clear from the diagram ; but what is not always realised is that the continuance of such a process results in a broad flat-floored valley with a deposit of gravel, sand, silt or alluvium. Such a flat floor is liable to flood when the river is in spate and so one gets a flood plain. Land liable to flood occurs along the lower courses of most British rivers. When the flooding is uncontrolled, a film of mud is spread by each flood and results in the gradual building up of alluvial flats. There is thus deposition closely associated with erosion in the middle and lower courses of a river. When the river reaches its mouth with a load of fine mud in suspension this may be swept seawards, especially if the sea into which the river discharges has a marked tidal movement. This is the case round the British Isles where nearly all our rivers enter into estuaries with a strong tidal movement. Where tides and currents are less strong the sediment is dropped near the mouth of the river and a delta of alluvium is gradually built up, passing seawards almost imperceptibly into very shallow muddy water. Since deltas are not typically formed round Britain it is unnecessary to enter into the details of their formation though there are many good examples where rivers enter lakes such as that shown in the foreground in Plate 31B. It is important to note the leading role played by vegetation in fixing the mud and then acting as a trap to catch more mud. In this way, though not directly associated with river mouths, there is accretion of land in such areas as around the Wash and in Morecambe Bay and advantage is taken of the natural processes in reclaiming land by building dykes or retaining walls to hold sediment. The stages in silting up are well shown in Plate XXV. Inland, artificially controlled flooding has long been practised, using the waters of such rivers as the Trent and Yorkshire Ouse to spread silt over the land after the manner of the Nile in Egypt and so both to build up the level and to spread a fertile layer rich in mineral salts and organic matter and of excellent mechanical texture. This controlled flooding is known as warping and the mud deposited as warp.
The well-graded meandering river with its broad valley floored with alluvium is a familiar feature in the British landscape. But even in geologically recent times, certainly since the Ice Age, there have been several changes in the relative level of land and sea, slight it may be but significant. What happens to such a mature river system when the land is lowered or raised relative to sea-level? First, if the land sinks, the lower valley is invaded by an arm of the sea and one gets the familiar feature of a drowned valley or ria. The best example of a coastline of drowned valleys or ria coast is the south-west of Ireland. Soundings show that the floor of the ria, the old river talweg, slopes steadily seawards and there is no “lip” as there is in the case of a glaciated valley with a rocky or morainic bar at the entrance (as in many of the Scottish fiords (#litres_trial_promo)). Drowned valleys give rise to the picturesque winding creeks of south Devon and Cornwall—the estuary of the Fal and Tamar for example (Plate 26). It is clear that the branching tidal creeks shown in Plate 26 could not have been excavated by the action of the sea which now occupies them.
If, on the other hand, the level of the land is raised relative to the sea, the river undergoes rejuvenation; it is given new erosive powers and immediately begins lowering its bed. But such a rejuvenated river exhibits certain special features. It was, before the new uplift, a meandering mature river and the effect of the uplift is for it to follow its same meanders but to cut them deeply and so one gets the interesting and picturesque feature of incised meanders with a river winding in a gorge, it may be of considerable depth. If in such a case a meander is cut off one gets between the abandoned course and the new course a “meander core.” Incised meanders tend to develop where the rocks are relatively hard. Where a broad valley is excavated in relatively soft rocks the rejuvenated river develops for itself a new alluvial covered flood plain at a lower level than the old one and so fragments of the old one are left as gravel-covered or alluvium-covered terraces. Successive uplifts produce successive terraces at several levels. Those of the Findhorn in Scotland are well shown in Plate XXXI. The terraces of the Thames are not only well known but have been and are very important economically—for the dry sites they offer for settlement, for the water supplies once afforded by the gravels, for the excellent well-drained soils to which they give rise, for the brickearth they formerly supplied for brick making, and latterly for the supplies of gravel which, alas, is being excavated regardless of the future use of the devastated land. In the case of the Thames near London it is possible to distinguish one gravel-covered terrace at about 100 to 120 feet above present sea-level, though naturally varying in height with distance from the sea. This is the Boyn Hill Terrace and is very clearly marked in several areas. There is another terrace, of wide extent, at about 50 feet above sea level known as the Taplow or Middle Terrace. A third one is the Low or Flood Plain Terrace at some 10 or 15 feet above sea level. Then there followed a time when the Thames was lower than at present—or rather when the sea-level was lower and the river excavated what is now a buried channel so that to this extent the estuary of the Thames is a drowned valley. Actually the history of the Thames is much more complex than this, and such a complex history is typical of British rivers. Each change has some corresponding effect on tributaries. In the lower courses of a well graded river the effects of hard bands which may cross the valley have been eliminated and an interesting feature is found when the course of a rejuvenated river is followed upstream. There is found to be a point where there is a break in the longitudinal profile of the river. This is where it is still cutting back as a result of the change in level. Such a break of slope is known as a “knick point” and its development is to a large extent independent of any differences in the rocks of the river bed.
It must be remembered that the British Isles had a well developed river-system before the oneoming of the ice sheets of the Great Ice Age and that the effect of glaciation was to modify rather than to change completely the existing valleys and land forms.
A number of the plates in this book illustrate a few of the extraordinary complex character of British rivers. A drowned estuary such as that shown in Plate 26 may become silted up and a marshy plain may result—well seen in the estuary of the Glaslyn in Plate 20. Rejuvenation may result in gorges even in the middle courses of rivers—as shown in Plate 2. A mature landscape with well-rounded hills affected by rejuvenation is often more apparent from the air than on the ground and an example from the Southern Uplands is well shown in Plate XXII. The interesting case of “drowning” exhibited by the Norfolk Broads is shown in Plate XXIII.
CHAPTER 6 (#ulink_d78236eb-d549-51fb-9c9b-5afbdb5534c8)
THE WORK OF THE SEA
THE extraordinarily varied character of the sea coasts of Britain and the variety of habitats which they afford to both plants and animals, with the consequent enrichment of our fauna and flora, give a special interest and importance to the story of the work of the sea in the building of the British Isles.
It is now generally agreed that ocean currents play but a very small part in the erosive and transporting work of the sea and that the effects of tidal movements are limited to a few special cases—notably tidal scour in confined estuaries and straits. The work of the sea is primarily through wave action—to some extent through the hydraulic forces engendered by the movement of great masses of water, but far more through the arming of the waves with quantities of rocks, stones, gravel and sand.
The waves of the sea are primarily wind-waves ; they are caused by the disturbance of the surface by wind but, once formed, waves may travel far beyond the area where they were generated—hence “swell” or “ground-swell” unaccompanied by wind. It is, of course, well known that there is no forward movement of the water in wave action, except where the waves are breaking on the coast. The vertical range of motion, in other words the height which waves may reach, is commonly much exaggerated. Waves which are as much as 50 feet from trough to crest are decidedly large, probably quite exceptional even in the open ocean. At a depth of 100 feet the water is little disturbed, at a depth of 500 feet it is doubtful whether there is enough movement to disturb even the finest mud. There is thus a fundamental difference between sub-aerial denudation, which takes place at all heights from sea-level to the tops of the highest mountains, and marine denudation which acts on a very restricted vertical plane above or below the surface level of the water, The maximum effect is where sea meets land—between the tide marks and just above or below.
Consider what happens at the base of cliffs. Angular blocks of rock and stones fallen from the face of the cliff are picked up by the waves and hurled against the base of the cliffs which they thus tend to undercut,
FIG. 18.—Sections showing the Formation of Cliffs These sections illustrate the plane of marine erosion (see Plates 3 and XX A) and the way in which the cliffs are cut back and a submarine peneplane formed.
much in the manner of coal-cutting machinery at the base of a coal seam. Blocks from above then split off along joints and fall by the force of gravity ; where there is a dip of the rocks seawards great masses may slide down the bedding planes. The latter effect is well seen where rock overlies clay the surface of which becomes slippery and acts as a greased plane—hence the constant slips along the south coast of the Isle of Wight and between Dover and Folkestone, in each of which cases chalk overlies gault clay. Plate V shows the famous under-cliff, west of Ventnor in the Isle of Wight. On the shore between the tide marks the rock fragments are rolled against one another and quickly reduced to rounded boulders, pebbles and sand. These, rolled backwards and forwards between the tide-marks and later dragged below low tide-mark enable the sea to level off its wave-cut platform. This is illustrated in Plate 3. The particularly interesting case of undercutting of massive limestone shown in Plate XXA is partly due to the small tidal range and the consequent concentration of erosion along one plane.
Thus the effect of the sea round the coasts may be described as the
FIG. 19.—Section through a Raised Beach. This is a diagrammatic representation of the scene shown on Plate XI
creation of a platform, a wave-cut rock bench, on which is distributed a veneer of sediments made in the process. The process of its development is shown diagrammatically in Fig. 18. This shelves gently seawards under the water and passes imperceptibly into what is called the Continental Shelf. This is a great shelf found round most of the lands over which the sea is less than 600 feet deep.
The surface of the continental shelf is, normally, very gently undulating through relative resistance of the solid rocks. It is, in fact, a peneplane formed by the work of the sea. Even the slightly irregular denuding action of the sea may result in swellings of the floor which just give rise to shallow areas or may reach the surface as islands. Just as the sea in cutting back a cliff may leave a stack or an island, so in the age-long process of marine peneplanation certain upstanding masses may have been left as islands—it may be isolated and far from land. Where this is the case there is usually an explanation in the hardness or resistance of the rocks of which they are composed. The Scilly Isles are thus the protruding surfaces of an almost submerged granite mass comparable with that of Land’s End and from which the surrounding sedimentary rocks have been removed. The isolated mass of Rockall far away in the Atlantic off the north-west coast of Scotland consists of a particularly tough micro-granite and the same is true of Ailsa Craig near the entrance to the Firth of Clyde south of Arran. The celebrated St. Kilda is the largest of a group of sixteen islets rising from the continental shelf. They owe their origin largely to the resistant character of the Tertiary igneous rocks of which they are composed. Three of the small islands of the St. Kilda group are shown in Plate 32A and Stac Lee in Plate XXXII.
It follows that the floor of the “epicontintental” sea around the continents—that is the continental shelf—is sometimes interrupted by rocky masses which are, in fact, the “monadnocks (#litres_trial_promo)” in course of evolution. It is probable that the solid rocks crop out over considerable parts of the sea-floor, uncovered by sediments, and these “rocky grounds’” are well known to fishermen. Where the rocks are hard and jagged trawling becomes impossible because of the tearing of the nets on the projecting rocks. Over very large areas, however, the continental shelf is covered by a veneer of sediments laid down by the sea itself and derived both from the nearby land by wave action and from the smoothing of the shelf itself as well as from sediments brought down from the heart of the land masses by rivers or icesheets. There is normally a gradation from the coarse shingle and stones of the beach near high-water mark, to sand, becoming finer seawards, which in turn passes into silt and mud. This sequence, however, is frequently varied or even reversed: the coastline may yield little or no coarse sediment ; rivers may bring down quantities of mud which becomes spread over a wide area ; outcrops of rock on the sea floor may yield local spreads of coarse material; and one may get sandy beaches. There are also more local or special conditions which result in variation in the form of the sea floor and its deposits. An interesting case is where icebergs broken off from land ice and bearing a burden of boulders and stones, both on the surface and frozen into the ice, meet warmer water. The icebergs melt and their burden is dropped on the sea floor. This is happening to-day in the Grand Banks area off Newfoundland and it must have happened extensively in the seas round the British Isles during and after the Ice Age. Indeed, boulders floated by ice from distant sources are found in some of the raised beach deposits along the shores of the English Channel.
The variety of habitat for marine bottom-living creatures in the shallow water of the continental shelf, is more than paralleled by the variety of habitat along the sea-shore itself.
Broadly speaking, any stretch of coastline is either one of erosion or of deposition and along such a varied coastline as that of the British Isles the conditions change with great frequency. A cliff coast is obviously an erosion coast and a high or irregular cliff line may be taken as indicative of long-continued erosion. Coast erosion was the subject of an exhaustive inquiry by a Royal Commission which reported in 1911 and much attention was given to the rate of cliff erosion.
FIG. 20.—Diagrams showing the Drift of Shingle along a Shelving Beach. In each diagram the dotted line shows the course of a single pebble. It is thrown up the beach parallel to the direction of the prevailing waves but is dragged down the beach roughly parallel to the slope of the beach by the force of gravity. As the process is repeated the pebble works its way, with all its fellows, along the shore. If groynes are erected, this longshore drifting is partly arrested (Plate IX).
FIG. 21.—A Shingle Spit and Salt-Marshes Hurst Castle Spit, Hampshire
The chalk cliffs of the Strait of Dover appear to be receding at a rate which suggests the widening of the Strait by half a mile in a thousand years—an average of 15 inches on each side per year. In parts of the low soft Norfolk coast up to 5 feet per year is possible, whilst great cliff falls may give spectacular figures in cliffs which otherwise are being eroded at the rate of only an inch or two a year.
A change from erosion to deposition is seen with cliffs in front of which shingle banks are accumulating. Taking Britain as a whole, even without human interference more land is being gained at present than is being lost. The material from a yard or two of high cliff would spread over a large superficial area of mud-flat. Allowing for man’s action the gain is very much greater than the loss. Accumulation along a coast may take many forms—among the chief of which are shingle beaches and spits, sandy bays often backed by sand dunes, and mud-flats.
A study of shingle beaches and spits gives a clue to many of the essential features of coastal evolution. For long it was believed, and actively advocated by many writers, that longshore currents were mainly responsible for the drift of shingle and sand along coastlines. That there is a drift of material along shores is quite clear and is immediately apparent where groynes have been built to minimise the movement—the shingle is piled up on one side of the groyne and swept away from the other. This is clearly shown in Plate IX of the shore at Folkestone. In recent years, however, it has been shown, notably by W. V. Lewis, that the drift of material is occasioned by wave action and not by currents. There is a constant tendency for shingle beaches to be piled up so that the ridges are at right angles to the dominant wave direction—in other words the shingle ridges are built up parallel to the ridges and troughs of the waves that create them. Where, owing to the general trend of the coastline, the waves break obliquely, the pebbles are thrown up slightly obliquely but the undertow drags them back more nearly at right angles to the direction of the shore so that they travel gradually along the coast, as shown in Fig. 20. There is, in consequence, the familiar phenomenon of a spit being built out, often right away from the coast line, but at right angles to the direction of the waves. A well-known example is Hurst Castle spit on the Hampshire coast opposite the Isle of Wight. Elsewhere successive shingle beaches may be built up—especially in time of storm, as at Dungeness. Dungeness is remarkable for the extensive area of successive shingle beaches. The spits of shingle or sand which are associated with river mouths are rather different in character: a drift of material deflects the channel of the river. Behind shingle spits and sand spits conditions tend to be favourable for the accumulation of mud and the development of salt marshes.
With sandy shores, wind nearly always takes a hand. The dominant wave direction (as on the coasts of north Cornwall) is often that of the predominant westerly winds. Such areas as Saunton Sands and Perranporth illustrate the formation of sandy beaches at right angles to this direction. Further, these sandy beaches are backed by large areas of sand dunes. Above high-water mark the sand, quickly dried by the wind and sun, is blown inland to form dunes, in due course to be fixed in the usual way by Marram Grass (Ammophila) and other sand-dune plants. Wind action is especially important during short or neap tides when an expanse of sea sand below the high-water mark of spring tides is dried and blows easily.
FIG. 22.—An Atlantic Coastline with shingle beaches and sand-dunes at right angles to dominant wave direction (shown by the arrows). Rocky, hilly headlands separate the bays and indicate the east-west strike of the rocks.
Around the shores of Britain there are some remarkably large areas of silt and mud covered only at high tide. Around parts of the Wash high and low tide marks may be several miles apart and the same is true of parts of the Thames estuary, the Bristol Channel, Morecambe Bay and Solway Firth. In these areas there is a steady accretion to the land and the salt-marshes which are developed exhibit the well-known zonation of their vegetation. Reclamation of such areas goes on steadily round many parts of the coast: when the silting has gone on so that the mud-flats are covered only by the higher tides, they are enclosed by earth banks and at first water let in at high tides is allowed to deposit more mud and then to run off gently. Then the entrance of salt water is later prevented and gradually rain water washes the salt out of the soil so that fresh-water marsh replaces salt marsh. After some eight or ten years the ground is sufficiently free from salt for ploughing to be possible.
Such are the features associated respectively with erosion coasts and coasts of accretion. There are other features associated with eustatic movement of elevation or depression. A rising coast frequently shows raised beaches—wave-cut platforms on which rest gravels, sands and other beach- or shallow-water deposits and which are frequently bounded on the landward side by lines of old or “fossil” cliffs. Such raised beaches are well seen round many parts of Britain ; especially famous are the examples along the Clyde estuary. Along the rocky coast of Cornwall the raised beaches are but narrow platforms cut in the hard rocks. Of other striking examples from Scotland one has been chosen for illustration in Plate XI.
A sinking coast or drowned coastline is often highly indented for the sea naturally invades the mouths and lower sections of river valleys and runs up the valleys of tributary streams. Excellent examples are seen along the south coasts of Devon and Cornwall, as illustrated in Plate 26. Local features such as submerged forests are indicative of a sinking or a sunken coastline. A fascinating little book by the late Clement Reid on Submerged Forests described the very numerous examples round the British Isles. Summer visitors perhaps know best those along the coasts of East Anglia and Lincolnshire, West Cornwall or Cheshire.
A distinction is frequently drawn between the “Atlantic” and “Pacific” types of coastline, so-called because of their relative prevalence round those two oceans respectively. In the Atlantic type the “grain” of the country, the axes of the folds in the rocks, is at right angles to the dominant direction of the coast.
FIG. 23.—A Drowned Coastline The view shown in Plate 26A is taken from the point A, looking in the direction of the arrow ; the view in Plate 26B is from the point B.
This is beautifully seen in north Cornwall where the Armorican folds have east-west axes whereas the coast often trends more nearly north and south. Banks of resistant rock run out to sea as headlands or ridges of rocks and between them are the secluded bays which form such a delightful feature of the coast (Plate 3B). The same phenomenon is seen on a larger scale in south-west Ireland, where the hill ridges of Old Red Sandstone are separated by long narrow rias. In the Pacific type of coastline, less commonly seen in Britain, the coast is roughly parallel to the grain of the country. This may be illustrated from the part of the Dorset coast shown in Plate XXVI.
We have referred to the continental shelf and are perhaps little concerned with what happens beyond its edge. The continental slope to the depths of the open ocean are often very steep—so steep that deposits laid down may slide in great masses from the margins of the shelf to the deep ocean floors and give rise to phenomena of extensive slip faulting and puckering which some geologists claim recently to have recognised in the rocks of past geological ages.
CHAPTER 7 (#ulink_7cb173cd-f718-5bed-b28c-788df187830c)
THE SCENERY OF THE SEDIMENTARY ROCKS
THE GREATER part of Britain is made up of rocks which were laid down under water as sediments. They were originally deposited in almost horizontal layers or strata and when first raised up above the level of the sea they were approximately horizontal. Some have remained so, but for the most part they have been tilted and folded to a greater or lesser extent. Interbedded with the sedimentary rocks proper are limestones, as well as lava flows and beds of volcanic ash. Whilst some limestones are built up of fragments of shells and are thus sediments, others are of the nature of chemical precipitates and again others are coral reefs which have grown in situ. Hence the distinction from sediments proper.
Whilst some of the members of the sequence retain the same general characters through a great thickness—the Chalk, for example, in places is nearly 1000 feet thick—it is a common feature for strata of different types—clays, shales, sandstones and many others—to succeed one another with almost bewildering rapidity so that each bed may be only a few feet, or even a few inches, in thickness. This is of great importance since each bed may, and usually does, differ in its resistance to atmospheric and other forms of weathering. It is this variation in ability to stand up against the denuding forces of nature which is largely responsible for the intimately varied scenery of so much of Britain and the absence of monotony even in the lowlands. It is also largely responsible for the remarkable and rapid variations in the sea coasts of the British Isles.
Before considering the land forms which are characteristic of the sedimentary rocks it will be well to recall a few of the salient features of the rocks themselves. The majority show evidence of bedding planes, indicating that they were deposited in layers. This is generally true of the harder rocks such as the sandstones, but bedding planes may be absent in the clays which form an amorphous mass. A few sands and sandstones which were deposited in shallow waters under the influence of currents have a curious type of bedding consequent upon current action and hence known as current bedding or false bedding. It is best understood by reference to the picture, Plate 10 and the diagram Fig. 37.
The bedding planes tend to form planes of weakness along which the rocks split easily. When a hard sandstone splits easily in this way it is known as a flagstone. Because of this property the Caithness flagstones of Old Red Sandstone age were the favourite paving stones for London and other cities till the introduction of artificial stone for the purpose. Some rocks break into such thin slabs that they can be used for roofing and though true slates have a different origin (#litres_trial_promo) such “slates” as those of Stonesfield and Collyweston are actually thinly bedded limestones. The vogue for “crazy paving” in gardens has introduced many townsmen to a considerable variety of rocks which split easily along the bedding planes. Rocks which do not split more easily along the bedding planes than in other directions are referred to by masons as “freestones” because they are freely worked into building blocks. Even so, as the expert knows, if a freestone block is laid on its edge it is liable to weather more rapidly than if placed as it was quarried. The term freestone is applied both to sandstones and to limestones.
In addition to this character of planes of weakness parallel to the bedding, the majority of the harder rocks have cracks or planes of weakness at right angles to the bedding along which they split easily or along which they are easily attacked by denuding agencies. These planes are known as joints or joint planes and there may be more than one series. In the latter case the major series is known as that of the master joints. When a rock such as a hard sandstone is well jointed it often weathers into vertical crags—a feature exhibited to perfection by the Torridonian Sandstones of the north-west of Scotland (Plate IB). The craggy sandstone cliffs of such heights as Stac Polly afford interesting plant and animal habitats. Although the jointing in limestone is partly of a different origin (#litres_trial_promo), being due to some extent to solution of the rock, the effect is broadly the same (Plate 6). Very naturally jointing and bedding both influence to a great extent the character of sea-cliffs and river cliffs. A well-developed system of joints results in vertical cliffs ; where there is at the same time a dip of the rocks seawards or riverwards the combination of circumstances is such as to encourage the detachments of great blocks and the creation of landslides (#litres_trial_promo).
FIG. 24.—Diagrammatic Explanation of the Peneplanation shown in Plate 5A A typical example of submarine peneplanation, with a subsequent uplift of the whole surface. As a result the almost level surface of the ground is independant of the underlying geological structure.
Sedimentary rocks when laid down are almost horizontal so that the simplest case to consider is when horizontal strata are raised up and subjected to denudation. Rivers cut into them and between the young
FIG. 25.—Diagrammatic Explanation of the Flat-topped Hills of Plate 5B In contrast to Fig. 24, the flat tops of the hills here follow the bedding of the underlying Upper Greensand which rests almost horizontally, but unconformably, on underlying rocks which dip to the east.
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- Жанр: Научно-популярная литература
- Язык: Книги на английском языке
- Объём: 420 стр. 169 иллюстраций
- ISBN: 9780007406081
- Дата выхода книги: 17 мая 2019
- Версия: 📚 Электронная книга
Britains Structure and Scenery deals with the physical background, the stage on which the drama of life is played and which provides the fundamental environment for plants, newnaturalists.comIt would be difficult to find an area of comparable size anywhere in the world with such a variety of physical conditions, scenery and consequently of plant and animal life as the British Isles. Our homeland is indeed a geological museum, epitomising in miniature the geological history of the globe. Each hill and valley, each plateau and plain reflects the underlying geological structure or build; this volume attempts not only to describe the surface features, but also to sketch the long and complex series of events which have given the land its present form – the building of the British Isles. It thus deals with the physical background, the stage on which the drama of life is played and which provides the fundamental environment for plants, animals and man.
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