Книга - Mountains and Moorlands

Mountains and Moorlands
W. H. Pearsall


An invaluable introduction to the upland regions of Britain – their structure, climate, vegetation and animal life, their present and past uses and the problems of their conservation for the future. This edition is exclusive to newnaturalists.comMoorland, mountain-top and upland grazing occupy over a third of the total living-space of the British Isles, and, of all kinds of land, have suffered least interference by man. Mountains and moorlands provide the widest scope for studying natural wild life on land.Professor Pearsall died in 1964. This new edition has been revised by his friend and pupil, Winifred Pennington. The book remains an invaluable introduction to the upland regions of Britain – their structure, climate, vegetation and animal life, their present and past uses and the problems of their conservation for the future.











Collins New Naturalist Library

II




Mountains and Moorlands

W. H Pearsall










EDITORS: (#ulink_4cb8f203-aea8-56f3-b613-a091bd0c9a2b)







MARGARET DAVIES C.B.E., M.A., Ph.D.

JOHN GILMOUR M.A., V.M.H.

KENNETH MELLANBY C.B.E.



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 naturalists. 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, is best fostered by maintaining a high standard of accuracy combined with clarity of exposition in presenting the results of modern scientific research.




Table of Contents


Cover Page (#u72e8ef56-100a-507b-b36a-e28ba334e79a)

Title Page (#uc91e44d8-3397-578d-868f-a5c52dedc06a)

EDITORS (#u074e157e-91e6-57c6-ae92-4142bbd532dc)

EDITORS’ PREFACE (#u0ae46201-1854-55bc-a671-7001cfac8bb6)

AUTHOR’S PREFACE (#u052f54ca-92ed-5bec-b7cf-6d40ebaf2dc2)

CHAPTER 1 (#u8cfba985-d923-5c50-8c5f-8b4924104c09)INTRODUCTION

CHAPTER 2 (#u51f9e0bf-2241-55a3-9306-d47b141a8f3d)STRUCTURE

CHAPTER 3 (#u59c08fd3-73e2-560e-a232-ff080064d085)CLIMATE

CHAPTER 4 (#u06bfee12-838a-5706-b7d5-28d9db7830ee)SOILS

CHAPTER 5 (#litres_trial_promo)MOUNTAIN VEGETATION

CHAPTER 6 (#litres_trial_promo)THE LOWER GRASSLANDS

CHAPTER 7 (#litres_trial_promo)WOODLANDS

CHAPTER 8 (#litres_trial_promo)MOORLANDS AND BOGS

CHAPTER 9 (#litres_trial_promo)VEGETATION AND HABITAT

CHAPTER 10 (#litres_trial_promo)ECOLOGICAL HISTORY

CHAPTER 11 (#litres_trial_promo)UPLAND ANIMALS—THE INVERTEBRATES

CHAPTER 12 (#litres_trial_promo)THE LARGER MAMMALS AND BIRDS

CHAPTER 13 (#litres_trial_promo)ANIMAL COMMUNITIES AND THEIR HISTORY

CHAPTER 14 (#litres_trial_promo)THE FUTURE—CONSERVATION AND UTILISATION

CHAPTER 15 (#litres_trial_promo)THE NATURE CONSERVANCY

BIBLIOGRAPHY (#litres_trial_promo)

GLOSSARY (#litres_trial_promo)

INDEX (#litres_trial_promo)

Plates (#litres_trial_promo)

Copyright (#litres_trial_promo)

About the Publisher (#litres_trial_promo)




EDITORS’ PREFACE (#u86e587f5-cda5-55d2-886b-26760e4e243b)







THERE are really two Britains—two different countries, their boundary a line that strikes diagonally across England from Yorkshire to Devon. To the north and west of this line is the region of mountains and old rocks; to its south and east the newer, fertile land of the plains. These regions differ vastly in their climate, rocks, soils, scenery, plants, animals—and men.

It is of the mountains and moorlands that W. H. Pearsall writes. Moorland, mountain-top and upland grazing occupy over a third of the total living-space of the British Isles, and, of all kinds of land, have suffered least interference by man. Mountains and moorlands provide the widest scope for studying natural wild life on land.

In the present volume Professor Pearsall has brought together the results of over thirty years’ work among the high hills, the lakes and the moorlands of northern and western Britain. He is a botanist, but these pages show that animals have appealed to him almost as much as plants, a double interest that is rarer than it should be among naturalists. It is doubtful whether any other author could, single-handed, have presented such a well-balanced picture of the wild life of an area as Professor Pearsall has done in this volume.

Although he is now banished to the gently undulating south (he is head of the Botany Department at University College, London) the whole of Professor Pearsall’s previous working life has been spent within call of the mountains and moorlands about which he writes—at Manchester, at Leeds, and finally as Professor of Botany at Sheffield University. During this period he has made many outstanding contributions to ecological research, especially in the Lake District, but it is obvious from his book that the severely scientific discipline that these researches demand has by no means extinguished his deep love of the countryside in which they were carried out. On the contrary, the aesthetic and the scientific approaches have reinforced each other, as they should—but frequently do not—in any fully developed naturalist.

For many people, perhaps the most arresting point in the book will be the idea that since the end of the Ice Age our mountains and moorlands have been subject to a process of inevitable change, one of the trends being towards the growth of bog and peat-moss at the expense of grassland and woodland, and towards a general impoverishment of soils; further, that during the last 3,000 years or so this and other changes have been progressively accentuated by man’s interference, so that the difference between the natural history of our moorlands to-day and a bare two centuries ago is very marked. Indeed, work such as Professor Pearsall’s is putting the history of our country in a new light. His chapter on the future possibilities of our uplands is equally striking.

Before the growth of modern transport most people in the south of England had little chance of knowing how those in the rest of Britain lived; there was little opportunity for studying and appreciating the lives of those who lived in the unspoilt countryside of the north and west. But to-day the mountains and moorlands of Highland Britain are within reach of every one, and we hope that Professor Pearsall’s book will help to quicken—and guide—interest in those parts of this country which now provide the main (almost the only remaining) opportunity for observing and investigating wild life and human problems in Britain as it was before modern man’s heavy hand was laid upon it.

THE EDITORS





AUTHOR’S PREFACE (#u86e587f5-cda5-55d2-886b-26760e4e243b)







THIS book is an expression of many happy days in the field and is thus a tribute to the many naturalist friends who have consciously or unconsciously helped towards it by sharing their interests and enthusiasms. I should like to think that they may find some satisfaction in its dedication and that they would feel that they had in part contributed towards its creation.

When so much is owed to others, it may seem invidious to mention any by name. But the largest part of the animal population is composed of insects and for these specialised knowledge is unavoidable, even for a general review. I count myself extremely fortunate in having been able to obtain the sort of information I wanted, and also pertinent criticisms, from Mr. C. A. Cheetham, Professor J. W. Heslop Harrison and Mr. W. D. Hincks, the last of whom also verified the names according to the Check List of British Insects. Mr. W. H. R. Tams very kindly gave much help in the preparation of Plate 30. (#litres_trial_promo) The Editors too have been generous in help and criticism; and, finally, a tribute must be made to the photographic skill of Mr. John Markham.

In spite of this, I fear that this may be thought an odd book, remarkable more for its omissions than its scope. It tries to integrate certain aspects of upland biology of which it may safely be said that about ten years’ intensive work would be required to do them reasonably well. The real integration, perhaps, is that it tells about some of the things that have interested its author.

W. H. P.

University College, London




PREFACE TO REVISED EDITION


Professor Pearsall died in October 1964. A lifetime of active work as a field ecologist, university teacher and administrator, ecological advisor and one of those most concerned with the foundation of the Nature Conservancy, had left him too little time to write. Mountains and Moorlands remains as his major work for the general reader; it is a classic, and must not be touched by a lesser hand. But in the twenty years since its publication, further research has inevitably modified certain concepts. When I was asked by Collins to revise Mountains and Moorlands for this edition, it seemed to me that Chapter 10, on ecological history, should be largely rewritten in the light of new work and the emergence of the technique of radiocarbon dating since 1950. Chapter 10 was partly based on the work of Dr. Verona Conway and myself, and the revised edition of this chapter has been approved by Dr. Conway and by Mrs. Pearsall. I am grateful to Miss Clare Fell for advice on changes in the interpretation of the archaeological record in North-west England since R. G. Collingwood’s account of 1933, which formed the basis of Professor Pearsall’s discussion of the ecological history of North-west England. In other chapters I have changed only a few sentences, to conform with new discoveries, and have provided additional bibliography to cover relevant work published since 1950. Chapter 15, a brief account of the work of the Nature Conservancy in Highland Britain, has been added to the book because of Professor Pearsall’s concern with the Nature Conservancy—he was for many years Chairman of its Scientific Policy Committee—and because of the relevancy of the work of the Conservancy to the matters discussed in Chapter 14 of Mountains and Moorlands, which was written before that work had begun.

The nomenclature of plants has been revised to conform with current usage; on the advice of various colleagues, the revised nomenclature, with two exceptions, conforms with that found in Clapham, Tutin and Warburg’s Excursion Flora of the British Isles, Second Edition; the Census Catalogue of British Mosses, by E. F. Warburg, Third Edition, published by the British Bryological Society in 1963; the Census Catalogue of British Hepatics, by J. A. Paton, Fourth Edition, published by the British Bryological Society in 1965; and A New Check-list of British Lichens, by P. W. James, in The Lichenologist, Volume 3, 1965. The exceptions are that Scirpus caespitosus has been retained, instead of Trichophorum caespitosum, as in Clapham, Tutin and Warburg, and that Cladonia sylvatica agg. has been retained, as it includes the two species, Cladonia arbuscula and C. impexa, of the Check-list.

W. P.

University of Leicester and The Freshwater Biological Association




CHAPTER 1 (#u86e587f5-cda5-55d2-886b-26760e4e243b)


INTRODUCTION






“The grounde is baren for the moste part of wood and come, as forest grounde ful of lynge, mores and mosses with stony hilles.”

(LELAND)

A VISITOR to the British Isles usually disembarks in lowland England. He is charmed by its orderly arrangement and by its open landscapes, tamed and formed by man and mellowed by a thousand years of human history. There is another Britain, to many of us the better half, a land of mountains and moorlands and of sun and cloud, and it is with this upland Britain that these pages are concerned. It is equal in area to lowland Britain but its population is less than that of a single large town. It lies now, as always, beyond the margins of our industrial and urban civilisations, fading into the western mists and washed by northern seas, its needs forgotten and its possibilities almost unknown.

Nevertheless, to the biologist at least, highland Britain is of surpassing interest because in it there is shown the dependence of organism upon environment on a large scale. It includes a whole range of habitats with restricted and often much specialised faunas and floras. At times, these habitats approach the limits within which organic life is possible, and they are commonly so severe that man has avoided them. Thus we can not only study the factors affecting the distribution of plants and animals as a whole, but we can envisage something of the forces that have influenced human distribution. Moreover, in these marginal habitats we most often see man as a part of a biological system rather than as the lord of his surroundings.

This book, then, deals primarily with mountains and moorlands as habitats for living organisms. Many plants and animals are mentioned, usually without detailed descriptions, except where they can be seen to be a characteristic part of the environmental system as a whole, or where they illustrate typical relations between organisms and environment. For this reason also no attempt is made to give full lists. It is also inevitable that the plant-soil relationship occupies in outline a large part of the story because this is the feature which links the animate with the inanimate.

It would hardly be possible to frequent upland Britain without becoming an admirer of its beauty. Its scenery is due to the interplay of its geological structure, of its climate and vegetation, and of human influences. It thus becomes important to the biologist as an integration of the interplay of these habitat factors and often his first interest will be to look keenly at the scenery for clues in the analysis of the environmental factors. As Professor Dudley Stamp has pointed out in his volume Britain’s Structure and Scenery, the scenery of the British Isles is remarkable in its diversity, and this conclusion applies with special force to the British Highlands. Diversity of aspect means diversity of habitat and of biological pattern. It offers a fruitful and as yet hardly explored field for the naturalist’s work and one which is particularly attractive because very valuable results can be obtained without highly specialised knowledge or apparatus.

While the study of the relations between organism and environment is no new aspect of biological inquiry, it is nowadays dignified by a special name and is called the science of ecology. The ecological study of mountains and moorlands may be in its infancy but their fauna and flora have long been objects of interest to naturalists. It is evident from the routes they followed and the lists of plants they collected from 1660 onwards, that John Ray and his associates were no strangers to the high northern hills, but the first record of an ascent of a British mountain we owe to another botanist—stout Thomas Johnson—whose account of the ascent of Snowdon in 1639 all naturalists will enjoy, especially perhaps the concluding sentence: “Leaving our horses and outer garments, we began to climb the mountain. The ascent at first is difficult, but after a bit a broad open space is found, but equally sloping, great precipices on the left, and a difficult climb on the right. Having climbed three miles, we at last gained the highest ridge of the mountain, which was shrouded in thick cloud. Here the way was very narrow, and climbers are horror-stricken by the rough, rocky precipices on either hand and the Stygian marshes, both on this side and that. We sat down in the midst of the clouds, and first of all we arranged in order the plants we had, at our peril, collected among the rocks and precipices, and then we ate the food we had brought with us.”

Johnson lived in troubled times and he was later to die of his wounds as a Cavalier soldier. If interest in the Alps may be said to have started as a result of de Saussure’s scientific expedition to Mont Blanc in 1787, we may perhaps fairly regard Johnson as a British de Saussure at a far earlier date, though on a more modest and unassuming scale.






FIG. 1.—Map of areas discussed, showing some of the Nature Reserves in Highland Britain.





CHAPTER 2 (#u86e587f5-cda5-55d2-886b-26760e4e243b)


STRUCTURE








THE British Highlands are composed of blocks of hard and old rocks that occupy the north and west of these islands. While the biologist is not primarily concerned with the manner in which these rocks originated and attained their present condition, the geological structure of the uplands is a matter of some importance to him because it determines the character of the soil and the nature of the habitats available for living organisms. Consequently, a slight acquaintance with geological structure and processes forms part of the necessary background of the present subject and one, moreover, which is of interest in helping us to understand the great scenic and biological diversity of different parts of Highland Britain.

Almost every mountain observer has been struck by the evidence of decay which centres around the larger peaks. There is shattered rock round their summits (see Pl. III (#litres_trial_promo)) while below every crag we find scree and from every gully there runs a stone-shoot (see Pl. 2b) formed from débris coming down from above. In the mornings the ceaseless downward trickle of stones or the occasional rock-fall proclaims the constant attrition to which the steeper hills are subject. Thus to the distant observer the mountains may seem to be permanent, “the immortal hills,” but to those who know and move among them a different impression is formed, in which breakdown and change play by far the most prominent part.

The causes of this constant weathering of the rock surfaces are primarily the uneven contractions and expansions of the rocks caused by fluctuations of temperature, the action of rain and frost and the force of gravity. Any cracks that develop become filled with water and are expanded when the water freezes and widened when it thaws. The actions of rain and of gravity tend to remove the smaller rock fragments so that nothing accumulates to protect the constantly exposed surface. Thus the surface continues to be weathered away until the slope approaches an angle of rest (usually between 30° and 40°). Rainwash, soil-creep and the gradual downward movement of larger stones continue, long after this angle is reached, to move the materials towards the valley.

Still more important in the long run are the effects of running water, for this not only tends to move materials downwards, but it may also remove them from the area altogether. In the course of time, therefore, every mountain torrent erodes a gully of its own construction, and the coarser materials eroded are deposited below the gully on a gravel fan or delta (see Pl. 2a (#litres_trial_promo)) while the finer materials are carried ultimately to the plains beyond the mountain area. It follows that the land-forms in the uplands tend to be those which have survived the processes of weathering and erosion. In terms of these ideas, the uplands, whether moorland or mountain, have survived because the rocks of which they are formed are harder or have resisted removal rather than because they are being or have been lifted up. At the same time there must have been some original mountain-building process.

Many facts can be used to illustrate this argument. Almost every visitor to the English Lake District becomes familiar with a type of scenery in which there is a foreground of lower and rounded hills, backed by a skyline of larger and steeper mountains. This is well shown in the photograph of Esthwaite Water in Pl. 3b and this type of scenery has a simple explanation. The rounded hills in the foreground are composed of rocks less resistant to weathering and the high mountains of harder or more resistant rocks, in this case the hard volcanic “tuffs” or ashy beds of the Borrowdale Volcanic series which make up much of the mountain core of the Lake District. Both to the north and the south of these hard rocks lie softer slates and grits. Although to the uninitiated these rocks present a generally similar appearance, they exhibit considerable difference in hardness. As the harder Borrowdale rocks have weathered much more slowly, they now generally form much higher ground than do the adjacent softer rocks. The actual junction of the harder and softer rocks is shown in Pl. I (#litres_trial_promo) where it will be seen that the harder rocks are mountain slopes, the softer being soil-covered and cultivated.

There are many equally good examples elsewhere of the influence of the hardness or softness of rocks upon land forms. A second illustration may be taken from Scotland, where, north of the Highland line, there are to be found long stretches of steeply inclined and metamorphosed grits, hard and resistant rocks which give the line of the summits, Ben Lomond, Ben Ledi, and Ben Vorlich. The valleys intersecting this area lie on beds of softer rocks, shales, limestones and phyllites, which have suffered correspondingly greater erosion and so have been cut down below the general upland level.

The general concept of mountain structure thus illustrated can be applied on a larger scale, for it has been pointed out already that the distribution of mountains and moorlands in Britain is essentially that of the older and harder rocks. These lie, as we have seen, to the northwest of the British Isles, and their range covers all the main mountain areas. The causes of this distribution lie in the far-distant past when large-scale earth movements were taking place, and there seems to have remained since a tendency for the north and west of the British Isles to stand as a raised system. While any attempt to trace these mountain building movements lies outside the scope of the present discussion, it may be interesting to indicate something of their effects on upland structure.

In the simplest cases, of which the Pennine range in Northern England is a good example, the upland area represents essentially a fold in the earth’s crust. In the Pennines, this fold runs approximately north and south and a transverse section through it would show the general arrangement represented in Fig. 2, with the newer rocks (including the Coal Measures) represented on either side of the fold, that is in Lancashire and Yorkshire, but absent from the top of the Pennines themselves. There is evidence of various types which points strongly to the probability that newer rocks, from the Coal Measures upwards, have in fact been removed by erosion from along the crest.

Thus the Craven Uplands, including Ingleborough at their southern end, are separated from the main block of the Southern Pennines by the great Craven Fault system. Just south of this fault, at Ingleton, coal was formerly mined from strata lying above those which correspond to the rocks on the top of Ingleborough. The assumption seems clear, therefore, that these Coal Measures have been removed by erosion from the area north of the Craven Fault.

The Craven Uplands are also interesting in another respect.






FIG. 1.—General character of Pennine anticline. (Diagrammatic.) a, Coal Measures; b, Millstone Grit; c, Yoredale Shales, etc.; d, Carboniferous limestone; e, Ordovician and Silurian.






FIG. 2.—Dissection of Craven Pennines by river valleys: 1, Yoredale rocks; 2, Carboniferous limestone; 3, Ordovician and Silurian. (Diagrammatic.)

The three parallel summits of the mid-Pennines, from east to west—Great Whernside (2310 ft.), Penyghent (2273 ft.) and Ingleborough (2373 ft.), but also, to the north-west, Whernside (2414 ft.) and Grey Gareth (2250 ft.), reproduce almost identical characteristics in structure and altitude. They are separated by a series of river valleys, Wharfdale, Littondale and Ribblesdale, then also by Chapel-le-Dale and King-dale, which have very obviously been cut down through the original rock formations, here almost horizontal (see Fig. 2). A reconstruction of the mid-Pennines on an east-to-west section thus shows these mountains as the surviving elements of the Pennine fold, resting on a mass of the still older Silurian rocks, upon which also the rivers now run. North and south of these Craven Uplands, the Pennines have commonly the character of a high moorland plateau (see Fig. 28). It is only when these plateaux have been greatly dissected by erosion and by river action, that distinctive mountain peaks are frequent. This has happened not only in Craven but also at the southern extremity of the Pennine range where dissection has also split up the plateau into peaks such as Kinder Scout and Bleaklow.

Now whatever their origin may otherwise be, it is extremely common to find that mountain masses have the character of dissected plateaux. There is perhaps no better example of this in Britain than the Cairngorms as a whole. The observer standing at any considerable distance from these mountains (so as to be able to see most of the major summits) will inevitably be struck by the fact that the group as a whole presents a nearly level or gently domed-shaped profile. This may be seen particularly well from near Aviemore (see Fig. 3), and it is suggested by the skyline in Pl. 5 (#litres_trial_promo). In other words, these peaks, so impressive at close quarters, are due to the cutting up of the high plateau by deep and steep valleys. Even on isolated peaks like Lochnagar, the vast summit plateau clearly indicates the remains of one still more extensive.






FIG. 3.—Silhouette of the northern face of the Cairngorms—a dissected plateau.

Imagine the processes of erosion and dissection proceeding over many square miles of nearly horizontal strata, until much more has been removed than is left, and it will be possible to understand the origin of the extreme examples of mountain or plateau dissection to be seen in Western Ross and Sutherland. Here, formerly, nearly horizontal layers of Torridonian sandstone covered an ancient surface of hard and resistant crystalline rocks. To-day, such mountains as Suilven and Canisp represent the last remains of these sandstone masses, most of which have long since vanished. In this category also must no doubt be placed Lugnaquilla (3039 ft.) in south-eastern Ireland—the last remnant of rocks overlying a large boss of granite.

There is one other point about the effects of erosion which is worthy of brief mention. If a mountain mass or ridge were composed of uniform materials and if it were equally eroded on all sides, the shape of the mountain would tend to approach more and more closely, as time went on, to that of a perfect cone. This generalised type of mountain is not perhaps very common in Britain—though isolated hills like Muckish and Errigal in Donegal are of this general type as well as many of the rather lumpy mountains in the Scottish Highlands, especially perhaps Schiehallion. The Paps of Jura, illustrated in Pl. 3a (#litres_trial_promo), show the disintegration of a quartzite ridge in this way. A common British variant of this simple type is one in which the summit is distinctly flat-topped or tabular. This is particularly to be seen in some of the examples already mentioned. The three most prominent Pennine summits, Cross Fell, Ingleborough (see Pl. 17 (#litres_trial_promo)) and Kinder Scout all have this form as do the Sutherland mountains Suilven and Canisp, and MacLeod’s Tables, west of Dunvegan in Skye. It is due to the presence at the summit level of a horizontal stratum of hard and resistant rock, usually Millstone Grit in the Pennines and Torridonian Sandstone, capped by Cambrian quartzite in Sutherland, the latter containing so much white quartz that the rock may be mistaken for a snow-cap when seen from a distance (see Pl. III (#litres_trial_promo)).

The Craven Uplands show in a particularly striking manner the dependence of mountain scenery and vegetation on the geological structure. The rocks are horizontally stratified and they consist of an upper zone, mainly of Yoredale sandstones and shales, below which lies a great thickness of Carboniferous Limestone, once called the Mountain Limestone from its association with upland areas in Britain. Where the overlying rocks have been removed by erosion, the hard limestone may form extensive plateaux, and because it is almost pure calcium carbonate, it yields practically no soil on weathering. It is traversed in all directions by deep vertical fissures and is consequently dry (see Pl. XXII (#litres_trial_promo)). The surface, aptly called “limestone pavement,” is usually devoid of vegetation except where traces of glacial drift occur, but a luxuriant flora lives in the shelter of the fissures. The limestone plateaux are often bounded by almost vertical “scars” (see Pl. XVIII (#litres_trial_promo)). A very striking type of scenery is thus produced, a feature not only of the Craven Uplands and mid-Pennines in general, but also of large areas in Western Ireland (Clare and Mayo).

In contrast, the Carboniferous Sandstones (including Millstone Grit) and shales are non-calcareous and are almost always covered by the moorland vegetation which is so characteristic a feature of the high plateaux of the northern and southern Pennines. In Craven, where these rocks are exposed along with the limestones, the contrast between the two sorts of rock is often very striking, and is well illustrated in Pl. 4 (#litres_trial_promo). Thus both the physical and chemical qualities of the rocks may affect the scenery and vegetation.

The simple conical form that is to be expected where rocks of approximately uniform texture are equally eroded on all sides, is lost not only when the harder rock strata occur, but also wherever the mountain is composed of strata that are not horizontal. Thus both Blencathra and Dow Crags in the Lake District show one gently sloping aspect (see Fig. 4) which is that of the “dip” or slope of the rock strata, while on their steep faces the rock weathers into blocks, more or less at right angles to the dip of the strata, so that a






FIG. 4.—Some types of mountain form. A, Symmetrical weathering of uniform rock; B, Recent oversteepening below ancient upper form; C, Ridge with softer interbedded rock; D, Dip and scarp slopes.

steep angle tends to persist. In Wales, Tryfan also shows this type of structure in a still more spectacular manner (see Pl. II (#litres_trial_promo)) and it is very generally to be seen in the different mountain areas, often recurring, again and again, wherever the rock strata act as guiding planes for the inevitable erosion.

Sometimes hard rocks arranged in this manner overlie much softer ones. Such is the essential structure of Mam Tor, the “Shivering Mountain” in Derbyshire, and also of Alport Castles not far away. In both cases, hard sandstones and grits in the upper part of the escarpment have below them soft shales which are constantly washing and weathering away. Thus, on Mam Tor, the upper parts of the escarpment is constantly being undermined and so are constantly falling. Alport Castles, in contrast, represent an immense wedge of the mountain detaching itself from the face behind and falling outwards with infinite slowness, pushing before it into the valley a great wave of earth, as is well shown in Pl. 6 (#litres_trial_promo). There are no better examples in Britain of the instability of mountain structures than these two Derbyshire hills.

In other British mountain areas, the comparatively simple arrangements of rocks seen in the examples already discussed are obscured and other considerations become important. An upfold like that met with in the Pennines is called an anticline (see Fig. 1) and a corresponding valley-shaped fold (or depression) would be called a syncline. Now it is a striking fact that the mountain summits very often represent the remains of a syncline. Naturally this is only in areas where great erosion has taken place. The reason for the persistence of the synclinal folds as mountains is that when folding takes place as a result of lateral pressure, the synclinal folds will be compressed and so will tend to become harder. Anticlinal folds, on the other hand, will come under tension and so will tend to crack.

Thus when weathering and erosion takes place, the anticline, being shattered, is more easily attacked and suffers more, while the syncline, being compressed and hardest, therefore tends to be more slowly affected. It is thus logical, if somewhat unexpected, to find that great peaks or perhaps particularly ridges often represent the remains of a syncline, though the synclinal structure may not always be evident because the main ridge of the mountain often represents the long axis of the synclinal fold.

The classical example of synclinal mountain structure, of which a fine picture exists in Lord Avebury’s Scenery of England, is that of Y-Wyddfa, the main peak of Snowdon, as seen from between the Crib Goch and Crib y Ddysgl under suitable conditions—with a powdering of recent snow. This face is usually in shade and not easily photographed to bring out the rock structure, but the essential features are shown in Fig. 5.

Another fine and well-known section illustrating synclinal structure is exposed on the Clogwyn du’r Arddu, to the north-west of the main






FIG. 5.—Rock structure showing syncline on Y-Wyddfa—the Snowdon summit.

summit, where a great synclinal fold makes up the whole of the precipice. These rather simple illustrations serve to illustrate a very important fact that where great earth-movements have taken place the contortions of the rock strata may greatly affect their hardness and resistance to erosion.

Snowdon itself represents the bottom of a great fold whose crest lay somewhere to the south-east. In that locality some 20,000 ft. of rock must have been removed by erosion. The human mind can hardly appreciate the length of time, not less than hundreds of millions of years, which erosion on this scale must have taken. The rocks now exposed belong to two ancient systems which we have already encountered in discussing an earlier illustration (see Pl. 3b). They are in geological terminology of Ordovician and Silurian age (see Britain’s Structure and Scenery by L. Dudley Stamp). The central core of Wales, as of the Lake District, consists of Ordovician rocks which are solidified volcanic ashes and stones (tuffs) and lava flows, with interbedded marine strata indicating a submarine origin. These make up some of our boldest mountain scenery, though there is nothing to suggest that the individual mountains such as Snowdon, Cader Idris or Scafell have ever been volcanoes. Associated with the Ordovician tuffs and lavas are extensive sedimentary rocks of later Silurian age which are mainly fine grits or shales, and these, though generally softer, are as a rule rather poorer in bases like lime. They form somewhat more rounded hills (sometimes described as moels, their Welsh name), to-day almost always grass-covered like the lower slopes of the Ordovician crags. The general appearance is well shown in Pl. XXIII (#litres_trial_promo). Together, the Ordovician and Silurian rocks make up some of the most extensive areas of British upland country, characteristic not only of Wales and the Lake District, but also of the Southern Uplands of Scotland and Southern Ireland.

The mention of volcanic action should not necessarily suggest an identification of parts of a particular mountain with the cone and crater of an extinct volcano. The correct interpretation of signs of volcanic action among British mountains is usually possible only if one keeps clearly in mind the fact that most mountains are likely to be the remnants of larger structures. Usually then it will be vain to look for anything so obvious as the cone and crater of a Vesuvius or a Stromboli. The nearest approach to this sort of structure that we are likely to find in Britain is seen in some of the Laws of Southern Scotland.






FIG. 6.—A Scottish “Law”—eroded remains of ancient volcanic vent. The shaded areas are basalt (lava flows)—the laminated areas are volcanic tuffs (ashes).

These usually represent the vents of small volcanoes which have become plugged with solidified lava whilst the surrounding cone has been more or less completely removed by erosion. A simplified section is given in Fig. 6. One of the most complete examples, Largo Law in Fife, is essentially similar but has two main vents. The figure shows the position of the vents and the lava flows which are marked by “basaltic” rocks. Around these are the remains of the cones formed by tuffs or solidified volcanic ashes and stones. The mineral composition of these volcanic tuffs is characteristic, so that they can be recognised where no volcanic cone is evident. It is this type of identification that is used in the case of the Ordovician tuffs already mentioned, where the scale of output was immeasurably larger and no certain vent can be found.

Igneous rocks apart from volcanic lavas more usually fall into one of two main morphological types. The largest areas are occupied by plutonic rocks, representing enormous masses of molten rock which has solidified without reaching the surface. There are secondly “dykes” and “sills,” both representing intrusions of molten rock among other pre-existing strata. In the case of dykes the intruded material runs through cracks or planes at right angles to the general stratification—in the case of sills the molten rock follows between the bedding planes and therefore runs parallel to the general “dip” of the rock. Sills are often more resistant than the rocks into which they have been intruded, and when this is the case they may form striking cliffs. Especially well-known examples are some of the sills in the Edinburgh district, of which perhaps Salisbury Crags are the most impressive. In Northern England the Whin Sill not only forms natural escarpments on which part of the Roman Wall stands, but it is associated both with a remarkable flora and with a series of majestic cascades in and near Upper Teesdale. Far away on the western side of the Pcnnines, it outcrops again on the great western escarpment, particularly at Roman Fell and in the spectacular amphitheatre of High Cup Nick, where it is eighty feet thick.

Dykes are often on a much smaller scale, but when found among resistant rocks they often give rise to striking gullies and cols. Perhaps the best-known mountain structure of this type is Mickledore, the great gap separating Scafell from Scafell Pike.

The larger intrusions of igneous rock are very often great bosses of granite which may be many miles across. Classical examples are those in Galloway, which give the mountains of Criffel and of Cairnsmore of Fleet. To this type of structure belong the summit of Crib Goch and also Penmaen Mawr in Wales, the latter familiar to every one who drives along the coastal road. Generally similar is the huge granite mass of Dartmoor. In all these cases the granite boss is harder than the surrounding country rock and so has been left more elevated than the areas around. Where the surrounding rocks are hard, however, granite bosses may contribute no noteworthy structure to a mountain region, and this is the case in the Lake District, for example, where the Shap, Eskdale or Ennerdale granites are all relatively inconspicuous among the hard slates into which they were intruded.

Along the western seaboard of Scotland granite intrusions occur among other traces of volcanic or plutonic activity. The Western Isles and many of their mountains include the remains of vast flows of basaltic lavas which formerly stretched from Antrim, through Staffa, Mull and Arran to Skye, and, indeed, as far north as the Faeroe Islands and Iceland. Geologically, these lava beds are of Tertiary Age and very much more recent than the tuffs of the Lake District and Wales. Even to-day the beds lie nearly horizontal, and though they form the well-known columns of Staffa and the Giant’s Causeway and are often exposed in sea-cliffs (those of Eigg and of Portree Harbour, for example), they do not as a whole contribute much to our mountain scenery. Nevertheless, the familiar view of the mountains of Mull, Sgurr Dearg and its neighbours seen from Oban, consists almost wholly of rocks of this type, forced upward by later volcanic action in Central Mull. Still farther north, in Skye, the Storr Rocks (see Pl. IV (#litres_trial_promo)) and the Quirang are, moreover, both composed of these Tertiary lavas overlying softer Jurassic shales, and the whole of the coastal scenery is dominated by them.

Much more important scenically were the great subsequent upwellings of molten igneous matter in this area, which are associated with the noble mountain scenery of Skye, Rhum and Arran. In Skye, the principal contrast is between the Black Coolin and the Red Hills. The crags of the former are composed mainly of a hard and basic rock called gabbro, with a coarsely crystalline structure that delights the climber’s heart. The Red Hills, in contrast, are granite and this has weathered far more rapidly and uniformly to give mountains of smooth and rounded aspect. The contrast, known to every visitor to Skye, is extremely well shown in the fine photograph (Pl. 1) (#litres_trial_promo) of Blaven and Ruadh Stac, the former of gabbro and the latter of granite. The gabbro is intersected by igneous “dykes” which, running mainly north-west and south-east, serve to accentuate the differences, for these are more easily eroded than gabbro and so tend to form the gullies in the great gabbro ridges. Pl. VII (#litres_trial_promo) gives an excellent impression of the distant aspects of the rock and the ridges.

Somewhat similar contrasts are to be seen in Rhum, where the outstanding peaks of Hallival and Askival are composed of ultra-basic and coarsely crystalline rocks of an unusual type. Their craggy outlines contrast noticeably with the grassy and rounded appearance of the hills farther west, such as Fionchre and Bloodstone Hill, both mainly built of more easily weathered basalt. A similar contrast is seen between the peaks of igneous rock and the gentle moorland contours of the Torridonian sandstones in the northern part of the island, which form a foreground as seen from Skye. In northern Arran, too, there were great intrusions of igneous rocks. The granite of Goatfell stands out boldly, as seen from Brodick Bay, against a foreground of softer sandstones.

The igneous geology of these western mountains is extremely complex and cannot adequately be discussed here except where it plays a part in determining the characteristic features of a mountain mass. But a few words may perhaps be spared for Ben Nevis (4406 ft.) which, as the highest mountain in Britain, deserves at least a passing mention. Ben Nevis represents a central plug of rock, surrounded by two cylinders of intrusive granite, that is presumably by two cylindrical faults, filled up from below by molten rock. The cap of the mountain core consists of ancient lavas (Old Red Sandstone Period) overlying Dalradian schists, and it is supposed that this central core of rock must have sunk considerably into the molten rock now represented by the granite cylinders. Going east from Ben Nevis, Carn Mor Dearg lies on the inner cylinder of granite and Aonach Mor (3,999 ft.) on the outer cylinder. From the north-west, both types of granite can be distinguished on the route from Fort William to the summit of Ben Nevis.

A similar complex system centres round Glen Etive, with the Buchailles of Etive representing a cap of rhyolites and tuffs on a core surrounded by cylinders of granite. Ben Cruachan lies wholly on one of the granite intrusions and so too does the greater part of the Moor of Rannoch.

From the point of view of their influence on the animal and plant life, a highly important property of the volcanic and igneous rocks is whether or not they are rich in basic substances like lime, potash and magnesia.

The geological classification expresses these features inversely in terms of the amount of the non-basic material, silica, which is present, as shown in the following table:

Table 1 SILICA CONTENT OF IGNEOUS ROCKS






Biologically, the basic and ultra-basic rocks provide habitats which are generally more interesting largely because they yield richer soil. The favourable feature of a high base content is, it is true, often partly counteracted by the hardness of the rocks and an accompanying resistance to weathering and erosion, as in the examples already given from Skye and Rhum. But many British basalts are not only basic but they also weather especially easily to yield a comparatively rich soil. The Ordovician tufls are often intermediate in character and may include much andesitic material. In contrast, most British granites contain on an average over 70 per cent of silica and they yield soils which may consist of little but sand and which, as a result, are correspondingly infertile. The biologist thus soon learns to regard granite areas as a distinctive upland type, just as they are geologically and scenically. On the other hand, he has learnt to approach areas dominated by basic or ultra-basic rocks with a certain amount of optimism. Their more varied vegetation and fauna runs parallel with the higher base-status of the soils and rocks, and the latter, indeed, often contain large amounts of bases such as potash, magnesia and iron oxides instead of the lime that prevails in many sedimentary rocks. By analogy with other parts of the world, it is probable that the presence of certain plants and animals on the basic and ultra-basic rocks is associated with these peculiarities of chemical composition of the latter.

The great variety of rock type and of rock arrangement which runs through the Western Islands is less apparent on the Scottish mainland. There the mountain masses of the Grampians are mainly composed of hard and ancient rocks, so greatly contorted by subsequent earth movement that their arrangement is often obscure and it is consequently less easy to describe in broad general terms their relation to mountain structure. They are geologically, for the most part, schists or gneisses (which are, respectively, metamorphosed and distorted shales or sandstones and grits) or finely crystalline igneous rocks. But the simple principles which have been stressed above are generally applicable when the structure of any individual mountain or upland area is considered. Without considering these in detail, it may be noted that the Grampians include three main areas of differing structural type, which have biological interest. Towards the south and west there is an area in which mica-schists predominate. This is a rock which weathers easily, yielding an open and uniform soil. It is marked by a group of characteristic and somewhat lumpy, grass-covered mountains lying roughly along a curved line between Ben Lawers, Ben Doireann and Ben Alder, which possess a well-recognised biological type.

The chief contrast in the Grampians is, however, between the eastern and western halves of the country. The former, exemplified particularly by the Cairngorms, is mainly a high though deeply dissected plateau, which constitutes the greatest continuous area of high ground in the British Isles. The Cairngorms are evidently in the early stages of a new erosion cycle, and their typical outlines, already discussed in connection with Pl. 5 (#litres_trial_promo), contrast remarkably with those of Dartmoor, for example, also a granite mass, but one characterised by land-forms indicating far advanced weathering and erosion (see Pl. XXXI (#litres_trial_promo)).

In the western part of the Highlands, erosion and dissection have proceeded far more effectively, so that more often the mountains are partly isolated peaks or broken ridges. The change has undoubtedly been hastened not only by greater precipitation and glacial erosion in these areas, but also by the presence of numerous faults, running roughly from north-east to south-west, which have offered full play to eroding influences and have given us a series of loch-filled valleys. The most notable of these fault-lines is that of the Great Glen. Nevertheless, in spite of the much greater amount of erosion, the general level of the summits among the western mountains is very uniform and is indicative of that of the original plateau from which they must have been derived.

The Scottish Highlands illustrate very well a point that was emphasised a long time ago by the late Professor J. E. Marr. In general, as upland surfaces recover from disturbances, they will tend to develop systems of gentle slopes and to approach, as Dartmoor is doing, characteristic forms of “subdued relief.” Among the upper levels of our British mountain regions it is possible to see a large proportion of land forms which are predominantly those of subdued relief. This implies that these forms must be of great age, for on account of the great hardness of the rocks, it must have taken an enormous time for the outlines to have “softened” in such an extreme manner. From arguments such as these, it may be assumed that the general form of our mountain regions is often ancient, and this usually applies particularly to the positions of the main summits and the river valleys. Superimposed on these ancient features we have also features which are the result of comparatively recent agencies. Foremost among these are the effects of ice and of glaciation.

Much British mountain scenery is that characteristic of a glaciated and ice-eroded country. That there is a marked contrast with other regions will be at once apparent if one compares a typical British upland scene with one, for example, from the Grand Canyon of Colorado.






FIG. 7.—Ice movements in the British Isles. GLACIATION

The most striking feature in the recent geological history of the British Isles was the series of great Quaternary Glaciations, which terminated only some 10,000 years ago. For biologists this is a convenient starting-point for recent biological history, but it was scenically of equal or greater importance. In order to obtain a picture of what Britain was like during the Glacial period, we should have to try to imagine it buried beneath a great ice-sheet many hundreds of feet thick, and covering, at its maximum extent, almost the whole of these islands north of the River Thames. The centres of ice formation were the areas with greatest precipitation (then snowfall, now rain), particularly the greater area of the Highlands of Scotland, centring on Rannoch Moor, to a less extent the Southern Uplands from Merrick outwards and the smaller Lake District, and also, but still less, Snowdonia. From these and other smaller centres the ice flowed outwards, though very slowly. A huge existing ice-sheet, that in Antarctica, to-day is still moving at the rate of a yard and a half a day when it reaches the sea as the Ross Barrier, hundreds of miles from its source.

We can trace the main directions in which our British ice-sheets moved, because they carried with them all the soil and rock detritus that had accumulated on the surface of the land in the preceding ages. Any unusual types of rock are readily recognised and, because they have characteristic fossils or special mineral constituents, limestone and igneous or volcanic rocks are especially useful for this purpose. Rocks thus found far from their place of origin are termed erratics, and the photograph in Pl. VI (#litres_trial_promo), shows a well-known example, one of the Norber boulders in West Yorkshire, slate rocks carried by ice from an adjacent valley and left on top of the Carboniferous Limestone which normally overlies the slates (see Fig. 2). In England, erratics of the Shap granite, coming from a small area in the eastern Lake District, have been particularly valuable in tracing the movements of Lake District ice. A magnificent boulder of this rock some ten feet in cube, standing in the main quadrangle of the University of Manchester, illustrates the fact that ice from the Lake District left debris as far south as Cheshire. Farther west there was an ice-flow carrying Galloway granite to Flint and Shropshire. Similar evidence shows that some Lake District ice went east over Stainmoor, leaving boulders of Shap granite as far away as the Yorkshire coast. The accompanying map (Fig. 7), constructed mainly from evidence of this type, shows the main lines of ice movement in Britain during this period. It will be noticed from this map that the ice movements did not always follow the obvious lines of outward radiation. In Lancashire and Wales, for example, the ice was deflected southwards and eastwards by Scottish and Irish sea-borne ice. In Scotland particularly, and to some extent in Yorkshire and Northumbria, the outward-moving ice was dammed up and deflected by Scandinavian ice coming across the North Sea. Moreover, in the partial northerly deflection of the northern ice there is evidence that it overrode mountains 3,000 ft. high. On the west coast of the Highlands, the ice-marks not only reach this altitude, but, allowing for the depth of adjacent lochs, it can be estimated that the ice-field must at times have been some 4,000 ft. thick. Similarly in the Lake District, where the area of high precipitation is much smaller, the ice-fields were some 2,000 ft. thick. Indeed, on Scafell and Helvellyn the marks of glaciation may be seen up to a height of 2,500 ft. It is not easy to imagine the scale of this ice-covering. The nearest thing to it at present may be the Greenland ice-cap; that in Antarctica is apparently larger.

Even at its maximum extent, it did not wholly cover the country (see Fig. 7). There could not have been much ice south of the Thames, and Dartmoor seems to have been quite unglaciated. Moreover, the highest mountains, and, indeed, many of the lower outlying ones, projected through the ice as nunataks. They can often be recognised by their greater altitude and bolder shape, which contrasts markedly with that of the lower, rounded and glaciated hills. In the later stages of the Ice Age, at least, considerable areas of the Southern Pennines may have been generally ice-free, though no doubt supplied with local snowfields. The main ice-flow at this time seems to have been deflected by the Howgill and Bowland Fells, or westward down into the Cheshire Plain. The existence of ice-free areas makes the comparison with Greenland more valuable and it allows us to assume that there were probably at least some plants and animals there.

In their movements, the ice-sheets not only scoured away existing soils and rock debris, but they also scraped away rock. Thus in glaciated regions, every projecting rock tends to be smoothed and scratched on the exposed side, even if it retains rough surfaces on the lee side. Such rocks are termed “roches moutonnées,” and often they allow us to infer the direction of local ice movements even better than do erratics. Although it did not invariably do so, the moving ice tended to follow existing valley lines and hence these were scoured out and deepened, particularly towards the valley heads where the ice was normally deeper. Often rock basins were formed which now contain lakes (see Pl. 8 (#litres_trial_promo)). The form of these glaciated valleys (and of the lakes) is very characteristic: they tend to be “canal-sided” in plan and U-shaped in section. The effect of these great ice-sheets is not only to deepen and broaden the main valleys but also in doing so to remove the lower and gentler slopes on each side. Thus spurs are cut off and lateral valleys are cut short, while the lateral streams they contain now tend to enter the main valley by sudden rapids or waterfalls. “Hanging valleys” of this type and “truncated spurs” are a characteristic feature of British mountain scenery. The photograph of Loch Avon in Pl. 8 (#litres_trial_promo), shows a fine example of a hanging valley, while truncated spurs can be seen in Pl. 10 (#litres_trial_promo).

Additional results of this form of erosion are, first, that vast quantities of detritus are removed and scattered over the adjacent country; and, secondly, that the existing forms of low relief are “sharpened” as it were and made more mountainous in aspect (see Pl. 23 (#litres_trial_promo)). The gentle plateau-like profiles of some of our mountain areas, as seen from a distance, give no hint as to the steepness and wildness of the ridges and valleys they prove to contain. At the same time, the removal of pre-existing soils and gentle slopes has generally left the valleys in what is essentially a “montane” condition of bare rock or rock detritus in marked contrast to the deep and long-established soils that must have prevailed in pre-glacial conditions.

As to the materials left behind by the ice, the most obvious are usually coarse rock-waste in the valleys, often material which had fallen from adjacent hills on to the ice during the last stages of its retreat. The effects of these last remaining valley glaciers were comparable indeed to those observed in existing high alpine regions, but they were insignificant as compared with those of the great inland ice-sheets. The detritus accumulated by these sheets in their ground moraines and left behind when they finally melted, has largely determined the appearance of the existing British lowlands and indeed of the lower valleys, obliterating all pre-existing soils or underlying rock, and often smothering the pre-glacial features under layers of “drift” 30 or 40 ft. thick (see Pl. V (#litres_trial_promo)). Although it may contain rock-waste of almost every type, the drift usually consists of some form of boulder clay in which ice-worn and scratched boulders are mixed with what is really rock-flour. The prominent clay fraction of this drift, whatever its general texture, usually suffices to make it “set” readily, and accordingly it often tends to be impervious to water. Not only has this type of material been scattered to great depths all over the lowlands adjacent to our mountain areas, but, in thinner sheets, it will also often be found to be plastered over almost any area of low relief in the mountains themselves. Consequently most gentle slopes in the upland districts, if not left scoured clean by ice action, are covered with drift which is often of a clayey nature.

These drifts and morainic deposits usually contain material carried from a distance and, like the erratics, unrelated in characters to the locally underlying rock. Pl. 9 (#litres_trial_promo), for example, shows some glacial ground moraines of a non-calcareous nature bearing moorland vegetation, although the underlying rock is limestone, on which such vegetation does not normally occur. A similar effect is clearly shown in Pl. 17 (#litres_trial_promo), where drift overlying limestone has blocked up the drainage so thoroughly that peat, bearing moorland vegetation, has developed over the limestone. Of course the reverse can also take place. In many places, calcareous clays have been distributed over non-calcareous strata, and thus as a general rule considerable caution is necessary in glaciated regions in attempting to relate soil or vegetation types to underlying rock.

Of course, at the close of the glacial period, there was a great deal of resorting of the rubbish left behind by the melting ice. The vast quantities of moving water which must have resulted during the melting process obviously redistributed the morainic materials on an enormous scale. Consequently to-day we often see streams running down valleys that now seem far too large for them or issuing across deltas which, quite obviously, they could never have produced in their present condition. A particularly striking condition at that time must have been the numerous large lakes held up among the ice-sheets, often in what are now quite unexpected places. At least one of the high limestone hills in Yorkshire, Moughton Fell, has lake silts on its summit, and such sediments or delta cones deposited in water are not uncommon on the flanks of the wider valleys. A classical British example, Lake Pickering, lying to the south of the Cleveland Hills in North Yorkshire, is associated with the name of the late Professor P. F. Kendall, who showed how the existence of the ice-dammed lake could be recognised. While we are not concerned here with the detailed history of these bodies of water, it should be recognised that they left behind various sediments, including impervious ones, that helped to create local stagnation of the drainage system; and, as we shall see, so gave a definite character to the sites they had occupied.

The great glaciation merits more attention than we can really give it, and this not only because it moulded our scenery. It is far more important because it represents a characteristic phase in the history of our mountains and moorlands as we now know them, as well as the agency which has perhaps more than anything else determined their present biological character. Whatever may ultimately prove to be the underlying cause of such an ice-age, it cannot effectively develop, as Sir George Simpson has emphasised, without the heavy precipitatation (then as snow, now as rain) that characterises British mountains. From this point of view, the Ice Age seems to be as characteristic of British highlands as is the present climate. From the historical point of view, the Ice Age is important because it means that the starting-point from which our present fauna and flora is derived must have been largely an arctic-alpine group of organisms. From still a third point of view—of particular interest to the botanist—the glacial epoch left behind it an upland country largely sterilised by the removal of existing soils and fertile deposits. Some areas, such as large parts of the Hebrides and of Sutherland, were in fact left sterile, and even to-day remain as an almost bare and undulating rock surface occupied only by small tarns and moorland of the bleakest type. This is a common condition among the mountains. But speaking generally, even among the mountains and particularly among the foothills round each mountain group, areas of gentle slope were usually plastered over with clayey drifts or sediments. The result was a great deterioration of the natural drainage in many areas. Even the limestone hills, as we have seen above, Plates 9 (#litres_trial_promo) and 17 (#litres_trial_promo), though naturally extremely porous, often suffered in this way, and in many cases cannot be superficially distinguished from the non-calcareous rocks around them in this respect. Elsewhere, and particularly in the valleys, the erosion of the lower slopes by ice and by glacial streams left behind a sharpened relief of a much more montane character and, incidentally, often paved the way for a new cycle of erosion.




RECENT EROSION


The forms of the mountains we see to-day are clearly the results of three agencies — of the original rock structure as modified by large-scale earth movements, of the long continued erosion which, acting on the original structure, has fixed the positions and outlines of the main valleys and summits, and, lastly, of the sharpening of land forms and the removal of pre-glacial screes and soils by ice action. As a habitat for living organisms, the surface of a mountain is as important as its skeleton, and this is affected not only by the legacy of slope and structure already described, but still more by the recent or post-glacial effects of erosion. Speaking generally, the upland surfaces are either physically stable or unstable, and it is the large proportion of unstable surfaces which is particularly characteristic of upland areas. Nevertheless, even the stable surfaces, those more comparable in slope and form with lowland areas, show peculiarities, for they are often rock surfaces, either scraped clean during glaciation (as in the Northern Scottish examples just mentioned) or sometimes, like limestone pavements, composed of rock which yields little or no soil on weathering. Even the soil-covered stable surfaces are often areas covered by poor glacial drifts or with impervious rock strata beneath, and now mostly peat-covered.






FIG. 8.—Arrangement of main zones below a rock outcrop.

The unstable surfaces naturally tend to lie along the main lines of erosion, and they include both the places over-steepened by ice action as well as those showing the immediate effects of stream erosion. The most widespread type of unstable surface is the steep scree-slope with its capping of crag. This generally shows a gradation of form and composition such as is illustrated in Fig. 8. The upper part is usually the steepest, and consists of coarser detritus, while the lower part shows finer detritus and gentler slope. As on any steep slope, rainwash leads to the accumulation of the finest materials on the lower parts.

The downward movement of material continues long after an angle of primary stability (usually between 30° and 40°) is reached; and there are numerous interesting manifestations of this movement. The larger stones, in particular, are usually persistent “creepers,” expanding more on the lower side when the temperature rises and contracting more on the upper margin when cooling takes place. They often continue to move downwards long after the rest of the surface has been stabilised by vegetation. When such stones are elongated in shape they generally tend to progress with their long axes more or less parallel to the slope, as may be seen in Pl. 25 (#litres_trial_promo). Although the larger boulders move most persistently on partly stabilised screes, they usually move more slowly than the finer material on loose scree slopes, where the finer materials often accumulate around the upper side of the boulders, giving a step-like arrangement. Obstacles such as tufts of grass lead to a similar effect, so that some form of terracing is particularly characteristic of steep mountain slopes, even after they have been partly stabilised by vegetation, and one has only to look down on a steep grassy slope under suitable lighting conditions to see what are apparently innumerable more or less parallel “sheep-track” terraces, due mainly to the agencies of soil-creep and rain-wash, though nowadays much accentuated by the movements of grazing animals.

While the characteristic features of crag and scree may occur at almost any level, there are other types of instability which are particularly characteristic of the higher altitudes above 2,000 ft., and generally most clearly shown on the high summits. The high mountains are generally but little affected by the action of running water, and their erosion is due far more to the effects of frost and snow, sometimes collectively distinguished as nivation.

The surface of the higher and steeper summits is commonly covered with rock detritus, sometimes to a depth of several feet (see Pl. III (#litres_trial_promo)). This material, often called mountain-top detritus, is formed by the disintegration of the native rock by the action of frost. The size of the individual fragments, as in the case of screes, depends largely upon the hardness and the physical character of the underlying rocks.

The frost detritus or mountain-top detritus is the most characteristic of summit surfaces. Its appearance is well illustrated in a number of the plates included here: Pl. 11a (#litres_trial_promo) Pl. 12 (#litres_trial_promo) Pl. 25 (#litres_trial_promo) and its loose surface indicates the constant struggle between the stabilising effect of vegetation and the instability due to wind exposure and the action of frost, snow and gravity. In the plates given here, the striking instability of very slight slopes at high levels is clearly shown. In the examples pictured in Pl. 11a (#litres_trial_promo) and Pl. 25 (#litres_trial_promo) the slopes have an inclination of only about 10° to 15°, although the surfaces show little tendency to be fixed by vegetation. A slope of 30° at lower altitudes would quickly become completely covered by vegetation and hence more thoroughly stabilised.

The instability of the surfaces at high altitudes is not confined to those that are predominantly or wholly stony. It is equally evident on many of the more rounded mountains (“moels”) and on those on which the friable nature of the underlying rock has permitted some soil formation. Here solifluction effects may become extremely marked. When soil highly charged with water first freezes and then melts, the expansion accompanying freezing makes the soil very unstable when it thaws, so that downward movement on even the gentlest of slopes becomes possible, the semi-fluid surface slipping easily over the frozen sub-soil. On steeper slopes, large volumes of muddy detritus may be stripped off the flank of a mountain through this agency, and at high levels soil-covered slopes, however slight, almost invariably show signs of movement produced in this way (see Pl. XIb). The most frequent signs are different forms of terracing, and these occur on quite gentle slopes and where vegetation is present. The swollen soil behaves almost as a series of fluid drops, each partly restrained by the turf, which prevents complete movement, bounding the whole on the lower side in the form of a step, the earth being exposed on the upper flatter part.

Of the reality and importance of these influences, no one who has frequented mountain summits in spring can have any doubt. The mountain soils at that time are “puffed up,” as it were, so that the foot sinks deeply into them. The frequent freezing and thawing has the effect of mixing the soil surface, and, in particular, it causes frost-heaving by which the stones present are extruded, so that the surface is commonly more stony than the material beneath.

The processes seen at work on the higher mountain summits bear a considerable resemblance to those observed in arctic regions. On flat or nearly flat surfaces in the Arctic, solifluction effects are associated with the production of curious “stone polygons” in which a central area of mud, often associated with smaller rock detritus, is surrounded by a polygonal boundary of larger stones. Possibly because of the prevalent slopes, polygons of this sort are not very common on British mountains, although they have been recorded by Professor J. W. Gregory from Merrick in the Southern Uplands and by Dr. J. B. Simpson from Ben Iadain in Morven. An interesting area may be seen at about 3,100 ft. on the broad saddle connecting Foel Grach with Carnedd Llewellyn. This shows that the polygons are found only on a flat surface, giving way to “stone stripes” as soon as the






FIG. 9.—Distribution of materials below “stone stripes.” (Diagrammatic.)

surface acquires an appreciable slope. Stone stripes, or the somewhat similar “striped screes” which appear in coarser and more sloping material, were first described in this country by Professor S. E. Hollingworth from examples in the Lake District, where, once one learns to look for them, they are not uncommon. The larger stones collect in rows parallel to the slope as is shown in Fig. 9. The stone stripes, like polygons, overlie soil, and presumably the stones have been extruded from the soil by the movements due to freezing and thawing. Apparently both polygons and stripes occur where frozen layers of soil persist below the thawed surface. The British mountain polygons and stone stripes are often quite small. Those shown in Pl. XI (#litres_trial_promo), were only about a foot apart, though where the movements are on a larger scale they may be three or four times this size. There are especially striking ones on the eastern face of Yr Elen in Snowdonia which can easily be seen from a distance of over a mile.

Many of these solifluction areas illustrate the general feature that the unstable areas on high mountains are often as characteristic of gentle slopes as of steep slopes. Thus the average angle at which an equivalent degree of stability is reached seems to be much less at 2,500 ft. and upwards than at, say, 1,000 ft. No doubt the slower growth of vegetation at higher altitudes also contributes to this condition. Nevertheless, if it is invariably the case, the difference must have a considerable influence on the shape of a mountain. Wherever the rock structure allows comparable rates of weathering, we might perhaps expect to get a shape of the type illustrated in Fig. 4, rather than the simple cone. The preliminary steepening of the lower slopes must, of course, be due to more remote causes.

There are probably other processes by which rock and soil movement may be brought about at high levels, though they do not seem to have been much studied in this country. Some are undoubtedly associated with places where snow lies long. Where such a slope persists below a region of surface instability, rock-waste may move rapidly downward across the snow surface, collecting in a band at its base. Further, long-persistent snow-banks almost always terminate below in erosion channels, which, at the higher levels, may give permanent drainage channels cutting back towards the mountain crest. The interest of these features is not only biological (see here (#litres_trial_promo)), but it lies also in their possible bearing on the origin of the high-level corries (or cwms or cirques) so often found in the larger British mountains. In the extreme form these are rock basins and undoubtedly relics of the small high-level glaciers and nevé which must have lingered on for long after the main ice-sheets had passed away. Corries seem to be most frequent on the east of a main summit or ridge, and it may be that in the first place their position was the result of a semi-permanent snow-bank which started an erosion system. In the later stages it has been supposed that the upper nevé exerts a plucking action on the frost-shattered mountain face, through the periodic filling and downward contraction of the bergschrund, if one may use this term in such a case. In this way continuous over-steepening of the head of the erosion system may have resulted in the formation of the crags encircling the corrie. It seems probable that corrie-formation was most vigorous during and just after the Ice Age, but as it usually lies above the other main erosion effects of the ice-sheets it may be appropriate to regard it as an extreme effect of persistent snow-lie.

In attempting to summarise what has been obtained from this survey, it becomes clear that physical instability is the most noticeable feature of upland surfaces, and it is equally evidently a chief characteristic of the high-level or montane region—although it also accompanies any steep slope as well as the borders of active erosion systems such as streams. Physically stable areas in the uplands differ little from lowland areas, except in other features such as those of climatic origin.

We also see that British mountains are often likely to show an upper zone of comparatively gentle slopes, representing the ancient land forms, moulded long ago, but often kept alive or unstable through the agencies we call nivation. The lower slopes have often been over-steepened in comparatively recent times as a result of glaciation or of the extensive erosion which must have been associated with the melting of the ice. This common plan, if we may call it so, results in the appearance of numerous rather round-topped mountains, although it is modified in innumerable ways as a result of the varieties of rock which make up the mountain blocks and of the different sorts of bedding planes which may be found in different areas.

There is still another way of looking at these matters. The present cycle of erosion as it affects the upland surfaces may be considered to have started at the end of the Ice Age. The upland surface at that time, except where covered by drifts or morainic materials, must have been very different from what it is to-day. It must have been mostly exposed rock which, presumably under the sub-Arctic post-glacial conditions, quickly developed frost-shattering and the characteristic erosion forms found to-day in the Arctic and at high altitudes. To-day much of the corresponding surface is soil- or peat-covered, and only the montane or unstable areas preserve what must have been a widespread condition in the immediate post-glacial period. It will be seen, therefore, if this argument is correct, that the study of the montane areas is likely to be of especial interest. We shall expect to find that the biological character of the unstable areas is widely different from that found elsewhere and perhaps in some respects reminiscent of a condition that was more widespread in post-glacial times.





CHAPTER 3 (#u86e587f5-cda5-55d2-886b-26760e4e243b)


CLIMATE








THE differences between upland habitats and those of the lowlands are only partly structural. Partly they are climatic and this aspect must now be considered. British mountains are only of moderate size but they lie near the sea and across the path of the strong Atlantic breezes from the west. For this reason, wind and cloud and rain play a large part in the weather conditions and they combine to give a characteristic “atmosphere” to British mountain scenery, something of which is conveyed in the photograph of Glen Einich in Pl. VIII (#litres_trial_promo). Equally familiar to inhabitants and noticed by many visitors is the building up of evening cloud after sunset (see Pl. XXIX (#litres_trial_promo)), while even in the finest weather the day is likely to break beneath a curtain of morning mist, well shown in the charming photograph of Llyn Padarn, (see Pl. 10 (#litres_trial_promo)). The visual impressions we thus carry with us can readily be confirmed from the precise data collected by meteorologists, and to them we may now turn.

We are fortunate in having detailed records which enable us to assess these effects over long periods and thus to present them as the main features of mountain climate in Britain. They were made between 1884 and 1903, when an observatory was maintained near the top of Ben Nevis (4,406 ft.), and though they thus give the extreme climatic limits for British mountains, they enable other more scattered observations to be checked and utilised.

In the first place, the records confirm the impression that strong winds are frequent. During thirteen years, an average of 261 gales a year with wind velocities exceeding 50 miles an hour was recorded at the summit of the mountain. This large number should be compared with the conditions at sea-level, when, even on the exposed western seaboard, few places average annually more than forty winds of such a velocity. The comparison between montane and lowland conditions may, however, be made in another form. A more recent estimate of wind-speeds has been made on Crossfell (2,930 ft.), a much lower summit in the Northern Pennines. There, it was estimated that the average wind velocity was at least twice that prevailing in the adjacent lowlands, a result comparable to similar estimates on Ben Nevis.

The Ben Nevis records also serve to illustrate the cloudiness of the mountain sky, for during the years of observation the summit was clear of mist and cloud for less than 30 per cent of the time and, as the table shows, had correspondingly low figures for exposure to sunshine (Table 2). These are, however, only different aspects of a more fundamental feature, the great humidity of the atmosphere. The average relative humidity of the air on Ben Nevis was 94 per cent of saturation with water vapour, showing little variation throughout the year, except in June, when it fell temporarily to 90 per cent, still an exceptionally high average figure.

As might be expected, this high atmospheric humidity was associated with high rainfall. Over a long period this averages 161 in annually at the summit, and it was rather higher during the thirteen years of comparative observations given in Table 2. The maximum recorded was 242 in. in 1909, and as much falls on Ben Nevis during the three “dry” months, April, May and June, as would represent the whole annual rainfall in Eastern England. High rainfall is, of course, a general feature of British mountains. Thus there is the well-known example of the Seathwaite District in Cumberland where Stye Head Tarn, east of Great Gable (2,900 ft.) has an annual average of 153 inches with a recorded maximum of 250 inches in 1928. The computed average for Glas Llyn (2,500 ft.), 500 yards north-east of the summit of the Snowdon ridge, is 198 inches. The Snowdon summit, Y-Wyddfa, in fact, competes with the head of Glen Garry (in Western Inverness), east of Sgurr na Ciche (3,140 ft.), for the distinction of being the wettest place in the British Isles. Both are considered to have an average annual rainfall of some 200 inches. Ben Nevis or Scafell and its Pike, have more of the character of isolated peaks, so that the prevalent winds can slip around them and less rain results.

The last feature of the Ben Nevis records to which attention must be directed is the range of temperatures, also given in Table 2, where they are compared with those at Fort William (at the base of the mountain).

In this table, the figures given at the foot of the columns for the year are averages in the case of temperature, and annual totals for hours of sunshine and rainfall. As there are many summits between 2,000 and 2,900 ft. to the south and west of Fort William, the rainfall there is already much higher than it would be on the outermost seacoast, and sunshine records are accordingly lower, so that the contrast between the lowland and montane conditions is much diminished.

Table 2 METEOROLOGICAL DATA OVER THE SAME 13-YEAR PERIOD






The temperature figures given in the table are for mean monthly temperatures and they bring out very clearly the striking difference in temperature conditions which higher altitude entails. At the summit, the mean monthly temperatures are at or well below the freezing point of water for eight months in the year. Even during the four “summer” months, June to September, the mean monthly temperatures barely rise above those experienced during winter at the foot of the mountain. The temperature conditions are therefore severe.

It may justifiably be urged that this represents the extreme case among British mountains and that we need a more general method of representing the usual effects of temperature. Roughly speaking, an increase of altitude of 300 ft. entails a fall in the mean temperatures of about 1° F. Assuming now that 2,000 ft. represents an approximate lower limit to the mountain zone in Britain, we can obtain representative temperatures at this altitude by taking the average of the Ben Nevis and the Fort William temperatures and adding 0·6° F. to reduce the values approximately to those at 2,000 ft. The results are included in Fig. 10.

It is interesting to note, however, that essentially similar results can be obtained for different parts of the British uplands using the varying records of temperatures made at various altitudes and calculating from them the probable values at 2,000 ft. The following table summarises the mean temperatures so obtained for January and July:

Table 3






The Dun Fell and Moor House stations are two set up in the Northern Pennines by Prof. Gordon Manley, for which the data are less complete, though it will be seen that they suggest that the temperature conditions are essentially similar to those at Braemar, which represents the Eastern Scottish Highlands. The conditions at 2,000 ft. are generally similar therefore, with lower summer temperatures in the west. They may perhaps be regarded as sub-Arctic, resembling those just above sea-level in South Iceland.

The graphs in Figure 10 thus serve to illustrate what are for practical purposes the upper and lower limits of temperature for the British mountain climate. In effect, the increases in altitude produce little relative change in the levels of summer and winter temperatures but they sink, as it were, the whole temperature curve, in relation to any temperature level which may be chosen. Such a level, for example, is that represented in the graph by the horizontal line at 42° F. This level is given, because it is a temperature level which has been used






FIG. 10.—Mean monthly temperature in °F. at Fort William and at the summit of Ben Nevis (4,406 ft.)—continuous lines. The broken line gives calculated figures at 2,000 feet. The circles are summer temperatures in West Greenland and the crosses are data for Vermont (U.S.A.).

by meteorologists to represent the mean temperature above which the normal crop-plants of cool temperate climates start to grow. While the choice of such a level is somewhat arbitrary and does not by any means deal satisfactorily with the physiological problems involved, it is convenient to use this convention for the purpose of making comparisons. We could thus estimate that for plants of the type named above, the growing season at Fort William would be about eight months, say 243 days, while at 2000 ft. it would be about 142 days, and at the summit of Ben Nevis it would be quite negligible.

One of the difficulties of using such a simple method of treatment is that the higher summer temperatures at low altitudes have also a strong and cumulative effect on the rate of plant growth, as indeed do other features of the temperature cycle, such as freedom from frost. Thus lower summer temperatures markedly reduce the intensity of growth and hence the total annual amount is also very greatly affected. The strong westerly winds on British mountains tend to regulate the temperature and in particular they help to maintain lower summer temperatures than obtain on continental mountains like the Alps. Prof. Gordon Manley has drawn attention to another difference associated with the temperature curves. In spite of their moderate size, British mountains become treeless at comparatively low levels, usually below 2,000 ft., and in the same way the zone up to which useful cultivation can extend is comparatively low, often less than 1000 ft. While this is partly due to the operation of other climatic factors, it is also associated with the nature of the annual temperature cycle. If we were to go to some place such as New England, where the mean annual temperature in the lowlands is of the same order as that in Northern Britain, about 46° to 47° F., it would be found that on the mountains, e.g. on Mount Washington, the treeless zone would not be reached below altitudes of some 5000 ft. Although in Switzerland the mean annual temperatures are more widely different from our own, a similarly high timber-line is to be found in the Alps. Prof. Manley points out that this feature can be associated with the temperature conditions, for the average July temperature on Dun Fell (2,735 ft.) in Northern England is almost the same, about 48° F., as that on Mount Washington in New England at 6,284 ft.

A biological explanation of this difference is seen in the form of the temperature curves, and to illustrate the fact an additional temperature curve is given in Fig. 10. This is a typical curve for the lowlands of New England (Vermont) taken from Prof. Manley’s paper. The effect of adjusting this for changes in altitude would be to lower it to an appropriate extent. To give the equivalent curve for a height of 4406 ft., that of Ben Nevis, would require a reduction throughout of 14·7° F. Even if this were done, a large part of the annual temperature cycle would remain above 42° F. There would be a growing season at this altitude of at least 60 days and a mean July temperature of 53·8° F. Thus a considerable amount of plant growth, even from crop-plants, would be possible under New England conditions, while none could be expected with the temperature cycles obtaining in the Western Highlands of Scotland.

This method of considering the matter emphasises the importance of the low summer temperatures in British mountains as an obstacle to plant growth and as a feature which distinguishes them from localities of comparable altitude in continental areas either in North America or in Europe. In fact if we wish to find a climate equivalent in summer to that of our high mountain zone in temperature and in humidity, we must go to places in Arctic regions, preferably to those near the sea and remarkable for their frequent summer fogs, like West Greenland. But even these only rarely attain the constantly high air humidity which was observed on Ben Nevis. This, it is true, probably represents the extreme in Britain, though, judging from the rainfall records, it must be closely paralleled on the other main mountain masses in the west. Farther east and notably in the Cairngorms, where rainfall is less, air humidity is probably more variable and hence more like the Arctic stations of which we have record.

It is rather striking that the low summer temperatures found on British mountains are not associated with the presence of permanent snow, although on the highest peaks drifts may persist throughout the summer on north-facing slopes and in deep gullies. Two such drifts are well known and almost permanent, one on the north face of Ben Nevis and the other in the great corrie of Braeriach (4,246 ft.) in the Cairngorms. The latter, after having been known for some fifty years, finally disappeared for a time in the summer of 1935.

On Ben Nevis the top is usually free from accumulated snow for about 75 days in the year, though some snow may fall on about one day in ten, even in July and August. Thus, though even the highest summits are below the permanent snow-line, they are evidently very near to it. In these circumstances it might be expected that the extent and duration of “snow-lie” in early summer would have a good deal of influence on the distribution of living organisms in the highest montane zone. No detailed study of this matter has, however, yet been made in Britain.

Just as the temperature conditions differ very greatly in Britain and in the Alps, so there is also a considerable difference in other conditions. Speaking generally, British uplands lie wholly within the range of altitudes in which rainfall rises as the height increases. At higher altitudes, however (above about 5,000 ft. in these latitudes), rainfall would diminish with further increases in elevation, and this is the condition obtaining in the Alps and Pyrenees. Further, the lower layers of air are denser as well as more humid. Hence they absorb light strongly, and so higher altitudes receive much larger proportions of the sun’s energy, particularly of the ultra-violet and blue rays.

Thus Alpine conditions imply not only lower precipitation of rain or snow, but also a clearer atmosphere and intense insolation. The average summer temperatures and the illumination are much higher in the Alps, but they are accompanied by the possibility of strong radiation at night and by the certainty of great diurnal and seasonal variations of temperature, in great contrast to the small variations of temperature observed on British mountains. It is evidently better to distinguish upland climatic conditions in Britain as montane rather than alpine, and, as we have already noted, there is a great similarity between these montane conditions in summer and the corresponding features of Arctic coastal regions.




RAINFALL


The influence of the high humidity that is characteristic of British hills is not easily assessed. Atmospheric humidity undoubtedly has a considerable direct effect on plant and animal life, so that most biologists would be able to point to facts of distribution, such as the greater abundance of mosses and lichens in the western hills, which can reasonably be attributed to greater air humidity. But climatic humidity expresses itself not only through its direct effects on the distribution of living organisms but indirectly by affecting the character of the soil, and in the British Isles these indirect effects are extremely important. The only climatic data available for examining them on a sufficiently extensive scale are the rainfall data, and these we must now consider to see how far it is possible to use them in defining climatic limits. In doing this it will be necessary to adopt the following rather rough method of analysing climatic effects.

In the southern part of the Pennines, and probably generally among their eastern foothills, the average annual loss of water by evaporation is equivalent to a rainfall of about 18 in. This figure has been obtained partly as the estimate of the average amount of water lost by evaporation from a 6–ft. Standard tank, and the figures given in Table 4 are actually monthly estimates of the average losses so obtained (as inches of rain). But a similar annual figure (about 18 in.) can be obtained by comparing the rainfall over a given river basin with the “run-off” down the river, and access to much unpublished data has shown that this figure is fairly representative for the eastern Pennines. The difference between rainfall and run-off (assuming no loss into the ground or taking an average over many years) gives the net amount lost by evaporation, and we shall assume it to be distributed seasonally as in the figures given. It may be noted in passing that the problem of estimating evaporation losses may be considerably more complex than this. Empirical formulae have been worked out for estimating these losses in which it is usually assumed that they increase with increasing rainfall as well as with rising temperature.

Table 4 gives, in addition to the monthly figures for evaporation, the average monthly rainfall, also in inches, for two adjacent stations. One of them, Doncaster, lying in the Plain of York and at an altitude of 25 ft., represents a typical lowland station in Eastern England, with an average rainfall of about 25 in. per annum. The other, Woodhead, lies among the high Pennines and is surrounded by “cotton-grass” moors. It therefore represents fairly well the climate of these high moorlands, with an annual average rainfall of about 50 in. The actual figure on the hills is probably more, rather than less, than this, say 55 in.

Table 4 MONTHLY EVAPORATION AND RAINFALL IN INCHES AT DONCASTER AND WOODHEAD






The figures show very plainly that there is no month in the year when the average rainfall at Woodhead does not exceed the evaporation. In contrast, at Doncaster, there are five months when evaporation approximately equals or exceeds rainfall (the rainfall figures in Table 4 are italicised when this is the case). Consider the implication of these facts, and particularly their effects on soil conditions. During the summer, at a station like Doncaster, the soil gradually dries out. This means that the water in the soil interspaces is replaced by air. The drains cease to run until the autumn, when rainfall once more exceeds evaporation and the water-level begins to rise in the soil.

At a station like Woodhead, on the other hand, the same filling of the soil interspaces will take place in winter, but the soils will have no opportunity of recovering and of drying out in summer, for any evaporation will be balanced by the higher rainfall. It follows, therefore, that as a whole, soils will usually be waterlogged in a rainfall of the Woodhead type and only those on considerable slopes will have a chance of becoming drained and well aerated. We may thus recognise that in a rainfall of this type and magnitude there will be a strong tendency towards bog-formation, and it may perhaps be useful to note that in Britain a rainfall of 50 to 55 in. (that is, about three times the evaporation figures) will apparently suffice to give conditions favourable to bog-formation. This is a useful measure, even if a rough one, of the effective humidity of an upland climate.

The influence of high rainfall is exerted in another manner also. When rain falls on soil and percolates through it, the water naturally carries away in solution and into the drainage system any soluble mineral salts present in the soil. These will include most of the substances valuable as plant food as well as the lime which prevents a soil from becoming sour. The process is called leaching, and the rate of leaching will obviously depend very largely on the rainfall. When this only just equals the evaporation losses there will be little or no leaching, but the higher the rainfall becomes in comparison with evaporation, the more rapid leaching will be. Very roughly, then, we shall expect little or no leaching when the rainfall is about 18 in. per annum, but where the annual rainfall is 54 in. we may expect leaching to proceed at about twice the rate expected under a rainfall of 36 in. It will be realised that these rough comparisons as to leaching apply only to porous soils through which water can freely percolate and there is obviously no need to stress the numerical comparison, although it serves to emphasise the high rate of leaching found in upland areas, where a rainfall exceeding 54 in. per annum is common.

The analysis carried out in the preceding paragraphs gives us one method of obtaining a significant boundary of humidity which must have pronounced biological effects. It is perhaps worth noting that a similar figure, an annual rainfall of about 55 in., has been obtained by noting the rainfall at upland sites where reclamation of moorland has proved just possible or has failed. If allowance is made for the nature and porosity of the underlying rock, this is roughly the altitude at which habitation ceases, and in the northern Pennines and eastern Cumberland it is stated to lie very near to the point at which rainfall exceeds 55 in. Of course, this is an extremely indirect method of approaching such a problem, for the result must be greatly affected by the nature of the prevailing occupations in the district examined; nevertheless in this particular instance the relation is clearly one which operates through soil effects, so that it agrees with the conclusion already reached in suggesting that the rainfall indicated is one of distinct biological significance.

The examples already quoted indicate that high rainfall and high altitudes are associated in a general manner, much depending on slopes and topography. No hard-and-fast rule can be given as to the increase of rainfall in relation to altitude, but it is useful to note what is commonly observed in different parts of upland Britain. Along the western margins an annual rainfall of 35 in. is generally found near sea-level, while one of 55 in. would occur at 500 ft. or even less. Where the slopes rise fairly uniformly, as on the west of the Bowland Forest area, the rainfall rises steadily as the height increases, as shown in Fig. 11. The curve given in the figure is contrasted with similar data for the eastern Pennines, showing the much lower rainfall at corresponding heights in the east. The gradient of increase in rainfall






FIG. 11.—Rainfall and altitude on a western slope, B (Bowland Forest), and on the corresponding eastern slope of the Pennines, E.






FIG. 12.—Altitude in Great Britain. Altitudes over 800 feet shown in black.

with altitude rises much more steeply elsewhere, however. Thus, for example, an average rainfall of 150 in. per annum may be assumed at 2,800 to 3,000 ft. in the Central Lake District, in Western Wales and in parts of Western Scotland. In contrast, the rapid decline in the rainfall on the eastern slopes of mountainous Britain is equally striking, for there a rainfall of 55 in. would not be found much below 2,500 ft., and indeed so high a figure is often not reached. A rainfall of 35 in. is not often found below about 700 ft. There is thus a marked difference between the westerly and easterly aspects of British uplands, a point worthy of emphasis because the change-over in the effective climatic conditions often takes place very rapidly in passing in an easterly direction from a watershed.

Moreover, there are indeed large areas in the eastern uplands where a maximum rainfall of between 45 and 50 in. is reached at about 1,500 ft. and no greater rainfall is observed at higher levels. For practical purposes, then, we may say that the western uplands above 500 ft. lie almost wholly above the rainfall limits of the bog-forming climate, while a large proportion of the eastern uplands is below these limits.

The general truth of this statement can be illustrated by a comparison of the maps in Figs. 12 and 14, which show that the zone of high rainfall by no means corresponds with any particular altitude. Further, if the map (Fig. 13) showing moorland and waste lands be compared with that of rainfall, it will be found that a considerable part of the eastern moorlands lies outside the zone possessing a “bog” climate. The distinction is particularly clear in the Scottish Highlands. The importance of this type of relation has hardly received the emphasis it deserves, perhaps because the climatic index is not one it is easy to employ in the field. Indeed, average annual rainfall alone cannot be a reliable guide to the distribution of this type of climate, for the essential feature is the normal absence of soil-drying in summer, and this must depend on evaporation rate and hence on other factors such as mean temperatures, cloudiness and air humidity as well as on local topography. But the field ecologist learns to recognise the certain signs of the existence of local variations in rainfall, of which the most valuable is usually the local distribution of cloud. Some areas are persistently under cloud, while others not far away may be as frequently cloud-free. Generally, rain-showers show a similar distribution, and these are both things which can be noted even in a brief visit.






Left, FIG. 13a.—Moorlands in the British Isles.






Right, FIG. 13b.—Distribution of Rainfall. Areas with over 50 inches of rainfall per annum shown in black.






Left, FIG. 14a.—Distribution of Palaeozoic rocks in Great Britain.






Right, FIG. 14b.—Distribution of sheep in the British Isles.

Very good examples of considerable local variations in climate which can thus be detected are to be found in the eastern Pennines—particularly in the Teesdale-Baldersdale-Stainmoor district just south of Mickle Fell. Stainmoor itself is a well-known bog area (see here (#litres_trial_promo)) which has a rainfall near to 55 in.; but this rainfall decreases very rapidly towards Lune Forest and Baldersdale on the north and east respectively, where other very different types of moorland vegetation hold sway. Very striking is the frequency of cloud-cover or showers over the Stainmoor bogs in contrast to the clearer skies of the drier and more easterly areas.

On a far grander scale, similar contrasts may very often be seen in the central Scottish Highlands. The eastern mountains, and perhaps especially the Cairngorms, may stand out cloudless or with small fair-weather clouds when the big western Bens are sunk in mist or dwarfed by rain-clouds. The contrast seems to become noticeable about a line drawn north and south through Loch Ericht or Dalwhinnie.




ALTITUDE AND ORGANISM


The influence of climate on upland organisms has so far only been considered in the most general way. We have observed that there is a correlation in distribution between certain types of soil condition and certain types of climate. Thus we assume that the bogs of the Western Highlands are associated with the wet climate. In a similar manner we may observe that there are some plants and animals found only at high levels, the special montane species, and we assume that they are there because they are in some way more suited to the severe climate existing at high altitudes. We have little evidence as to how the climatic factors are effective and it will be useful accordingly to discuss this matter a little more fully.

The distribution of plants is obviously a very important factor in animal distribution, not only for grazing mammals but also for the insects which live on and in plants. In such cases the influence of altitude may be indirect, and there are, as we shall see, instances of the distribution of the animal following that of the plant. If we are to consider plants, the influence of the soil needs to be taken into account, and we have already seen reason to believe that the wet climate may be effective through its influence on soil conditions. But climatic humidity varies greatly in different parts of the country—being high in the west and lower in the east. If this were the effective montane influence then we should expect to find a richer montane fauna and flora in the west. It is well known that on the whole there are on the eastern mountains more of the species restricted to high mountain life; so that in one aspect at least humidity cannot determine the altitudinal zonation. However, the fauna and flora of upland country as a whole is very different from that of the lowlands, in proportionate representation if not always in the individual species, and a large part of this upland fauna and flora is associated with the ill-drained and wet soils. What humidity does do is to give great areas dominated by a limited fauna and flora of this type, which is upland rather than montane and which is evidently related to the soil conditions induced by humidity.

The more common view and one which has been referred to and used already in this chapter, is that temperature largely controls the altitudinal zonation, and we may look at this problem as something which would repay attention from naturalists and as a subject which requires little in the way of special equipment.

The principal biological effect of temperature is that it greatly affects the rate of biological processes. Thus a lowering of temperature such as would be experienced at a higher level would retard growth and development so that there would be less likelihood of a given developmental process being completed within the shorter period available in a montane summer. Some upland organisms do in fact appear to take longer over a given process of development. A well-known case is that of a moth, the northern eggar (Lasiocampa callunae), which spends two years in the larval stage instead of the one characteristic of the original woodland race, the oak eggar (L. quercus). It is unlikely, however, that the difference is due to the lower temperature of the upland habitat. To double the period of development, or to halve the rate of development, would require a reduction of temperature of about 7·5° C. or 14° F., equivalent to an increase in altitude of about 4,500 ft.! The lengthened larval period may be just too long to fit into one growing season, but it seems more likely that the change in the length of the life cycle is either genetical or mainly due to nutritional differences imposed by the moorland habitat.

There are, of course, other ways in which lower temperatures may affect distribution. Where two organisms are dependent on one another for success, but possess life-cycles of different duration, an alteration in temperature may put the two life-cycles “out of step” with one another, as it were. A case which might involve something of this nature is one in which an insect mined or fed on a plant organ at some particular stage in development, as in an example discussed later in this chapter.

Lastly, of course, alterations in temperature may produce qualitative effects on plant and animal metabolism (in the widest sense), and it is perhaps in this direction that we have to seek an explanation of the tendency of certain insects to be represented by short-winged races at higher altitudes (see here (#litres_trial_promo)). In plants, the effects of temperatures approaching the freezing point are often to induce the conversion of insoluble food-reserves like starch to soluble sugars. To this type of change has been ascribed the immunity of some evergreen plants from frost injury, which is attributed to the difficulty of freezing cells containing a high sugar-concentration. Undoubtedly the presence of these sugar solutions does confer on plant tissues a certain immunity from frost injury and the effect may easily help to account for the over-wintering of arctic and montane plants, just as it would undoubtedly be advantageous in helping to promote the rapid growth and early flowering observed in arctic climates. Dr. Scott Russell has verified the existence of high sugar-concentrations in spring in arctic plants collected on Jan Mayen Island and in the Karakorum mountains.

The only clear effect of this general type I know of in animal tissues is the very characteristic production of orange-coloured and fat-soluble pigments in certain aquatic copepods during the winter months and commonly also in cold, high-level tarns.

When one goes on to consider the ecological effects of these factors in nature, it is generally difficult to dissociate the effects of temperature and humidity. Thus the presence on mountain-tops of certain spiders usually found in damp cellars might plausibly be attributed either to high humidity or low temperature. A clearer example of the influence of temperature on animal distribution is that of the alpine flatworm, Planaria alpina, for this lives in water and is not therefore subject to the great variations in humidity which may effect mountain-top habitats. Planaria alpina is a small creature about a quarter of an inch long, resembling a somewhat flattened grey slug. It is a carrion feeder, living under stones in the margins of streams and in mountain runnels. In this country, these little water-courses usually contain a second, much darker species of flatworm, Polycelis nigra. The two species are always distributed in the same way, P. alpina at the higher levels, certainly at least to 2,000 ft., and P. nigra in the lower reaches of the water-course. This distribution is mainly a matter of temperature. Numerous observations in Britain and on the Continent have shown that P. alpina is never found in nature where the temperature exceeds 14° C., while P. nigra may be found where the water reaches as much as 20° G. Further, prolonged observations on the animals under controlled conditions by Mr. R. S. A. Beauchamp have shown that P. alpina cannot long survive temperatures exceeding 12° C. Thus in nature it occupies the high-level runnels and cold springs, occurring at high levels in mountain districts. There are reasons for believing that other animals confined to high-level streams and soils owe their distribution to similar effects, particularly perhaps certain insect larvae.

It is less easy to point to instances in which similar effects are produced on plant distribution, though they doubtless exist. Plants are not able to change their positions readily, and most of the high-level species are perennials, which means that the effects of the environment if not immediately lethal are likely to be the integration of the prolonged effects of the given habitat factor or factors. In some cases, perhaps especially in grasses, a given species is represented in the montane zone by separate races, often it may be not very distinct in form, but possessing some ability to live under the especial montane conditions. The common upland grass, the sheep’s fescue (Festuca ovina) is thus represented in the montane zone by an allied highland species (F. vivipara) which has the ability to produce young plants in place of the floral structures. This feature is much accelerated by, if not wholly dependent on, the existence of humid conditions, and this is probably the reason why the viviparous form of this plant is found at low altitudes along the seaward margins of Western Britain.

It seems that in order to get some idea of how climatic factors affected upland plants one would have to consider the influence of whole seasons upon the growth of a chosen plant. After making observations upon a number of plants it became clear that there were good practical reasons for using a relatively common plant like the moor-rush (Juncus squarrosus) as material for estimating these effects of altitude. This plant has certain practical advantages for work of this type. It occurs at almost every altitude in Britain and it prefers the wet and base-deficient peaty soils which predominate in the uplands.

The plant consists of a rosette of rather fibrous leaves just above ground level with a long flower-stalk bearing an upper group of brownish flowers or fruits Pl. IX (#litres_trial_promo). The latter contain numerous small seeds. At ground level there is a woody stem having numerous roots. The flower-stalks are numerous, they are tough and so can be collected rapidly and transported for subsequent measurement. The fruits, small brown capsules about a sixth of an inch long, are also tough and numerous enough to give suitable numerical measurements. The inflorescence is laid down as part of a bud in summer. It develops the following year, and its length may be taken as a partial expression of the conditions favourable to growth in the preceding summer and






FIG. 15.—Effect of altitude on moor-rush, Juncus squarrosus: L, Length of flower-stalk; N, Number of flowers produced; R, Number of mature capsules.

also in the summer in which it has developed. These conditions affect reproduction in addition by controlling the number of flowers and, later, of fruits and seeds. The only method by which the plant is distributed is by the numerous small seeds.

If one studies the performance of such a common moorland plant at different altitudes, it is apparent that the amount of growth and the production of flowers, or better still, of fruits and seeds, both diminish as the altitude increases (see Fig. 15). But fruit production is affected far more than growth in length, so that a point is reached, generally about an altitude of 2,500 ft. to 2,700 ft., above which fertile fruits are not usually produced, although the plants may form inflorescences of considerable size and in other ways be capable of making satisfactory vegetative growth.

This effect is evidently due mainly to the retardation of the development of the flowers and fruits. Thus in the Lake District in 1942, flowering was completed during June at 700 ft., but it had not begun at the end of July at 2,000 ft., and, at 2,500 ft. to 3,000 ft., it was not complete by the end of August. Thus at these highest levels there was little or no chance of most of the fruits becoming mature and they did not in fact do so. Again, in late September, 1943, only one mature capsule per 20 plants was found on the summit of Ingleborough (2,373 ft.). These and similar facts thus suggest that viable seeds are not usually formed above about 2,500 ft. to 2,700 ft., although large and healthy plants can be found up to at least a thousand feet higher. Until 1947, viable seeds had not been collected from above 2,700 ft., but the exceptionally long and warm summer of that year led to very abundant seed production—so much so that viable seeds were obtained from 3,400 ft., on Ben Wyvis.

In view of the infrequency with which such seeds are formed at high levels, the presence of moor-rush plants at 2,700 ft. and upwards is interesting. They are certainly very long-lived (twenty years or more) and possibly originally due mainly to transported seeds. It is noticeable on some mountains that the plants are not only sporadic but also are often collected in colonies, suggesting a group of individuals centred round a parent plant which has fruited only at rare intervals. The fruits are, perhaps, distributed in the wet wool of sheep, for, as far as is known, no mammals eat the inflorescences although snow-buntings habitually eat the dry fruits in winter and so may help to disperse seeds. The rush is commonest on sheep-infested mountains, and although it occurs to at least 3,700 ft., I have looked for it in vain on the high and grassy Scotch summits where deer habitually graze.

However, it seems certain that the effects of altitude are differential, affecting the seed-production most, flower-production less and vegetative growth least. The analysis of these effects shows that they vary little as between districts receiving great differences in rainfall, and they can thus be attributed mainly to the diminution of mean temperature with increasing altitude. Thus temperature, though it actually operates by controlling the relative rates of development, affects the distribution mechanism.

It is interesting to carry this problem a little further by considering how these things affect a little rush-moth, Coleophora caespititiella (see Pl. 30 (#litres_trial_promo)), that lives in association with the moor-rush and also with the common rush. Its life-history is not very well known, but moths are mature and the eggs are apparently laid in June–July, on or near the flowers of the rush. The larvae then feed on the growing seeds inside the developing fruit. By about the end of August, the infection of a fruit capsule becomes noticeable because of the presence of the larval case, a small cylindrical and white papery object in which the larva may live (see Pl. XI (#litres_trial_promo)). The larvae, possibly usually with the case, leave the rush-heads in late autumn and hide in the surrounding vegetation until the following summer. With certain obvious precautions, the presence or absence of the white larval cases can be used to study in an approximate way the extent to which the population of heath-rush is infected by the moth. The data also give a picture of the altitudinal distribution of the moth. This is much more restricted than is that of the rush on which it lives. In the central Lake District, in 1942, the frequency of the larvae decreased rapidly from a maximum infection of about 40 per cent of the capsules at 700 ft. and no signs of the moth were seen above 1,800 ft., although in that district the moor-rush goes up to 3,000 ft. Now at first it was thought that the larval cases might become more frequent at a higher level later in the year. In fact larval cases were never seen above this level except in the abnormal summer of 1947, when some were found at 2,000 ft. on the south-facing slopes of Saddleback.

It seemed obvious at first that at higher altitudes the lower temperatures would retard the development both of rush-flowers and of the moth growth-cycle, for both last a year. When no infection was found above 1,800 ft. it was thought that the lower average temperatures might so retard the development of the larvae from the egg to the case stages, that the cases were not produced at higher levels even although there was infection. In this case the larvae might fail to over-winter or the whole growth-cycle might take two seasons. However, no evidence of a later infection at higher levels could be found.

A possible alternative explanation was that, as suggested earlier, the whole growth-cycle of the moth might get “out of step” with that of the rush, so that mature moths and “infectable” rush-flowers (i.e. in the young stage when they are infected) might not coincide in time.

This does, in fact, happen, though not quite in the manner expected. It was found, in samples from the higher levels, that only the early maturing fruits were infected by Coleophora. It followed that there was normally no infection above 1,800 ft. because no rush-flowers were normally open in July above that altitude (1944 and 1945). Even in the abnormal summer of 1947, no sign of infection was seen above 2,000 ft. (and this on a south slope) in the Lake District, and in the Eastern Highlands (Ben Wyvis and Rothiemurchus district) none was noted above 1,400 ft. On the whole, then, it seems as though the main population of mature Coleophora individuals comes out at one time, about June–July. It may then infect any rush-flowers which are then open. This severely limits its altitudinal range, for as we have already seen, the high-level flowers are not mature at these early dates. One difficulty about these findings is that there seems no reason why the cycle of development of the moth should not be retarded somewhat at the higher levels just as that of the rush-flowers is. If this were the case, a small number of late-maturing individuals should appear at higher levels. No individuals of this type have been seen, nor has it been possible to find signs of rushes which might have been infected in this manner. It seems to be only possible to explain this apparent absence of the mature moths at higher levels by assuming also a temperature bar to their development such as we have already encountered in the flatworm Planaria alpina.

There are many further observations that could usefully be made on this matter. It appears that Coleophora is generally confined to lower altitudes on the Eastern Highlands as compared with the Lake District, and, at first, it seemed that lower mean temperatures might explain this. However, I have not seen this moth at all in the Western Highlands or Islands—perhaps because I have generally been too early or too late for the moth and too early for the larval cases. Certainly the creature seems to be much less common in this area of high rainfall, a result that could not be easily explained on the grounds of temperature alone.

The brief summary of upland climates and the analysis of their possible effects on animals and plants suggests that temperature has much to do with the zonation of plants and animals we observe on ascending a mountain. It controls the distribution of some organisms because they are not able to live in the higher average temperatures of the lowlands. In other cases, it seems that the low montane temperatures so lengthen the life-cycle that it cannot be completed in the short mountain summer. Perhaps more often low temperature retards some part of the developmental cycle, so that we get short-winged insects (see here (#litres_trial_promo)), or plants unable to produce flowers and fruit. For these reasons, some zonation of organisms is inevitable as altitude rises and temperature falls.

In practice, the most widespread influence of altitude is the change in the character of the prevailing plant communities, with all that it implies in its effect on animal habitats. Most noticeable is the disappearance of woodlands and trees with their varied faunas and ground floras. As this commonly takes place at about 2,000 ft. and as the restricted montane species appear above that level, we may take it as a convenient altitudinal separation of montane and sub-montane zones.

Within the limits thus defined by temperature other factors must play their part. Every naturalist knows that shelter from wind is often vitally important, so that here and there among the mountains there are oases in which the frequency of plant and animal life is altogether different from that found on the exposed and wind-swept faces. Within the limits imposed by temperature, humidity also exerts its restrictions, not only by presenting a range of habitats running from pool or rivulet to desiccated rock, but by influencing the character of the soil. It is to the consideration of these soil conditions that we must now turn.





CHAPTER 4 (#u86e587f5-cda5-55d2-886b-26760e4e243b)


SOILS








THE second important group of factors in upland habitats is the nature of the soil covering—or perhaps more strictly, of the surfaces available for plant growth. Geologically, as we have seen, these surfaces may be classified either as stable or unstable, depending on whether they are still subject to active erosion or not. As habitats for plants there is a more profound difference between these two classes. Most of the unstable surfaces are rocky or are covered by rock fragments in various stages of disintegration, and even their physical properties differ greatly from those of fertile lowland soils. They are, in fact, soils in the making, and it is one characteristic of upland areas that they exhibit in profusion all the varied stages of soil-formation. We see the native rock breaking down under the action of frost and other weathering agents to rock fragments, which become progressively finer as the process is longer continued, ultimately to yield the small mineral particles which form the basal material of most soils. The weathered material may remain in situ, covering the original rock surface, or it may be removed by erosion and redeposited elsewhere by streams and rivers as banks of silt or alluvial plains, by solifluction or rain-wash, or formerly in Britain by widespread glacial movements.

The raw mineral material is, however, comparatively sterile. It is converted into what we call a soil partly by chemical modifications resulting from the presence of water, often charged with carbon dioxide or humic acid, and partly resulting from the gradual accumulation of organic materials derived from plant remains. This latter material is called humus, and is particularly important because it forms a medium upon which can grow various micro-organisms, mainly bacteria, moulds and protozoa. With the accumulation of humus and the gradual colonisation of the material by these organisms comes a final stage, when it is usual to imagine that the original particles of mineral substance have become covered by a jelly-like mass of colloidal material—in part gelatinised minerals but also including humus—on and in which the population of soil micro-organisms lives.

It will be evident from this brief summary that upland soils can usefully be considered as belonging to a developmental series. But it is true of any soil that one of its outstanding characteristics is its capacity for change. Soils are inherently dynamic systems even when they are developed in physically stable situations, and to a far greater extent is this true of mountain soils, most of which are of geologically recent origin, even if not physically unstable.

Five types of environmental factor control the development of a soil mantle. First comes the nature of the rock or other parent material, from which soil is formed by physical and chemical weathering. Climate also exerts a marked effect on the weathering process, affecting both its physical and chemical parts, and, in particular, determining the amount of rain-water percolating through the soil in any season, a process known as leaching, which is responsible for the removal of soluble substances, bases like lime as well as plant nutrients like nitrates. Relief influences the lateral movement of percolating water down a slope, the degree of drainage and the stability, and thus affects the degree of leaching. But none of these effects is instantaneous and so there is a time-factor to be considered. Lastly, there are the obvious biological factors, of which the action of vegetation is most significant. Vegetation derives part of its sustenance from the soil and so incorporates a portion of the soil material which is returned to the soil on the decay of the plant tissues. The fertility of a soil is the result of this cyclic exchange. An efficient type of plant which draws heavily on the soil nutrients keeps them in a form of biological circulation which mitigates the losses due to leaching. Thus there is a natural mechanism for maintaining soil fertility, which, by drawing on the deep layers of the soil, is capable even of increasing the fertility of the surface layers provided the leaching factor is not too intense. Further, in any environment where the climatic factors have remained reasonably stable for a long time, it is possible for the soil-vegetation system to achieve a measure of temporary stability. In upland Britain, however, the soils are generally in dynamic states moving along definite trends of soil development. The trends due to a severe climate are particularly marked, and they operate during the different stages of soil development in the following manner.




SKELETAL AND IMMATURE SOILS


The initial stages of soil development in which rock fragments predominate are what we can only call skeletal soils. They are found principally where the surface is unstable or where further development is retarded by hard rocks or low temperatures. Of these factors, the low temperatures have also distinct qualitative effects, because while they greatly retard chemical modifications, the associated physical disintegrations caused by frost and solifluction are especially vigorous. There may thus be much physical commination of rock fragments with little chemical change. Thus the soils, even if finely divided, are immature in the developmental sense because of the deficiencies in their chemical and biological equipment. Soils of this general type occur on mountain-tops, where they are found under the mountain-top detritus except, perhaps, where it is especially coarse and deep. Generally, however, the detritus seems to be a superficial layer of stones extruded from below during the frost-caused or solifluction movements of the materials. Beneath the stones there is commonly a sandy loam, generally brown in colour and little leached. When vegetation is present this merges at the top into the almost black humus that collects among the surface detritus. The depth of the soil varies with the nature of the rock and the degree of erosion, but it is usually between one and three feet, and then comes disintegrating rock.

Parallel to these summit soils in general quality may be the scree-slopes of finely-divided material which occur lower down a mountain, often approaching stability but still subject to soil-wash and soil-creep, and so often distinguished as creep-soils. These show great variability in detail, but, like the mountain-top detritus, they often show coarse material at the surface and finer below. As they approach stability, they merge into the woodland soils described below, but in the earlier “gravel-slide” stages, a vertical section usually reveals a sequence of more or less alternating sandy or stony layers parallel to the slope (see Fig. 8). All soils of these types are alike in possessing a high base-status because they consist mainly of rock particles as yet not greatly modified by chemical change.

In all upland habitats there are in addition the overall trends caused by continual washing by rain, and as a result every exposed and porous surface will be more or less leached. Wherever leaching has taken place there must be corresponding areas that receive the products of leaching. The water that carries away lime or other bases from the higher upland surfaces must produce elsewhere lime-rich or base-rich habitats. Areas of this latter type may be distinguished as flushed or enriched habitats to distinguish them from the leached or impoverished ones.




FLUSHED SOILS


In general, of course, leaching will preponderate in upland regions and enriched soils will be commoner in lowland regions. Nevertheless enriched soils are always to be found occupying characteristic localities in mountain areas. Thus there is a flushed area around every springhead and around every rivulet. However, the water need not emerge as a separate spring but may perfuse the surface soil—a type of flush that can be recognised by a zone of greener vegetation. The various types of “damp flush” may be associated with a soil of almost any physical category. Enrichment by water from a higher level is greatest when such water has penetrated into the rock by means of structural fissures, and permeated the rock strata on its way down; a mere receiving area for surface run-off from acid upland soils is often as severely leached and as acid as the upland soil itself.

Parallel with enrichment by water (“damp flushes”), there is enrichment by presence of freshly-weathered rock particles, and areas of this type might be called “dry flushes.” The lower part of any steep slope is constantly enriched by such particles washed down from above. Screes and gravel-slides, in which the breakdown of new rock by weathering continually yields a supply of bases, could thus be considered among the enriched or flushed habitats. In this category also comes any unstable surface, crag, gullies and the like, where new rock surfaces are being exposed by erosion.

It will be observed that the flushed habitats are determined by a diversity of factors producing enrichment and have thus few physical characteristics in common. They tend to fall technically into four categories:



1 Bare rock or oversteepened slopes with soil particles washed away or present only in narrow fissures. Enrichment by continual weathering of freshly exposed rock surfaces.

2 Block scree in which leaching tends to preponderate over weathering, although the latter nevertheless does continually refurnish some of the bases lost, especially in the case of more rapidly weathering and base-rich rocks.

3 Unstable scree-slopes and solifluction areas with movement and accumulation of weathered soil particles, often below the surface layer of coarse detritus; enrichment both by weathering and particle accumulation.

4 Accumulation areas—nearly always showing fine and deep soils, with enrichment mainly by accumulation from above.


Upwelling of base-rich waters may occur in conjunction with any of the four categories above, although it is rarest in a and commonest in c and d. In both c and d the total “flush” effects normally counteract losses by leaching unless the soils are deriving from base-poor rocks.




LEACHED SOILS


On the whole, the unstable surfaces are areas of enrichment, and as such show distinctive types of vegetation. When they become stable enough to permit a complete vegetation cover, they inevitably tend to become leached—a tendency that is in some slight measure accelerated or retarded according to the nature of the plant cover. Because heavy rainfall is the rule and because plant remains tend to accumulate on the soil surface, it is characteristic of upland soils, once soil-creep and the forces of erosion are sufficiently arrested, that they rapidly develop the stratification or “profile” indicative of the more advanced (or “mature”) stages of soil development.

The extreme form of stratification found is that of the soil type known as a podsol, in which, under a surface layer of peaty humus and of humus-stained soil, there is a grey soil-layer from which almost all of the available bases (especially lime and iron) have been removed by leaching. With them also have gone the finest particles of clay and the humus colloids. These accumulate at a lower level, commonly two or three feet below the surface, as a dark-brown layer of “humus pan,” while immediately below this can usually be seen a red- or orange-brown precipitate of iron compounds (“iron pan”). Still lower is the little altered parent material. Thus these highly leached soils show a characteristic soil profile, with the following layers:

A. a Surface peaty humus and “litter”

b peat-stained inorganic soil

c leached grey ” ”

B. d humus accumulation zone (“pan”)

e iron ” ” (“pan”)

C. f little altered parent material

In continental areas, podsols characterise the cold temperate climates on stable and porous substrata receiving moderate rainfall. They are there associated with the northern evergreen forests of coniferous trees (firs) and with an abundance of small shrubs like the heathers.

In the British uplands, podsols are most characteristic of the eastern regions where the annual rainfall is relatively low (say about 35–40 in.) and the parent material is often sandy or gravelly morainic material. They are often now associated with heather-moor and pine forest, and originally this association may have been still more widespread. In the wetter western areas, although podsolic features in the soils are frequent, good examples of podsols are infrequent. Often this is because of great irregularity or diversity in the composition of the parent material. In particular, the deposits of iron or of peaty colloidal matter which suggest the appearance of a podsol are often due to lateral seepage from higher up the slope and are not necessarily derived from the soil-layers immediately and vertically above them. Varying porosity, in particular, leads to very irregular local accumulations of the humus and iron colloidal layers, which may appear in blotches along the lines of seepage and at a varying distance below the surface. But it is probable that most of what were once free-draining forest soils have now been transformed to bog (see below and Chapter 10) (#litres_trial_promo), and that those which are left are but transitional stages.

Between the extreme podsols and the young or slightly leached soils, there is a range of soil profiles usually classified together as “brown earths” from their ochreous brown colour, and in the lowlands characteristic of forests of deciduous trees, oak, beech and the like. The surface layers have been somewhat leached of bases but retain a brown colour along with a base-status sufficient to give a moderate fertility. This is the characteristic soil-type of lowland Britain. In the uplands, however, this condition can only be long maintained, where the original rock fragments are base-rich, where flushing of some sort maintains the base-supply, or where the vegetation is such as to renew the base-supply in the surface layers. The latter condition would be favoured, for example, by oak woodlands rather than by a covering of birch or of pine, for the lime contents of the leaves of these trees differ considerably: that of oak approaches 3 per cent, while those of birch and of pine are only about 1·5 and 1·0 per cent respectively. Thus by absorbing more lime from the lower layers of soil and returning it to the surface, oaks would maintain the base supply of the surface soil-layers for a longer time and so would tend to retard the effect of leaching.

The upland soils of brown earth type are usually recently stabilised “creep soils,” flushed glacial drifts, or soils derived from base-rich rocks. Almost always they were until recently under woodland, normally of oak though often with much ash. Now they are almost always cleared of trees and are covered by grasslands of various types. The removal of the original tree-cover was often followed by destruction of the surface humus through its oxidation and by the removal of the surface layers by rain-wash. Thus many upland soils of this general type, like the “frydd” soils in Wales, have been considered now to show “truncated profiles,” the top strata having been removed, often by erosion. In some cases, however, it is also probable that the profiles are immature and that the soils owe to this their comparatively high base-status.

It is quite clear that generally in the uplands the process of leaching can only be retarded and not completely stayed. The high summer rainfall in particular ensues that leaching will continue under the most favourable conditions of temperature. In lowland climates, in contrast, there is in summer an excess of evaporation and drying in the surface layers of soil so that base-rich water ascends from below by capillarity. This opportunity for replenishment is lacking even in a well-drained upland soil so that the soils as a whole must tend towards the leached condition.

Thus it seems inevitable that any porous soil, once stabilised, must ultimately develop towards a podsolic stage. In the majority of cases it seems that the process has not ceased at this stage. The downward movement of fine particles of clay and humus which characterises podsol development leads to the formation of impermeable pan-layers which impede drainage. Thus under conditions of high rainfall the upper layers of soil become waterlogged, seasonally if not permanently. This leads to peat accumulation, which in turn accentuates both leaching and poor drainage. Thus in mountainous Britain, podsols have almost always tended to become peat-covered bog soils, such as are described below, and it seems probable that the characteristic podsolic profile becomes modified when this change takes place, and ultimately disappears.

These trends of soil development favourable to bog formation are undoubtedly much accentuated by the topographic relations of the mountain soils. Once bog conditions have been established at any point on a hillside—a process which in most cases clearly must have taken place at an early stage in the physiographic history of the area—there must inevitably be a tendency for bog seepage to extend downward and to produce a general degeneration in the soils below it. The greater the area of peat accumulation in the upper bog zone, the more rapidly would this influence extend downwards. Moreover, because a mountain is a gathering ground with a considerable excess of rainfall over evaporation, it seems likely that the lowest slopes will often also show a marked trend towards the establishment of bog, the degree to which this happens being controlled by the geological structure of the mountain and the amount of mineral bases (lime, potash and so on) the mountain yields to the seeping water. So soon as the mountain-side becomes stable enough to show signs of leaching these trends are strongly in favour of it passing over to bog. Thus the upland soil problems are very complex, affected not only by the immediate characters of the soil but by lateral movements, by adjacent relief, rock and vegetation.




CHEMICAL STAGES IN LEACHING


The chemical stages which can be distinguished during the leaching process can now be outlined. In the first instance, most British soils are principally calcium soils, or, putting it in another way, they have lime as their principal base, and are lime-saturated. Agriculturally and ecologically, soils of this type possess high fertility. As leaching goes on, most of the lime and other bases are removed, being replaced by hydrogen, so that the soil finally becomes acid and sour (e.g. to the taste). For ecological purposes there are, however, two intermediate stages in this process which can usefully be distinguished: a state of partial lime-deficiency and one of higher lime-deficiency. Most British soils also contain a good deal of iron oxide—to which their colour is largely due—and under conditions of good aeration this is removed much more slowly than lime. The following four types of soil can conveniently be recognised as stages in the leaching process:



1 Lime-saturated;

2 Lime-deficient;

3 Base-deficient, iron oxide becoming mobile and relatively more important than lime;

4 Acid, with podsolic profiles in stable soils, often masked by peat accumulation.


Of these b and c represent the stages usually referred to as brown earths. The ecological value of this series lies in the fact that very decided transitions in vegetation occur at a point between b and c (which is also approximately half-way between a and d)—that is, a point of half-saturation with bases.

The technical methods of distinguishing these soil-types depend on the methods of measuring the percentage of base-saturation, which decreases from a to d, or of measuring the increasing acidity. The latter is perhaps a mode of expression more familiar to biologists. Estimates are made of the hydrogen-ion concentration and expressed in the following way. Concentrations, varying, for example, as 1/100, 1/1000 or 1/10,000 gm. per litre of hydrogen ions can be written either 1/10


, 1/10


or 1/10


, or 1 x 10


, 1 x 10


or 1 x 10


g/l, and for convenience these are termed pH 2, 3 or 4 respectively. The notation extends over a range of 1 to 14. In terms of this notation, pure water has a pH value of approximately pH 7, lime-saturated soils have a somewhat similar pH value, of above 6, while natural soils which are about half-saturated with bases have a pH value of approximately 5. The characteristic acid soils in the ecological sense lie below pH 3·8.

These soil types can, however, often be distinguished by their appearance and biological characters. The grey and leached zone in a well-developed podsol is likely to be mainly a hydrogen soil, as is the humus-stained layer on the surface. In many upland soils, the leached but still brown layer of inorganic materials below the surface humus has a characteristic orange-brown colour—not grey as in a proper podsol. This condition is associated with the removal of most of the lime and the mobilisation of iron, at first perhaps dissolved from the soil minerals near the surface by humus compounds, but then reoxidised on the surface of the soil particles in a state which accounts for the characteristic colour. This type of soil is probably very definitely associated with periods of waterlogging, such as are frequent in upland areas, and it normally has a lower base-status than the more typical soils of brown-earth type.

In these and in other ways, therefore, the characteristic appearances of soil profiles give a good deal of information about the base-status of the soils. It is perhaps worth emphasising also at this stage that two factors in particular are especially effective in removing iron and other bases during the final stages of the leaching process.

One of these is the increased acidity and especially the effects of acids derived from plants like oxalic and citric acids, in which iron salts are especially soluble. The second is the establishment or development of waterlogging, which, by eliminating the oxidising effects of air or oxygen, permits the reduction of iron to the ferrous state, in which it is very much more easily soluble, as well as more readily replaced by the process known as base-exchange. In aerated soils, however, waterlogging can only be temporary even though it must be frequent in winter in all upland soils. When it occurs permanently the soil commonly acquires a blue-grey appearance which we associate with the presence of ferrous iron compounds, and this contrasts very noticeably with the reds and browns of the ferric salts in air-containing soils.

Of course, where there are extensive areas of waterlogged soil and especially where peat is abundant, large amounts of ferrous salts may be present in solution in the soil-water. Wherever this becomes exposed to air it becomes oxidised either to metallic iron or to ferric salts and so considerable amounts of ferric substances may be precipitated. The orange-brown or metallic films due to this process are familiar objects round any peaty spring, and, long continued on a large scale, it has in the past been responsible for the production of deposits of bog iron ore. The same process continues in any peaty flush soil which receives drainage from waterlogged surfaces and into which air can, at times, penetrate. To the ecologist, the colours due to iron compounds are of great importance as visual evidence of the existence and progress of leaching or accumulation and also as useful clues to the presence or absence of aerated conditions in the soil. There are, of course, many chemical tests which can be used to confirm and extend these visible signs.




WATERLOGGED SOILS AND PEATS


What has just been said about waterlogged soils serves as a useful preface to the further consideration of this subject. These soils are distinct in being anaerobic or devoid of oxygen, and this is reflected in their “sad” appearance and in the characteristic blue-grey colours of the mineral matter due to ferrous iron salts. Such mineral soils are usually called gley soils, and, in the British uplands, they almost always possess another characteristic. The absence of oxygen prevents the decay of the organic matter derived from the plants growing on the surface. Consequently, upland waterlogged soils are also normally covered by layers of peat, and these are deep wherever waterlogging has been long continued, while they are usually shallower where this condition is of more recent origin. As a result of these accumulations of peat, the mineral soil below may be stained and mottled by peaty material.

The waterlogged peats are of two main types: (i) bog-peats and (ii) flush-peats. The bog-peats are widespread, covering the majority of stable upland soils and characteristic of those of slight slope (see Pl.s 22a (#litres_trial_promo) and 24 (#litres_trial_promo)). They fall into two topographic types: those found on concave lowland forms, valley bottoms or lake basins, which have sometimes been distinguished as basin-peats, and, in contrast, those on long slopes and gentle ridges, for which Dr. H. Godwin coined the name blanket-bog, a term expressive of the way in which the peat covers all stable features of the original surface. Strictly speaking, basin peats are part of the blanket-bog in the uplands and it is only useful to separate them because they have, at times, a somewhat different and longer history as well as differences in present vegetation.

Flush-peats are also topographically conditioned, occurring only where water from a higher level impinges on the bog surface and brings to it a distinctive supply of minerals in solution. The three main types of dissolved materials (see here (#ulink_bb50f962-7cd1-5432-9723-e7dd0722279c)) give rise to the flush types: (a) lime-rich, (b) iron-rich, (c) peaty, but the first of these types is not nowadays common in our uplands, except where the bed-rock includes limestone. It is usually marked by the presence of certain mosses, and of gasteropod shells, elsewhere absent from the upland zone. Iron-rich flushes are, however, frequent, not only in the upper woodlands, but also around the bog margins. They are usually indicated by the presence of ferric-iron deposits, either on or in the surface layers of soil or peat. Moreover, if this peat is exposed to air-drying, the red-brown colour frequently becomes widespread. Certain types of vegetation (see here (#litres_trial_promo)) are characteristic of these iron flushes. Peaty flushes are topographically distinct within the bog area but generally only show variants of the general bog vegetation.

While the properties of the soil in wet flushes are determined largely by the inflowing water in bog soils, certain other properties commonly exist to which attention may now be directed. The development of a peat-covering not only marks a stage in the soil development but it also modifies subsequent development by acting as a blanket which insulates, as it were, the mineral soil from the plants growing on the peat surface. At first these plants are rooted and are drawing mineral matter from the soil, but they get less and less dependent on it as time goes on and the peat gets deeper, and the soil water becomes more and more that derived from rain. As a general rule, then, the vegetation might be expected to show a transition in its mineral salt requirements from eutrophic, with high demands, to oligotrophic, with low salt requirements. Two things result from this: first, a succession of vegetation types, and secondly, a resultant succession of peat types. We shall see later that these facts help us in the analysis both of moorland vegetation and of the history of moorland areas. For the moment, however, we are more simply concerned with its effect on the properties of the soils. There will clearly be a change in composition throughout the peat profile, and the amount of mineral matter present in the peat will decrease as the level rises above the mineral base. This is apparently a general rule in upland peats, though, locally, flush effects may disturb the normal sequence. It should be noted, however, that it does not usually apply to the actual surface peats. Moor-burning is an almost universal practice nowadays, and its effect is to destroy the existing vegetation, leaving the mineral matter it contains to enrich the surface peat. Similarly, any form of oxidation of the surface peat, due, for example, to drainage, must have a similar effect, for the oxidation products of the organic matter are mainly carbon-dioxide and ash—the former of which escapes to the air, leaving the ash to increase the amount of mineral matter in the residual peat. Thus the surface peat, where moor-burning is practised, commonly contains more ash than the layers below it. The table opposite gives illustrative figures from peat-profiles in different British areas.

There are, of course, other effects which appear to be associated with this distribution of ash. Thus the acidity of the peat almost always increases from the lowest levels upwards—showing a general correlation with the decreasing ash content.

It will be seen from Table 5, and it follows from the arguments used above, that typical upland peats are remarkable for the small amount of ash they contain, and when we seek to define the term bog, it is usual to regard it as referring to peat of this type supporting an extremely oligotrophic vegetation. In this use of the term a bog is mainly dependent on atmospheric water (i.e. rain) and uninfluenced by ground-water. The term bog contrasts in usage with the term fen—derived from the extensive peat deposits in East Anglia. In terms of this usage, fen-peat is characterised by its high mineral content and hence by its dependence on ground water. It usually shows signs of an abundance of lime and is always lime-saturated peat with a luxuriant and eutrophic vegetation of tall reeds and small trees, willows and alders. Peats of this type are almost non-existent in the British uplands to-day, although long ago they existed in some of the hollows where lime-rich waters accumulated in small ponds and lakes. These now often show their former character by an underlying bed of marl. Almost all these areas are now deeply buried beneath bog-peat, and only small areas of flush peat remain round the bog margins to illustrate the effects of this type of peat on the vegetation. Where the basal peats were originally calcareous the succession of peat types above usually shows a much more gradual decrease in mineral content, and the sequence of vegetation types was often different.

Table 5 ASH CONTENTS (AS PER CENT OF THE DRY WEIGHT) OF PEAT SAMPLES AT DIFFERENT DEPTHS






Finally, where a peat-profile generally shows signs of differences in botanical composition at different levels, it also usually shows differences in physical structure. The changes are due partly to alterations in the composition of the vegetation from which the peat was formed, and sometimes they may be due to changes in the conditions (of humidity or temperature or drainage perhaps) under which the peat was formed. As a general rule, however, the most important progressive change in the peat must be associated with its decomposition as it gets older. Two sorts of change are possible: partial oxidation, which occurs particularly in the surface layers, and the slower changes which can ensue in water-saturated peat from which oxygen is absent. We assume that these are mainly hydrolytic—that is, caused by the slow action of water on the organic materials present. These are the changes which are generally implied when we say that a peat is humified or that it is undergoing humification. They are thought to result in the plant-remains gradually becoming gelatinous, so that, although the peat appears to retain visible structure, it escapes as a jelly through the fingers when squeezed in the hand. In contrast, the more recent peats usually retain a firm and fibrous structure. Even when a peat bed appears, on first opening it up, to consist of more or less uniform material, the bottom layers will normally differ in the degree of humification from those at the top. One result of this is that if such a bed is cut and a profile exposed to the air, it will soon appear to consist of two different types of peat. The fibres quickly become prominent and some are exposed by weathering, while the upper layers which contain them are often more quickly oxidised and so may become darker in colour. The lower layers, however, remain as a damp gelatinous mass and show less alteration on exposure. After a period of exposure they may thus appear to be of quite different composition, although the apparent difference is really one mainly of different physical and chemical condition.




THE BIOLOGICAL CHARACTERS OF SOILS


The peaty nature of upland soils is one of their most easily observed attributes. It is by no means confined to waterlogged areas, but it is equally noticeable on leached podsolic soils and even in the early stages of soil-formation on mountain-top detritus. This clearly indicates that the climatic effects characteristic of these three extreme types of habitat—waterlogging, leaching and low temperatures—are alike in leading to humus or peat accumulation in the soils affected. They do so in a generally similar way by reducing the activity of the soil micro-organisms.

Their effects differ in some respects and it will therefore be convenient to consider them separately, although actually they overlap in nature to a considerable degree. The effects produced on and by the soil organisms in their turn affect the vegetation of larger plants, and hence, as later chapters will show, have much effect on the larger animals.

Numerically, the soil flora mainly consists of vast numbers of bacteria and fungi. These are colourless plants, mostly of microscopic size in the soil, that obtain their nutriment by chemically changing the remains of dead plants and animals. They are responsible for what we usually call decay, although the chemical processes involved are analogous to and, indeed, often identical with, the processes of food digestion and utilisation in animals. The breakdown of plant organic matter in soil is, however, very generally initiated by small invertebrate animals, sometimes by the larvae of flies but particularly, as Charles Darwin showed, by earthworms. These break down the cellular structure of plant-remains and partly transform it, making it suitable for further transformation by fungi and bacteria. Most soils also contain single-celled animals, protozoa, which browse on the fungi and probably serve to keep them in check. There are also insects, such as springtails and fly larvae, whose role in soil economy is usually less well known.

This large soil-population requires air, or rather oxygen from the air, for breathing and it is consequently said to be aerobic in character.

In a normal soil the chemical materials produced during these transformations by the soil organisms are substances containing oxygen, carbon dioxide, which escapes to the air, nitrates, which contain the nitrogen which was present in the animal and plant proteins, and other substances such as sulphates and phosphates. Of these, nitrates are generally regarded as most important because they are quantitatively the materials a normal plant requires from the soil in largest amounts. Thus the fertility of a soil is usually determined very largely by the rate at which it can produce nitrates, i.e. the rate at which the nitrogen locked up in the decaying organic materials can be released in a form available to plants. In actual fact, plants can also use ammonia, but the amounts of ammonia in a natural soil are normally small. This substance is the first simple substance to be formed by bacteria and fungi during the soil decompositions. It is converted by other soil organisms, the nitrifying bacteria, into nitrate. These processes of ammonia production and nitrate formation seem to be particularly sensitive to adverse soil conditions. So also are the parallel processes of nitrogen fixation by which a fertile soil is usually able to increase its nitrogen content (to an extent which balances leaching losses) by fixing the gaseous nitrogen present in the air. This process is brought about in most soils by bacteria, as well as by the nodule-forming organisms which are found living in the root-nodules of leguminous plants like peas, beans and clover. Adverse soil conditions almost always produce their effects by retarding these processes as well as the numerous other processes, e.g. of decomposition, going on in the soil. Special effects are produced by the different adverse factors.




AEROBIC AND ANAEROBIC—OXIDISING AND REDUCING SOILS


The principal result of a soil becoming saturated with water is that the amount of oxygen in the soil is reduced to vanishing point. Consequently as most of the soil organisms are aerobic, requiring oxygen, the soil population is reduced to the minimum, and there can remain active only a few anaerobic organisms with specialised methods of maintaining their existence without oxygen. The products of the decompositions going on in the soil also change in character. Instead of the formation of carbon dioxide, nitrates, sulphates and phosphates, all containing oxygen, there may be produced instead marsh-gas (or methane, CH


), ammonia (NH


), sulphuretted hydrogen (H


S) or other sulphides, and sometimes phosphine (PH


), a series of compounds devoid of oxygen. All of these products are associated with the activities of anaerobic types of moulds or bacteria, the latter usually being most abundant. The microflora of waterlogged soils is thus specialised in character as well as poor in numbers, while the products of anaerobic composition include substances, in addition to those mentioned above, which may be toxic to the larger rooted plants. Some of the products are also responsible for other manifestations peculiar to boggy soils, such as the “will o’ the wisp” and “corpse-light,” these being attributed to the burning of the highly inflammable marsh and phosphine gases.

In effect, in contrasting waterlogged and aerated soils in this way, we are contrasting two sorts of micro-biological activity—oxidising and reducing—depending on whether the organisms can form chemically oxidised products like carbon dioxide and nitrates or chemically reduced substances like marsh-gas and ammonia.

The particular value of being able to recognise these possibilities is because they give us information as to the effect of the soil conditions on the action of living organisms, and we may infer that the conditions which affect the soil flora will also affect its fauna (see here (#litres_trial_promo)) as well as the larger plants. Moreover, the level of oxygen content which produces these biological effects is low and it is not one which can be detected in the field with any certainty, if at all, by measurements of soil oxygen. For our present purpose, therefore, it is useful to think of upland soils as belonging to the two types named above.




It is useful also to realise that a large proportion of wet soils may be oxidising in drier periods and reducing in wet.




MULL AND MOR


In some respects the quantitative effects of leaching are similar to those produced by saturation with water—namely, a great reduction in the activity of the soil organisms. The qualitative effects of leaching on the soil micro-flora are, however, even more pronounced, and so much so that it is customary to give a special name, mor, to the peaty humus formed in leached soils, in order to distinguish it from the more fertile leaf-mould (or mull) typically associated with fertile forest soils. While mor is chemically different, as we shall see later, its most noticeable distinguishing features are biological and are easily recognised. There is a vegetation dominated by plants such as heathers, bilberry and wavy hair-grass (Deschampsia flexuosa), and normally an absence of earthworms. Usually, too, no tree seedlings are to be found except those of pine and birch. Moreover, leguminous plants such as clover are absent, while suitable tests show that the soil lacks nitrogen-fixing bacteria or, at least, effective strains of this type. Finally, the mor soil has a high and characteristic degree of acidity (see here (#litres_trial_promo)), normally marked by a pH value below 3·8. It yields low proportions of ammonia, while nitrates are absent. It is evident, indeed, that in mor the rate and extent of nitrogen transformation is greatly reduced, presumably by the reduction in the numbers of suitable bacteria. It is in fact usually considered that the micro-flora of this type of humus contains few bacteria and is mainly one of moulds and of other fungi like actino-mycetes and basidiomycetes, but the evidence is far from conclusive owing to the difficulties of identifying these microscopic organisms in a dark-coloured peaty soil. Moulds and other fungi are generally more tolerant of acidity than are many soil bacteria, which fail to develop either in acid or in lime-deficient maedia.

It may be useful to add here a note on the soil animals which are more characteristic of mor soils. They include certain mites, the larval stages of two-winged flies (Diptera), click-beetles (Elaterideae), and often centipedes and predatory beetles. In contrast, earthworms, snails and millipedes are particularly characteristic of good forest leaf-moulds.

Mor soils in the strict sense in Britain are confined to the acid types of soil (normally with a podsol profile) distinguished in an earlier section. Characteristic mull soils are found on lime-deficient as well as on lime-saturated soils. More strongly base-deficient soils generally have humus of this type which produces nitrates (for example) rather slowly and may, at times, be somewhat intermediate in other respects.

The third factor which must profoundly affect the properties of the soil organic matter is the low temperature of many upland habitats. There can be no doubt that the soil organisms, like other forms of life, have their activity greatly reduced by low temperature. Hence the rate of decomposition of soil organic matter declines very rapidly in cold climates and peat accumulates, for the rate at which higher and larger plants form organic matter is less affected by low temperature, depending rather on the carbon-dioxide content of the air and on light. So far as I am aware, little or no investigation of the characters of this peaty material has been attempted, and the matter would probably be most easily examined by studying the rather peaty humus that accumulates on and among the mountain-top detritus. This is usually a black and easily crumbling peat generally containing a good deal of sand. It differs very markedly from a typical mor, such as that from below heather, for example, which is generally red-brown in colour, closer in texture and more acid. On the other hand, both soil-types have certain features in common such as some resemblance in fauna, e.g. scarcity of worms and frequency of dipterous larvae. There is also a good deal of evidence, though it is mainly derived from studies in arctic regions, that the bacterial processes affecting the accumulation of nitrogen (by fixation) and its liberation in forms suitable for plant food are especially curtailed by low temperature. As a result, there is certainly a notable scarcity of soluble forms of nitrogen in mountain-top habitats, though the limited available evidence suggests that this scarcity is not as severe as is the case in either bog-peat or acid mor. In my own observations, about half the tested soils have given traces of nitrates in late summer, and higher proportions might possibly be observed at other times. The interesting feature of the samples of humus from among mountain-top detritus has been their high proportion of nitrogen, usually 4 per cent of the total humus content. This does in fact suggest that there has been a rather slow decomposition of nitrogenous materials, though this is not the only possible explanation. The following short table summaries a few values for the nitrogen proportions of characteristic humus types.

Table 6 NITROGEN CONTENT OF DIFFERENT SOIL TYPES AS PER CENT OF THEIR HUMUS CONTENTS




This table brings out the low nitrogen content of the upland bog-peats and mor when contrasted with the mull humus of an oak-wood. Still more evident would this contrast be if we included the humus of lowland woods or of agricultural soils. The well-decomposed humus of a fertile arable soil, such as a wheat field, has commonly a nitrogen content of between 4·5 and 5 per cent, a value which seems to be a characteristic of soils in the lowland north temperate climate.




SIGNIFICANCE OF NITROGEN CONTENT OF HUMUS


The lower nitrogen content which is characteristic of upland peats and humus is due partly to the slow breakdown of the original plant materials. These, as a general rule, are rich in carbon and poor in nitrogen, though these proportions vary with the plant species. Consequently undecayed plant remains reflect principally the low nitrogen content of the original material. On the other hand, the ultimate nature of the humus produced depends also on the relative rates of breakdown and removal of the two main components—which are the carbonaceous and nitrogenous compounds. In many peats the soil bacteria are unable to attack cellulose, one of the principal materials containing carbon—a substance which makes up the bulk of plant cell-walls and skeletal structures. Slow though the breakdown of nitrogenous matters may be, therefore, it still is often more rapid than that of the chief carbon compounds, so that the latter gradually become more abundant and the percentage of nitrogen remaining in the humus becomes less. This condition has a further harmful effect on the activity of the soil fungi and bacteria, for most of these organisms prefer a growth medium containing a low C/N ratio. Thus in order to facilitate the breakdown of dead leaves in a rubbish-heap, gardeners often mix with it nitrogenous manurial materials, a treatment that results in a greatly accelerated rate of bacterial decomposition of the plant remains.





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