FLOWERS, AND CHALK AND LIMESTONE SOILS

I MAKE NO apology for being an enthusiast over the flowers of chalk and limestone. As a schoolboy I soon realised that I found a greater variety of plants on the chalk than on other soils, and most of my botanising was done on the North Downs within cycling distance of my home in South London. An annual stay with grandparents near Reading offered opportunities of getting to the Berkshire Downs. Holidays in Somerset and Devon provided an introduction to the joys of the flora of the older limestones. Later on, when choice of places to visit was entirely my own, I found that chalk and limestone areas were automatically selected for a very large proportion of my field work.

Although chalk is a limestone—indeed the white Upper Chalk is one of the purest British limestones—it differs in its softer nature from the other, usually older, British limestones. Hence the sense in which “chalk” and “limestone” are used in this book is in accordance with popular usage.

My preference is by no means unusual. The majority of the thousands of people who flock to Box Hill or Ivinghoe Beacon, the Great Orme or Berry Head, Cheddar or Ingleborough, are not botanists, but many of them find the flowers in these places an additional attraction. Some of them doubtless realise that the plants differ from those of other soils. The botanist is so well aware of it that he may travel right across England to make for one particular bit of chalk or limestone where he knows that certain rare flowers are to be found.

Many of these rarities have never been found in Britain on other soils. The only places where the Monkey Orchid and the Military Orchid, Orchis simia (Plate 14b) and O. militaris (Plate 26), have been seen are on chalk. All our localities for the White Rock-rose, Helianthemum apenninum, are on limestone. A very long list might be given of scarce and beautiful wild flowers which are similarly restricted; they will be discussed in later chapters.

There are also a great many more widespread plants which are almost limited to chalk and limestone. Good examples are Chalk Milkwort, Polygala calcarea, and Round-headed Rampion, Phyteuma tenerum, which are locally abundant on much of the southern chalk though never found in the north of England. Conversely the Dark-flowered Helleborine, Epipactis atrorubens (Plate 41), is widespread on the limestone of the north and west. Horseshoe Vetch, Hippocrepis comosa, and Clustered Bellflower, Campanula glomerata (Plate 4), are plentiful on both chalk and limestone. In addition there are many flowers such as Salad Burnet, Poterium sanguisorba, and Carline Thistle, Carlina vulgaris, which are specially abundant on soils over these rocks but are also sometimes found on others. Finally there are numerous common species which are not exacting in their soil requirements—such as Common Birds-foot Trefoil, Lotus corniculatus—abundant there just as they are elsewhere.

Thus the flowers found on chalk downs like Box Hill or limestone crags such as the Great Orme are a mixture. Some could be seen equally well in many other places, but a great many, and this applies particularly to the rarities, grow specially on soils containing a high percentage of lime. These plants are called calcicoles—a word made up from the Latin calx, “chalk,” and colo, “I inhabit,” i.e. plants dwelling on chalky soils. The derived adjective is calcicolous. Botanists generally use these terms in a very much narrower sense than one might suppose from their literal meanings. They only include as calcicoles those plants which show not only the positive characteristic of thriving on calcareous soils but also the negative attribute of avoiding soils deficient in lime. Those which grow equally well elsewhere cannot be regarded as chalk or lime “lovers.” They are not calcicoles.

There are certain flowers which avoid the chalk and limestone. One of the best examples is Foxglove, Digitalis purpurea, which is common on other soils in most parts of the British Isles. Similarly, Broom, Sarothamnus scoparius, is practically absent from calcareous soils. These two plants are such excellent soil indicators that on train journeys it is often easy to tell immediately the line leaves chalk or limestone by their presence on the railway banks. The term applied to plants which avoid lime is calcifuge, from the Latin calx and fugio, “I flee.” It is the converse of calcicole.¹

Every serious gardener is well aware that some plants have these strong soil preferences. Even under the artificially easy conditions of cultivation, which frees flowers from the competition of their neighbours, they may often be a matter of life or death. It is hopeless to try to grow most members of the Heath Family, Ericaceae, on chalk, and if Rhododendrons or Ling, Calluna vulgaris, are planted in such situations they will probably soon die. On the other hand Buddleia, Buddleja davidii, and many other plants thrive on the shallowest of chalk soils; and limestone is commonly used for rockeries. Gardeners select the plants they grow according to the nature of their soil. This they can modify over a limited area by adding lime if it is too acid, or peat if too mild. Thus by taking the appropriate measures they can grow species which are quite unsuited to their natural soil.

Soils found in the country show contrasts similar to those of gardens. On moorlands and heaths they are acid, or, as gardeners would say, sour. Those formed where chalk or limestone are close to the surface are usually basic or mild. These differences are primarily due to the nature of the rocks from which the soils are derived.

Limestones are rocks which contain at least 50 per cent of carbonate of lime, which is known to chemists as calcium carbonate, CaCO3. This is a basic (alkaline) substance which neutralises acids, and farmers spread ground limestones on their fields to prevent the soil from becoming sour. Chalk has been used for this purpose since very early times and many small chalk-pits remain to remind us of the practice. The net result is similar to that attained by the use of lime (oxide of calcium, CaO) though there are important differences in the action. Calcium carbonate effervesces vigorously with cold dilute hydrochloric acid, and this is one of the tests by which limestones can be recognised.

Another characteristic of these rocks is that they are all soft enough to be scratched with a knife. This is true even of the hardest forms (known as marble). The purest of the limestones, such as much of the Upper Chalk which is commonly 95–98 per cent calcium carbonate, are white. Others are greyish, or stained yellow or red.

Flowers which grow on chalk and limestone do so because the conditions suit them better than those elsewhere, but the particular reasons are complicated and not yet fully understood. Some plants seem to be restricted by the chemical characteristics of the soil in which they grow. One example is the Privet, Ligustrum vulgare, which thrives equally on the driest of chalk downs and in wet East Anglian fens. In spite of the great contrast in other conditions both localities agree in being strongly basic owing to the presence of calcium carbonate. But the more common reasons are probably due to the physical characteristics of the ground. Limestones are dry, well aerated and warm. Plants which grow on them never have their roots waterlogged. They are able to commence growth early in the spring and many of them flower in May and June and remain more or less dormant during the driest summer months which follow. Others send their roots deep down into the well aerated soil to a level where they are assured of a more constant water supply even in times of drought: these include some of the later flowers like Wild Thyme, Thymus drucei, Perforate St. John’s Wort, Hypericum perforatum, and Small Scabious, Scabiosa columbaria.

Some of the finest scenery in the British Isles is to be found on chalk and limestone. The broad outlines of the views which attract and delight visitors are directly due to the nature of the rocks. Where these are soft, as in the chalk areas, the hills are rounded and undulating. Even the steepest escarpments, like those of the North Downs and Ivinghoe Beacon, have all their edges smoothed off. Cliffs are never formed naturally except by the sea, and the soft outlines of the chalk make ideal walking country. Here the wild flowers have often been preserved from destruction by the use of the land as sheep walks throughout the centuries. This, in turn, has produced a short dense springy turf with orchids and other characteristic flowers.

On the harder limestones there are cliffs and steeper slopes. Cheddar and the Avon Gorge (Plate XV), parts of the Wye Valley, and the Great Orme (Plate XVI), are examples which are particularly well known. In such places the choicer flowers grow on ledges where difficulty of access and immunity from any threat of cultivation has served to protect them.

The details of the scenery are supplied by the trees and flowers, and the vegetation in turn is dictated, like the topography, by the nature of the rocks. Visitors sometimes fail to appreciate how much the beauty of Box Hill owes to its famous Box Trees, Buxus sempervirens, or Selborne Hanger (Plate VIII) to its Beeches, Fagus sylvatica, or Cheddar (Plate 32) to its Yews, Taxus baccata, and Whitebeams, Sorbus aria. The downland turf is made up of plants which only grow together in this way on calcareous soils, and the association of flowers differs from those found on all other soils. With a little practice places where chalk or limestone is near the surface can be recognised immediately; the plants collectively give characteristic detail to the scenery.

In my travels all over the British Isles I have been impressed with the very gradual changes in the flora when the various chalk and limestone districts are compared. They are like a length of cinematograph film. Each frame differs only in detail from its neighbours, just as the chalk flowers of Reigate Hill, Box Hill and Hackhurst Downs are very similar. Pictures farther apart show much greater differences, and if the distance is sufficiently great they may at first sight seem to have little in common when examined as “stills.” Thus the chalk flowers of the Dover cliffs and the limestone plants of the Burren in Co. Clare may seem to be quite unconnected. But when the cinematograph film is projected and the pictures are shown on the screen in quick succession they are shown to form a continuous story. The links which form a series between the scattered frames are seen in their proper place. The chapters in this book cover habitats scattered all over the British Isles and each place described has its own characteristics. By bringing them all together in one volume they can be compared as a series gradually changing from east to west, and from south to north. It is the first time this has been attempted for the calcareous soils of any country.

The reasons for these differences between the flowers of various districts can be distinguished under two heads—variation in climate and the incidence of the geographical distribution of species. The second is to a great extent dependent on the first.

Even in a small area like the British Isles the climate varies considerably and the differences are more important to some kinds of flowers than to others. Just as the human species is adaptable and can live under conditions varying from tropical heat to polar cold, so there are some plants which can endure a considerable range of climate. Flowers like Common Birdsfoot Trefoil, Horseshoe Vetch, and Clustered Bellflower, are to be found growing just as well on the limestone of the north of England as they do on the chalk of the south. But many others are much more exacting. For example, the much wetter conditions of Westmorland and west Yorkshire suit Birds-eye Primrose, Primula farinosa, and Globe Flower, Trollius europaeus, which would never grow on the dry chalk of the South Downs even if they were planted there. Then there are plants like Maidenhair Fern, Adiantum capillus-veneris (Plate 48), which cannot stand much frost. They thrive on limestones of the west coast of England and Ireland but would be killed under the extreme conditions of the average east coast winter. There are many contrasts of this kind and they will be emphasised in later chapters.

But in addition there are plants which are so local that present climatic conditions can hardly be the only explanation of their rarity. For example, there are three small Sandworts, Arenaria ciliata, A. gothica, and A. norvegica. In Britain the first is found only on a short length of cliff in Ireland (see Chapter 15), the second occurs only on a few limestone tracks on the side of a Yorkshire mountain (see Chapter 13), while A. norvegica is restricted to small areas in north-west Scotland, the Hebrides, Shetland and Ireland. It seems certain that there must be other places where each of these plants could find the conditions under which it thrives, and the explanation of their present rarity must be looked for in the past history of the species. In this particular case the three Sandworts belong to a group with a circumpolar type of distribution. The scattered localities where they are now found are likely to be relics of a time when they were more widespread. Although we do not know for certain why they have disappeared from the intervening districts, it is highly probable that they were destroyed during periods of glaciation in past ages. Thus to explain the present distribution of plants in Britain it is necessary to plot the places where they occur elsewhere and also to study the geological and climatic history here and overseas.

From a careful study of the areas over which each species is found in Britain and abroad it has become clear that our country has been invaded by plants from more than one direction. For example, some probably came in from the south-east (see Chapter 4). Others found in the west of Ireland also grow in Spain and Portugal and the Mediterranean district (see Chapter 15). In some cases plants have not spread far from the districts where they entered our country; in others they have extended over more or less wide areas. Plants, and especially rare plants, become very much more interesting if we know something of the climatic conditions under which they can live and of how they have come to the places where they grow. Such facts are most conveniently considered if their habitats are arranged geographically.

But a purely geographical arrangement would not suffice. Limestones (including chalk) are rocks which are mainly composed of the base calcium carbonate. Such rocks can occur in strata of all ages and, far from being uniform, they vary widely in degrees of impurity and hardness. Limestones are sedimentary¹ and mostly of marine origin, though some were laid down in fresh water, and sand, mud and other impurities as well as calcium carbonate were involved in their formation. The purity of much of the Upper Chalk (often 98 per cent or more) is exceptional; most limestones are far less pure. It is obvious that these differences may be reflected in their floras, and although very little scientific work has so far been done on this subject it is one which deserves attention.

The problem is complicated by the fact that the occurrence of plants is influenced by the characteristics of the soil round their roots rather than by the geology of their habitats. The latter determines such features as steepness of slopes produced under given climatic conditions, and drainage. Geology provides the outline of the picture but other factors are also involved in deciding whether soil conditions are acid or alkaline.

Soil is the superficial unconsolidated layer of the earth’s crust and is a highly complicated mass of organic and inorganic material. The latter is itself the weathered products of rocks. It follows that in general limestone rocks are likely to give rise to calcareous soils, but there are many exceptions to this assertion. Examples will be given in later chapters, but perhaps the one which impressed me most was a shallow depression on the top of a block of limestone in Westmorland. This contained peaty soil only 1½ in. deep in which grew calcifuges, in marked contrast to the calcicoles on the adjacent rock. Although the example just given is an extreme one, less sudden soil changes due to various causes are very common, and hence it is only practical to produce reasonably accurate soil-maps of small areas. The distribution of the chief geological formations (which influence the soil in a general way, but not always in detail) is shown on the small-scale maps (Fig. 1) and in the Table.

It has already been explained that limestone rocks are largely sedimentary. At certain periods in the ancient past conditions in our part of the globe were particularly favourable to their formation, and for this reason strata of some ages are richer in limestones than others. The geologist marks his maps primarily according to the times when the rocks near the surface were made, and this he is able to do by a careful study of the fossils they contain and the sequence in which they occur. For example, there is the well-known Cretaceous system of some eighty million years ago. At the beginning of the period the fine sands and clays of the Wealden Beds were laid down in fresh water in the south of England. Then sea invaded the area and the sands and clays of the Greensands and Gault were deposited. These include only very local and small areas of calcareous rock (e.g. Kentish Rag), but towards the end of the period the sea advanced still farther and an accumulation of calcareous ooze formed the Chalk.

Geological maps show this as outcropping over considerable areas (Fig. 1a). In some places it can be seen as the familiar soft white rock of the Upper or White Chalk, which is often almost pure limestone. At others it outcrops as the less pure Middle Chalk, while some exposures of the Lower Chalk (and Chalk Marls) are very muddy and impure. Not only does the Chalk itself vary but also the higher ground is frequently covered with superficial deposits of very different rocks (see below). It will be evident that the indication of Chalk on small-scale generalised geological maps covers a wide variety of rock conditions. Nevertheless areas so marked indicate where limestone habitats are likely to be found and serve to link together in an orderly fashion the places where they occur.

Next in botanical importance to the Cretaceous, the Carboniferous is the period of geological time when limestones were laid down over wide areas of our country. Sands and shales were also deposited and much of the land-surface shown as Carboniferous has outcrops of these rocks (and very impure limestones) on which acid soils form. But once more the map is a useful guide. Its markings include all the places where limestone of this period comes to the surface and is the basis of calcareous soils.

Reference has already been made to superficial deposits which cover large areas of the limestone-bearing divisions of small-scale generalised maps of “solid” geology such as those just mentioned. These are “rocks” (often gravels, sands or clay) which may give rise to neutral or even acid soils. They have been formed in the following ways:

Residual Deposits. In the formation of these the original limestone rock has been broken up by frost and dissolved away by rain-water (in which it is soluble to the extent of about 1 in 16,000 parts). When the calcium carbonate is removed the impurities are left as sands or clays on which grow plants different from those of calcareous soils.
Drift Deposits derived from extraneous sources. These consist of material removed from other areas during the Ice Ages and dropped by the glaciers. Sometimes the drift has been carried for very great distances, and is very unlike the limestones over which it is deposited. Other drifts are calcareous.

Superficial deposits of both kinds are to be found on various limestones, but perhaps the Chalk is the best-known example to use as an illustration. This is the softest of all our limestones and weathers most quickly. When all the calcareous material has been removed the residue sometimes consists of fine particles of very sticky clay mixed with angular flints. The latter are similar to those which are often to be seen in chalk-pits or in cultivated fields on the downs. When this residue accumulates to any depth it gives rise to a soil on which calcifuges can grow. In part, it forms the basis of the “Clay-with-Flints” deposits which often extend over considerable areas on the flattish tops of the downs in south-east England.

FIG. 1
Maps showing the areas in which the most important limestones of botanical interest outcrop

Some of the Chalk was once covered with more recent beds such as those laid down in Eocene times, and here and there patches of these Tertiary deposits still remain. In places they form pebbly or sandy areas such as those in many places on the chalk of north Kent. In others they seem to have contributed to the beds which the Geological Survey mark as Clay-with-Flints. In most parts of the North Downs their flora provides a very sharp and sudden contrast to that of the adjacent chalk soils because they carry neutral or even acid soils. Good examples which will already be familiar to many readers are to be seen in the Box Hill district. Here Clay-with-Flints carries woodland on the top of the hill and on the higher parts of the spurs (see Chapter 2), and associated sands carry heathland composed of calcifuges at Headley and Walton Heaths.

Superficial and Tertiary deposits characteristically occur on flat or undulating country—in Surrey on the plateau above the 500-ft. contour. On slopes, provided the angle is sufficiently steep, loose material has long been washed away, leaving the chalk at or near the surface. On escarpments and on the sides of valleys this is usually the case.

There are, however, products of a process of soil development which are sometimes confused with superficial deposits. The latter are geological “rocks,” but areas of slightly acid soils (such as “chalk heath”) due to leaching are of a different nature. As rain-water percolates through the surface layers of permeable soils it carries off the soluble salts in solution. Calcium carbonate is one of the salts that is removed, and if the process is continued long enough the material left behind gradually becomes less calcareous and may be converted from a strongly basic condition into an acid one. In regions of heavy rainfall in the north and west of Britain such changes take place relatively quickly and leached areas are common: in the south and east the process is slower and of less frequent occurrence. Where they are found it is easy to mistake them for superficial deposits. In fact the formation of Clay-with-Flints is essentially an extension of this leaching process.

North of the Thames there is also the problem of Glacial Drift over the Chalk. Ice sheets came south from Scandinavia and Scotland as far as North London and brought many kinds of rock with them. They also eroded the chalk where they passed over it. As a result most of the Yorkshire, Lincolnshire and Norfolk chalk is now covered with material brought in this way. A great deal of it gives rise to soils which are not calcareous—or only very slightly so—and hence the flora of these areas is far less rich in calcicoles than the chalk south of the Thames (see Chapter 6).

As some sort of compensation the glaciers left behind them a deposit known as Boulder Clay which is calcareous over considerable areas. This is an important source of calcareous soils best known to botanists in parts of East Anglia, where it is the home of some particularly interesting and beautiful wild flowers such as Oxlip, Primula elatior, and Crested Cow-wheat, Melampyrum cristatum.

Although chalk has been chosen as an example, deposits also occur over other limestones. They are often loosely but conveniently referred to collectively as “Drift,” and they are marked on the “drift” editions of the Geological Survey maps. These should always be used by botanists in preference to the “solid” editions, which ignore them. But it must be remembered that the geologists have been unable to show all the very numerous small areas of drift deposits which occur over the limestones. Once again it must be stressed that geological maps should only be regarded as useful indications of where calcareous soils are likely to be found and not as evidence of them.

In order to be sure that a soil is rich in lime more reliable guides are essential. These are to be found in chemical tests of the soils and also in using the plants which grow on them as indicators.

One very simple test is to add a few drops of dilute hydrochloric acid to a soil sample. If an appreciable amount of calcium carbonate is present (say more than 5 per cent) strong effervescence will follow and continue for some time, just as it does when the same test is applied to limestone rock. When this occurs the soil may be accepted as calcareous. With a little practice it is not difficult to judge from the weakness or violence of the reaction whether the soil contains a little limestone or a lot.

Another method is to test the soil water to ascertain whether it is acid or alkaline and to what extent. If it is very alkaline it can be assumed that this is due to the presence of considerable quantities of calcium carbonate, since other bases are relatively scarce in British soils. The easiest way to make the test is to use the B.D.H. Soil Testing Outfit which is employed by farmers and gardeners, and may be obtained through chemists for a few shillings. The outfit includes a bottle of special liquid which is carefully made up to change colour according to the acidity or alkalinity of the soil water being tested. If this is very acid it turns red ; if neutral, green; if basic, blue or bluish-green. For a few additional pence a colour chart may be obtained, and if the outfit is used according to the instructions and the resulting colours carefully compared with the chart, the estimates of soil reaction will be sufficiently accurate for our purpose.¹ If the liquid turns a deep bluish-green the soil may be regarded as calcareous.

The plants themselves act as most useful indicators as to whether soils are calcareous or not, but unfortunately a great deal of experience is necessary before they can be relied on. The trained field botanist can tell at a glance in most cases when he passes from other soils to limestone. Often he knows to within a yard or two where the change takes place, but his deductions are based on the association of different kinds of plants which he sees rather than on the presence or absence of individual species. Attempts to use these as indicators are usually successful only in the case of the rarer flowers whose requirements are most exacting; efforts to show that widespread plants grow only on calcareous soils generally fail as wider experience reveals numerous exceptions.

It is, however, comparatively easy to state a negative case. If strong plants are found of the best marked calcifuges such as Ling, Calluna vulgaris, Fine-leaved Heath, Erica cinerea, Whortleberry, Vaccinium myrtillus, Foxglove, Digitalis purpurea, Sheep’s Sorrel, Rumex acetosella, or Heath Bedstraw, Galium saxatile, it is almost certain that the soil is not calcareous.

As positive indicators the most reliable guides are certain mosses and also grasses such as Upright Brome, Bromus erectus, and Tor Grass, Brachypodium pinnatum. Of the more conspicuous plants my own experience would suggest Traveller’s Joy, Clematis vitalba (Plate 1 ), and Dark Mullein, Verbascum nigrum (Plate IVa), but these do not occur in the north. The shrubs Wayfaring Tree, Viburnum lantana (Plate 9), Dogwood, Thelycrania sanguinea (Plate 8), and Whitebeam, Sorbus aria, are seldom found off calcareous soils, but these again are mainly southern. More widespread indicators include:

Common Rock-rose, Helianthemum chamaecistus.
Hairy Violet, Viola hirta.
Mountain St. John’s Wort, Hypericum montanum,
Bloody Cranesbill, Geranium sanguineum (Plate 39).
Kidney Vetch, Anthyllis vulneraria.
Dropwort, Filipendula vulgaris.
Mountain Bedstraw, Galium pumilum.
Small Scabious, Scabiosa columbaria.
Clustered Bellflower, Campanula glomerata (Plate 4).
Yellow-wort, Blackstonia perfoliata (Plate IVb).
Hoary Plantain, Plantago media.

If several of these are found together it is highly probable that there is a great deal of calcium carbonate in the soil. Such conditions can also occur off the limestone as, for example, in dunes where comminuted sea-shells provide calcareous material, but in such places the soil may be chemically very similar to that of the downs.

It follows from the statements made in the last few pages that the really characteristic plants of the limestone and chalk areas are to be found only where the soil is calcareous. These calcicoles include many of our rarest British wild flowers, and also some of the most common and beautiful ones. Their variety and interest are probably unequalled by those of any other habitat, and time employed in their search is spent in some of the most delightful country in Britain. There is certainly no need to apologise for being an enthusiast over flowers of the limestone and chalk.