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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.
