Fig. 4.—Sketch-map to illustrate river-capture.
A very simple example of this widespread phenomenon may be taken in illustration. The accompanyingsketch-map, drawn by Mr. Lionel Hinxman, shows part of the course of the River Feshie, one of the tributaries of the Spey, and part of the Geldie Burn, one of the tributaries of the Aberdeenshire Dee. It will be noted that the Feshie shows a very curious bend, or elbow. Mr. Hinxman points out that this curious condition can be explained on the supposition that the River Eidart, shown on the map to the north of the bend, once formed the headwaters of the Feshie, which cut its valley back until it captured the headwaters of the Geldie, and thus brought water which formerly flowed into the Dee into the Spey valley. The boundary between the two counties of Aberdeen and Inverness is shown on the map by a dotted line, and it is seen that the marchfollows the watershed, which between the present Geldie and the bend on the Feshie is very low. Formerly, however, this watershed lay much further to the west, and its shifting is due to the capture.
A careful study of large scale maps will show many examples of similar river-capture, some old and some recent. A sharp bend, the so-called elbow of capture, on a river in close proximity to another stream affords in itself a certain presumption that capture has taken place, though this presumption can only be verified by study on the spot.
It may be noted that before the capture is finally accomplished there may be an intermediate stage when the water has the choice of two channels, both of which may be utilised in a time of flood. A very curious case is that of the Casiquiare, a river in South America which connects together the two systems of the Amazon and the Orinoco, while another is the connection recently discovered by Captain Lenfant, a French explorer, between the systems of the Shari and the Niger in Africa. Such conditions are obviously unstable, for one stream must sooner or later predominate over the other, and deprive it even of flood water.
Another example may help to explain the evolution of a complex river system with many tributaries. A glance at the map of England (seediagram) shows that while the rivers of Northumberland and Durham flow independently into the sea, those of Yorkshire are united into a characteristic bunch, and all reach the ocean by means of the Humber. This estuary breaks through the high ground formed by the Wolds of Yorkshire and Lincolnshire, which consists of hard rock. At one time it is probable that the rivers of Yorkshire entered the sea separately, while the other great factor of the Humber, the Trent, mingled its waters with the present Witham. At this time the weathering of the land surface had not reached its present stage so the land would lie higher. In what is now the vale of York the rocks are much softer than where the Wolds now stand, and the present Ouse, which was at first a longitudinal tributary of a transverse stream, eating its way back through these soft rocks, tapped successively the streams flowing eastwards from the Pennines, and with the help of the abundant water so obtained was enabled to cut out the wide estuary of the Humber.
Fig. 5.—Sketch-map of northern England, to show the position of the Tyne and Aire Gaps, and the peculiar character of the rivers of Yorkshire. The black areas are heights above 600 feet.
One other important point in connection with river-capture has been already suggested in the account given of the Feshie. In the little sketch-map we see clearly the shift of the watershed to the east. The ultimate cause of this shift is doubtless the fact that inGreat Britain the rainfall diminishes to the east, so that, generally speaking, the westerly streams have more erosive power than the easterly. But the special interest of the case is simply that it may serve to suggest a fact not at first sight obvious, which is that water-partings are excessively unstable features. One set of streams is continually striving to encroach upon the others, and by capturing their headwaters to reduce their erosive power. A very striking example of capture on the large scale is seen in southern Patagonia, where the water-parting does not lie near the summit of the chain of the Andes, as might be expected, but considerably to the east, the western streams (or glaciers) having captured all the headwaters of the eastern streams, which lie in a region of much lower rainfall.
The net result is that running water not only scours valleys in the sides of mountain chains, but also, sooner or later, wears away the crest itself, and with the assistance of the other agents of denudation tends to reduce the mountains to plains—or at least “peneplains.” The deduction is, of course, old enough, but the recent emphasis placed upon river-capture helps us to realise it, showingus the actual “shift of the divide,” or, in other words, the wearing down of the summit levels.
This is a theoretical matter, but there is another point which has practical significance. Referring once again to the sketch-map onp. 43, we note that just at the sharp bend in the Feshie, that is, at the elbow of capture, there is a narrow region, crossed by the boundary line, which was once traversed by the headwaters of the Geldie, but is now a dry valley. Such “gaps,” as they are called, are present where recent capture has occurred, and where they occur in hilly country they sometimes form useful passes, permitting the construction of an easy road across the hills. A good example is the Aire Gap (seefig. 5) in the Pennine range of Great Britain, apparently connected with the fact that the Ribble has captured the headwaters of the Aire. Another interesting example is the so-called Tyne Gap, that breach in the Pennines which occurs near the present head of the South Tyne; it was traversed by the Roman wall, and is now crossed by the road and the railway from Newcastle to Carlisle.
As we shall see, ice appears to have thispower of cutting passes through mountain chains to a much greater extent than running water; but here, as in many other respects, there does not appear to be a sharp breach between the action of the two.
In the last chapter we have spoken of the moulding of the surface of the earth by means of running water and the agents summed up in the term “weathering.” The process is sometimes called “normal erosion,” to distinguish it from that other form of surface moulding in which ice and frost play a prominent part. At the present time ice, in the form of ice-sheets or glaciers, is confined to relatively small areas of the globe, so that we are justified in regarding its action as exceptional when compared with the work of running water. It is, however, well known that this limitation of the field of action of ice is very recent, and that during a period which geologically is only yesterday, a much greater part of the surface than at present was ice-clad.
In point of fact, much of Europe, especially the northern parts and those regions which lie close to the lofty mountain chains, much of North America, and, probably, considerable parts of the southern hemisphere, were subjected to the action of ice so recently that the processes of normal erosion have not had time to obliterate, hardly even to blur, the tracks which the ice left.
The results of the great extension of ice action in that period which geologists call Pleistocene were twofold. In the first place, as the result of the presence of the ice-sheet, we have vast accumulations of débris spread over the lower grounds. These accumulations sometimes form great sheets of boulder clay; sometimes they are collected into the curious sandy and gravelly mounds called kames which in parts of,e. g.Scotland, have a great extension; sometimes they have formed great heaps of material at the entrances of valleys. Again, these deposits have sometimes blocked valleys and so formed lakes, and they have supplied the post-glacial rivers with a vast amount of material which has been used to scour out the river-beds, and has been often re-sorted and re-arranged by running water.
Secondly, thefactthat the northern region and the high grounds further south, in both Europe and North America, have been recently clad in ice is associated with many peculiarities of surface form, some of which have exercised a marked influence on human settlements and ways of communication.
These peculiarities of surface moulding have been the object of singularly detailed study in late years, and from this detailed study many interesting facts have emerged. It may be well to state at once that this study has been largely stimulated by the fact that there is at present a great want of unanimity of opinion as to the exact cause of these peculiarities of form. According to one school ice is a more powerful eroding agent than water; according to another its action is largely conservative, and its power of erosion is slight as compared with that of water.
The beginnings of a possible solution of the problem are perhaps to be seen in the suggestions of those who seek the causes of the peculiar features of glaciated regions in the way in which running water works when it is controlled and modified by the existence of ice; but we must admit that, on the whole, the conflict is still hot and manymembers of the opposing schools will have no compromise.
To the geographer, however, the very fierceness of the controversy has been useful. The question as to the exact part played respectively by water and by ice in surface moulding is really a question for the geologist. It is, however, of great importance to the geographer that recently glaciated surfaces should be studied from every point of view, for from this detailed study are emerging many important generalisations. We shall, therefore, in this chapter only touch very lightly upon the actual points in dispute, but shall lay stress upon the interesting facts admitted by both parties.
When the conception of a just-vanished period of great glaciation was being established by the labours of many geologists, stress was naturally laid upon the obvious resemblances between parts of,e. g.Scotland and Wales, and those parts of the Alps which have been exposed by the retreat of the existing glaciers. Thus we find that most of the text-books emphasise the occurrence of perched blocks, of erratics,i. e.of blocks of rock which must have been carried from a distance, of the phenomenon of crag and tail,of giants’ kettles, and so on. All these are of more geological than geographical importance; they do not in themselves greatlyaffectthe distribution of other phenomena over the surface. We shall not, therefore, stop to consider them in detail. It is otherwise with those indications of recent glaciation which have been studied within the last few years, and they demand the geographer’s most careful consideration.
The most active discussion has taken place in regard to the peculiar features of the valleys in recently-glaciated districts, and we shall discuss especially this point.
We have already described the general features presented by valleys which owe their origin to running water. In such valleys, as we have seen, the longer the forces work the more nearly is the valley floor reduced to an even slope, whose angle decreases in passing from the mountain to the plain track. In the ordinary river valley the shape of the valley approximates to that of a V, that is, the valley narrows downwards, the river occupying the narrowest region.
Again, as a general rule there is no great difference of level between the tributaryvalleys—at least at their extremities—and the main valley, that is, there is no sharp discordance between the two. While, however, the “mature” river valley shows a gentle, continuous slope, we usually find that “young” rivers, at least in their mountain track, show an alternation of plain and gorge, which is very easily observed in any hilly region.
In other words, we find that, owing to the inclination of the rocks, or to their varying hardness, or to other causes, particular reaches are less easily eroded than others. These form waterfalls, which ultimately, as we have seen, give place to gorges. Beyond the waterfall the diminishing slope checks the rapidity of flow, and the stream tends to widen out, and also to throw down its load of débris, so that an alluvial plain may be formed.
One other character of an ordinary river valley may be noted. It heads, as we have seen, in a collecting basin, which receives the surface runnels and the outflows of the springs which form the beginning of the river.
Let us now turn to the valleys in a recently glaciated country. We omit any descriptionof existing glaciers; these will be found described in the volume on the Alps, and further, photography and the picture postcard have rendered the main features of a glacier familiar to every one. Almost every large railway station now shows fine coloured photographs of some of the important Swiss glaciers.
Taking, then, a valley known to have been occupied by a Pleistocene glacier, we find the following features. As contrasted with an ordinary river valley, the glacial valley is usually flat-bottomed, a condition described as U-shaped to point the contrast with the river valley. Examples in Great Britain and elsewhere are frequent, but some of the Alpine valleys show the phenomenon in a very striking form. Two good examples are the Aar valley at Meiringen, and the Lauterbrunnen valley at the village of the same name. Both have been rendered more or less familiar by constant photographing (seefig. 7).
The reason why they have been so much photographed leads us to consider another peculiarity of the glaciated valley. In both the cases named a steep cliff wall rises from either side of the broad, flat valley floor,and from the summit of this cliff the lateral streams leap into the main valley by often superb waterfalls. This is a very important feature of glaciated valleys—the fact that their tributaries are markedlydiscordant, that is, that there is marked difference of level between the beds of the side and main streams.
Because the side valleys lie high above the main they are said to “hang,” and are called hanging valleys, while the main valley is said to be over-deepened. The rocky height over which the water springs may be called the junction step, as an attempt to translate the French termgradin de confluencewhich is applied to it.
Incidentally we may note that in the Alps the junction step is of great human importance. Its presence gives the water the power which is used in lighting the Alpine villages with electricity, and in driving the trains which often carry the tourist to those villages. In the French and Italian Alps especially, the power is being more and more used to supply the motive force for various minor manufactures, notably for the production of nitrogenous manure from the air.
Fig. 6.—A diagrammatic cross-section of a recently glaciated valley. AB, the mountain slope which rose above the ancient glacier and has therefore retained the sharp, unrounded forms due to ordinary weathering. BC, the shelf or shoulder, formerly covered by the ice, and therefore strewn with glacial débris. It now usually forms a pasture or alp. The dotted line connecting CC shows the probable form of thepre-glacialvalley; CD, the rocky wall of the existing U-shaped valley on whose floor the river now flows.
Associated with the hanging valleys ofAlpine regions is the presence of a curious shelf, shoulder, or “bench,” which frequently lies on the top of the cliff from which the lateral streams spring (seefigs. 6and7). Any one who has done some walking in the Alps, must have noticed a peculiar and often trying feature of any walk which leads up the side of the valley. This is that thewalk begins with a very steep ascent, where the road or track zig-zags to and fro. After this steep and trying climb the walker reaches a broad shelf (BC in figs. 6 and 7), where the slope is much less, and where the extent of relatively level ground gives room for the erection of a huge hotel, or perhaps only of a group of chalets. This shelf is covered with fine herbage, destined to be cropped by the cows of the community.
Fig. 7.—An actual cross-section of the Lauterbrunnen valley. The vertical and horizontal scales are the same. B marks the edge of the cliff wall over which the streams leap in cascades. A is the position of the stream at the bottom of the U-shaped valley. BC marks the position of the shelf, largely occupied by the pastures or alps. Above them are rocky, unsmoothed slopes.
If the traveller continue his walk he will find that above this pasture ground oralpthe slopes are again steep up to the mountain summits. Possibly, however, his walk has been to see a famous waterfall from above, and he will find that the streams whichflow with relative slowness over the comparatively gentle slopes of the alp or shelf, will at some point tumble over the region up which he climbed, probably in a series of leaps or cascades.
The U-shaped valley, the “hanging” tributaries, the shelf or shoulder running along the upper part of the cliff wall which bounds the main valley, all these are striking features of glaciated regions. We shall not here discuss the probable causes of this striking “break of slope,” so different from the characteristically continuous slopes of an ordinary mature river valley. As has been indicated, it is here that active controversy rages. It is, however, important to note that the shoulder or bench of which we have spoken was almost certainly once covered by the ice, its gentle slope indicating the original valley floor, before over-deepening took place.
The reason why pasture now grows upon it is that it is covered with fine glacial débris, which makes fertile soil. The fertile soil, which is often irrigated by milky water from existing glaciers, combined with the effect of altitude upon the plants, produces rich pasturage, and makes cattle-rearing an important alpine industry.
The next interesting feature of glaciated regions is the occurrence of those curious mountain forms which have special names in nearly every recently-glaciated region. Those gigantic arm-chair-shaped notches, high up on the mountain sides, which the Welsh call cwms, the Scotch corries, the French cirques, and the Germans kare, are very widespread in the Highlands of Scotland, in the mountains of Wales, in the Tyrol, and in other parts of the Alps (though they are not common in the Central region), and in North America as well as elsewhere.
A cirque (fig. 8) is shaped something like an office arm-chair. The floor has only a gentle downward slope, and often lodges a lake; or in other cases it is marshy, showing that a lake was once present. The back and sides are steep and precipitous. In some instances, if several cirques occur near together, the side walls may be eroded through, so that a shelf is produced, as one might produce a bench by putting two chairs side by side, and cutting away the contiguous arms. Very often, as one may easily see in the Highlands of Scotland, a series of cirques occur, one above the other, so that a climber proceeding from the valley floor upwards hasa succession of steep “pitches,” to use a mountaineering term, alternating with easy if wet walks across the floors of the successive cirques.
Fig. 8.—Diagram showing two glacial cirques.
It quite often happens in the case of high mountains in the Alps that the topmost of such a series of cirques still retains a glacier, what is called a dead glacier, that is, one which has practically ceased to move.
In other cases, again, we may find thatwhat should be the flat floor of the cirque has been largely eaten away, as it were, by a huge rounded trough, which occupies what would be the extreme front of the seat of the arm-chair. In this trough a stream runs, and the trough has the characteristic U-shaped rounding characteristic of glacial forms. Further, at the top of the wall of the trough a bench or shelf exists, which is obviously the remains of the old cirque floor. In the case of all characteristic glacial cirques, however, the special feature is that the flat bottom of the cirque is discontinuous with the valley below; they are not parts of the same system of drainage. What we may call an unconformity appears between the two regions, more or less marked according as running water has or has not had time to begin the work of the removal of the unconformity.
The immediate human importance of these corries or cirques is not so apparent as in the case of hanging valleys, but they must be mentioned, if only because of their extraordinary abundance in glaciated regions, and especially in Great Britain. There are two views as to their origin, and we shall indicate both here without making any attemptto decide which is the correct one. A very full and clear statement of one position will be found in an article by Prof. Garwood in theGeographical Journalfor September 1910, while previous articles by Prof. Davis and others in this journal formulate the opposed view.
To the first school the corrie is simply in origin the collecting basin of a pre-glacial stream, such a basin tending to acquire, roughly speaking, a flattish bottom and somewhat steep sides. With the onset of the ice the floor of the basin was protected by the ice from further erosion, while the frost ate back the wall and so steepened it, and the glacier carried away all débris as it formed. At a later stage the lower part of the glacier disappeared and only the cirque glacier was left. It continued its protective action, while below the powerful torrents hollowed out a trough. This process was perhaps repeated several times, with the final result that the protected cirque was left as a much-modified remnant of pre-glacial conditions, while the valley below was powerfully eroded by the glacial torrents. Thus a cirque lying above an existing valley is to be regarded as the beheaded end of an oldvalley, preserved by its ice covering, while below the old valley has been fundamentally modified by the scour of the glacial torrents. On this view the sharp distinction between the two angles of slope marks the distinction between the work of ice (protective) and the work of water (erosive). A series of cirques means a succession of glacial and interglacial periods.
According to the other school, for whom ice is a more powerful eroding agent than water, the cirque was produced by the ice, its presence or absence, ine. g.the Alps, being determined by the shape of the pre-glacial mountains. Cirques are believed to have been produced by the ice wherever the form of the mountains conduced to the accumulation of snow, and the occurrence of a series of cirques, and of the troughs which seem sometimes to eat into their floors, is ascribed to the successive retreat of the great ice-plough,i. e.to the action of the retreating ice itself, and not to the water which flows from beneath it.
Another striking feature of many glacial valleys is a very marked want of continuity in the slope of the main valley. Not only dothe side valleys “hang” over the main valleys, but, further, this main valley itself often consists of relatively level reaches alternating with rocky bars, through which the river has sometimes later cut a gorge. Examples of this are very frequent. The famous gorge of the Aar above Meiringen is a river gorge cut through a rocky bar of this kind.
The Pyrenees are somewhat less familiar, both to tourists and in the form of pictures, but there, also, the same thing occurs. Above the health resort of Cauterets lies the little Lac de Gaube, whose mouth is blocked by a rocky bar through which the little torrent is cutting a tiny gorge. If the tourist crosses the lake in a boat and begins to walk up the valley above it, he will find that it has the form of a staircase, the huge steps being separated from one another by broad plateaux, which are flat and swampy, and have obviously been occupied by lakes not long ago. Above each plateau there is a rocky wall, almost precipitous, down which the stream flows in cascades. In other parts of the Pyrenees the same phenomenon occurs, and the lakes sometimes persist, lying one above the other in a series.
Fig. 9.—Profile of the Maderaner thal in Switzerland, to show the staircase arrangement peculiar to recently glaciated valleys. (From Garwood.)
The phenomenon is so common that it markedly affects human life in the Alps. The “landings,” as the French call them, usually afford good pasture ground, while those which lie at no great elevation can be cultivated. Further, as the ground is level there is room for houses or even for a considerable village. The intervening region or step is too rocky to give level ground for human habitations or for pasture and cultivation. Where the river has had time to cut a gorge, the road must leave the stream, and can often be constructed only with difficulty. The result is that an Alpine valley often consists of a chain of villages, linked togetherby a difficult mule track or path. The abundant water-power, however, makes mechanical traction relatively easy, and we have sometimes the curious condition that a mule track is replaced by a railway, without the intervention of a road fit for wheeled traffic.
We need not stop to discuss the probable cause of this step and stair arrangement, which presents much the same problem as the series of cirques at the head of the valley. It is enough to indicate that according to one group of physical geographers the flat landings are due to the way in which the gradually decreasing glacier protected its bed from erosion, while the torrent which issued from it eroded very rapidly below; according to another school the landings are due to direct glacial erosion. There are other observers, again, who lay especial stress upon the modifications of the erosive powers of running water, due to the presence of the ice. For us it is of interest to notice that, as has been already indicated, the staircase effect occurs also, though on a smaller scale, in the case of mountain streams generally, some of which must be post-glacial in origin. In other words, there seems to be fundamental similarity betweenthe work of ice and of water, the differences being differences of degree rather than of kind, and due largely to the varying fluidity of the two.
There is still one other feature of glaciated regions to which reference must be made. This is the occurrence of peculiarly open passes in considerable numbers across mountain regions which have been recently glaciated. In the geography books and in some maps, the Alps, for example, are represented as a great barrier, shutting off the fertile plains of Italy from the countries of Central Europe. But history shows that they have never been such a barrier, and the phrase of “splendid traitor” has been applied to the whole mountain range, in order to emphasise its total inadequacy as a barrier, either to armed or to peaceful invasion.
Since the time of Napoleon I public attention has been focussed upon a few great Alpine passes, notably the Mont Cenis, the Simplon and the St. Gothard, which are crossed by great carriage roads, now functionally replaced by railway tunnels beneath. But we must not forget that in addition to these and the other great passes there are almost innumerable ways of crossing the Alpson foot, and the presence either of Hospices or of small inns on many of the smaller passes shows that they are constantly used at the present time, in spite of railway tunnels and carriage roads elsewhere. Even a pass relatively so difficult as the Théodule, was used by very large numbers of Italian peasants during the time when work on the Simplon railway made great demands on Italian labour.
Any one of the passes, great or small, shows in outline the same characters. There is a steep ascent, often steeper on the Italian than on the other side, then a broad, windswept, open summit, sometimes almost level, below which the rapid descent begins. Not infrequently a lake, or lakes, may be found near the summit.
On a smaller scale the same phenomenon occurs in such glaciated regions as Scotland, the relatively low connections between one valley system and another greatly facilitating communication, and usually carrying both road and railway, where the latter exists. Such connections between two drainage systems (that is, the existence of a very low divide between the two) only exist on a small scale outside glaciated regions, so thatthey, with all their effects upon communications, must be largely ascribed to ice-action. We shall describe one case in a little detail, with the proviso that while no one denies the frequency of such passes in glaciated regions, some authorities believe that their production was due more to glacial torrents than to the erosive action of ice itself.
A very pretty example is the picturesque pass known as the Gemmi, which is traversed only by a mule path, and connects Kandersteg, and thus the lake of Thun and the town of Berne, with the Rhone valley, which the path enters at the village of Leuk. The walk proper is, however, over at the Baths of Leuk, a small health resort lying at the foot of the great Gemmiwand, a wall of rock over 1,600 feet in height on the summit of which is the Gemmi pass. Readers of Mark Twain’sA Tramp Abroadwill remember his interesting description of the crossing of the pass, which is part of the regulation tour in Switzerland.
The excursion may be very briefly described. The traveller starts from the village of Kandersteg, and almost immediately begins a steep climb, which after a rise of over 2,000 feet leads him over a ridge to a pasture, once swept by an avalanche. Another shortbut steep rise (note the staircase arrangement) leads him to the lonely Daubensee, a little lake which is frozen for more than half the year and has no outlet. It is itself fed by a glacier lying to the traveller’s right, the Laemmern glacier, which is shrinking and exposing more and more of its old bed. Even to the most inexperienced traveller it is obvious that this present day shrinkage is, as it were, the last remnant of a shrinkage which has been going on for a prolonged period, so that the route by which the traveller ascended from Kandersteg is but a remnant of the bed of the old glacier. The point of special interest, however, is that at the end of the Daubensee the traveller leaves the glacial valley by which he has ascended, and passing through a great notch or gateway in a wall of rock, begins the almost precipitous descent to Leukerbad, which lies at his feet, 1,600 feet below. It is this notch which makes the pass, and it is fundamentally a breach in the mountain wall which separates the drainage of the Rhine from that of the Rhone. Comparing small things with great we may note that this gateway presents some resemblance to the Tyne and Aire Gaps in the Pennines, already mentioned,which may also have been modified by ice-action.
The explanation given is as follows:—At the time when the glaciation reached its maximum height the mass of ice in what is now the Laemmern glacier was so great that it could not be contained within its own valley. The ice was piled up so high that it over-rode the watershed, rose up beyond the containing wall of its own valley, and pushed a long arm over the valley wall, down into the Rhone valley. This tongue of ice, either by its own erosive power, or because of the glacial and sub-glacial streams which it produced, wore out a notch in the wall as it crossed, and it is this notch which makes the pass. As the glacier gradually shrank, it could no longer send this tributary over the wall into the valley below, and was constrained to send all its drainage into its own valley, that is ultimately into the Rhine. But the Gemmi pass persists as a proof of its former magnitude, of the fact that once part of the Laemmern drainage reached the Mediterranean instead of the North Sea, that there was once a communication between the Rhine and the Rhone drainage systems.
Many at least of the great Alpine passesare believed to have been produced in this way, and therefore we must add to the peculiarities of recently-glaciated countries, the fact that passes are likely to be frequent across their hills and valleys, owing to the power which ice possesses, when enormously developed, of rising above valley walls, and streaming down into another valley system. Some of the great Alpine passes, perhaps, arose in other ways, but this brief description may be of interest as suggesting one, probably common, mode of origin.
If we sum up what has been said as to the special features of glaciated regions, we may note that their valleys tend to be U-shaped, and to be discontinuous with their tributary valleys, which “hang” over them. On the top of the cliff from which these tributary streams leap is a shelf, which is clearly a portion of the floor of the pre-glacial valley and is covered by glacial débris. At the heads of the valleys there are often cirques or plateaux, which again are markedly discordant, hanging high above the valley below. In the main valley itself there are similar discordances, giving rise to a staircase arrangement. Finally, different valley systems oftencommunicate with each other by passes, natural highways which hang high above both valley systems alike.
Obviously, however, we might replace this detailed summary by the simple statement that whereas in a region subjected only to the action of running water, there is a marked tendency to continuity of slopes throughout, a tendency more and more marked the longer the water acts, in glaciated regions there is an equally obvious discordance, a discontinuity of slope, most marked where water has not had time to begin its smoothing action. As every glaciated valley which we can study in detail has been subjected to the action both of ice and of water, it is a simple deduction that the discontinuity is due to the differential action of the two. This is the point of geographical importance, and to the geographer it is of minor importance to know whether it is the passive resistance of the ice which has caused the discontinuity, or whether it is the water which has been unable to keep pace with the activity of the ice.
There is one other point which must be alluded to even in this very brief considerationof the effect of the ice age upon the physical geography of the glaciated regions. This is the fact that it greatly modified the numbers and distribution of plants and animals throughout the areas affected. Obviously the covering of ice must have rendered a large part of Europe uninhabitable both for man and for the vast majority of animals and plants. In Europe, therefore, as also in North America, there must have been a southward sweep of all living organisms, driven from their original habitat by the onset of the cold period. But the conditions in the two continents differed greatly.
In North America, especially in the east, there are no transverse chains of mountains, there is no southern sea until the Gulf of Mexico is reached in lat. 30°, and even here Florida almost touches the tropic, and Mexico extends far beyond it. In this continent, therefore, the plants and animals, though driven far to the south, still found room to live and multiply, and had no great obstacle to cross either in their southward journey, or when they strove to re-annex their old territory as the cold conditions passed away again.
It is a curious fact that the forest trees of eastern Asia and of eastern North America show a remarkable resemblance to one another, and both regions are very rich in species and in genera. It is believed that this rich North American flora is a remnant of pre-glacial conditions, and that its persistence is due to the ease with which the trees obtained an asylum to the south during the period when the climate was most severe.
In Europe, in spite of the fact that the winter climate is much milder than in corresponding latitudes in North America, the number of kinds of forest trees is much less, there is little resemblance to those of Asia and the eastern United States, and the trees have generally a less southern aspect. This is the more remarkable in that trees of southern facies introduced from China and Japan and from the United States thrive admirably in Europe, showing that there is no climatic obstacle to their presence there. To mention only a few examples, the Tree of Heaven (Ailanthus glandulosa), so very common, even as a wild tree in many parts of the continent of Europe, was introduced fromChina, while the beautifulSophora japonica, so frequently planted in towns, comes, as its name indicates, from Japan, and the various species of those beautiful flowering trees known as Catalpa are either American or Asiatic. The western plane (Platanus occidentalis), another favourite town tree, comes from the United States, and other American trees which are found very abundantly in towns in the warmer parts of Europe are the black walnut and the honey locust (Gleditschia tricanthos). Perhaps more striking than any of these is the case of the so-called false acacia (Robinia pseudacacia), which is as common over a great part of the continent of Europe as hawthorn bushes or wild roses are with us, and yet is a North American species, introduced less than three hundred years ago. Generally, we may say that all the more beautiful trees now growing in the warmer parts of Europe come either from eastern Asia or from the United States. In other words, the Ice Age seems to have greatly impoverished the flora of Europe. To a less extent this is also true of western North America, which has fewer species of trees than the east.
Why had the ice this impoverishing effect upon Europe? The topography of the continent supplies the answer. In the first place, in Europe there are numerous transverse chains of mountains. The Pyrenees, the Alps, the Caucasus, each with its load of ice, each with glaciers deploying on the low ground at its feet, must have been obstacles in the way of the southern migration alike of plants and of animals. Again, even if these obstacles were passed or turned, the great inland sea formed another barrier further south. In consequence of this difficulty in finding asylums the pre-glacial plants and animals must have perished in considerable numbers, and thus a general impoverishment took place. One must not of course exaggerate. A proportion of the pre-glacial forms did succeed in living through the period of stress, but many must have been, as it were, squeezed out of Europe or out of existence by the unfavourable climatic conditions.
As the climate improved the lands swept bare once again became inhabitable, and there was a recolonisation by movements from the south and from the east. We shallindicate later how man himself came from the south and the east to colonise the west and north, but his movements were only part of a great series which included also those of plants and animals.
To the superficial observer those daily variations in the atmospheric conditions in any one locality which we sum up under the term weather, may appear to occur without order or regularity, but detailed quantitive study soon shows that even British weather displays constancy in its irregularity. The existence of such basal constancy, indeed, lies at the root of all intelligent utilisation of the soil. The irresponsible amateur gardener may lightheartedly assume that a particular spring will be “early,” but the professional is not easily induced to abandon his rule that such and such operations must not be undertaken before certain fixed dates. The farmer, if he is to avoid bankruptcy, must know within what limits the first autumn frost islikely to make its appearance, and when the last spring one may be expected.
Collective experience, then, whether expressed in the meteorologist’s figures or in a less accurate form, leads us to the conclusion that for every locality on the earth’s surface there is a certain fixed average succession of weather, which we sum up in the termclimate.
In the case of both climate and weather our knowledge may be summed up in such general terms as “wet” or “dry,” “warm” or “cold,” and so forth, or we may borrow the meteorologist’s notations, and express the facts in degrees of temperature, inches of rainfall and of pressure, percentages of humidity, and so on. But it should be understood that such figures can be used by the geographer with justification only when he is himself aware, and can assume that his audience is aware, of the significance of the figures in connection with the processes of erosion and the phenomena of life. To say that the mean January temperature of a particular place is 30° F., is only a convenient shorthand way of saying that in this place in winter plant life is arrested, water is ice-bound, and most animals sleep or migrate. In other words, the use of the figures assumesa certain knowledge of biology and of physics on the part of the audience.
We do not propose here to treat either climate or weather with any fullness, for there is a volume in the series specially devoted to these and kindred subjects. All that will be attempted, therefore, is to discuss one or two important climates with the object of considering later their respective effects on the distribution of other phenomena on the surface of the globe. This is the more worth doing in that the subject is one which has had a great deal of attention devoted to it in recent years.
Certain points in regard to climate,e. g.the fact that the regions of the earth near the equator get more solar heat than those nearer the poles, and that parts of the globe are subjected to variable winds, as contrasted with those regions where the extraordinarily regular winds called “trades” blow, have of course been known for long enough. But not till the latter half of the nineteenth century did the civilised nations begin regular meteorological observations, and these observations are still scanty for the uncivilised and partially civilised regions. The meteorological raw material necessary for the exact study of climates has thus only been availablefor a comparatively short period, and is still incomplete.
We may begin with that type of climate which has so profoundly influenced the civilisation of western Europe, and therefore also the new civilisations of America, Australia, South Africa, and so on. This is the type called Mediterranean, because it reaches its best development and has been most studied round the Mediterranean area. But it also occurs in California, in parts of Chile, in South Africa round Cape Colony, and in south and south-western Australia. Generally, it is characteristic of lands lying on the western side of continents, in the latitudes between tropical and temperate, and is therefore sometimes called the maritime sub-tropical climate. The term maritime is applied because, as we shall see, for some part of the year oceanic influences prevail, sub-tropical indicates the position in latitude.
A very curious illustration of the similarity of climate in the different regions named is to be found in the fact that in parts of the Mediterranean area two introduced American plants, the agave and the prickly pear, are more obvious and abundant than most native plants; while in California, Cape Colony andsouthern Australia the cultivated plants are chiefly of Mediterranean origin.
The main features of the Mediterranean climate may be briefly summarised. The most important character, next to the mild temperature, is the fact that no rain (or very little) falls in summer, the growing season further north, which is here largely a period of cessation of plant growth. The rain, which tends to be scanty or even absent in the interior of land masses,e. g.in Spain and Asia Minor, and also to the south,e. g.in the Desert of Sahara, in the Mediterranean region proper falls in the winter months. It is this winter rainfall and the summer drought which define the Mediterranean type of climate.
The reason for this seasonal distribution of rainfall is as interesting as the fact itself, and to understand it we must turn to the circulation of air on the surface of the globe.
In the following description we shall restrict ourselves, for the sake of clearness, to the Mediterranean region itself, the region where the Mediterranean type of climate is developed over the largest area, and where, for many reasons, it is most important. But it must be noted that the conditions which give rise to the Mediterranean type of climate arethe same wherever it occurs, though in the Mediterranean area they are greatly modified by the great inland sea of that name, which carries oceanic conditions far into the land.
We must note, first, that at all seasons those regions of the earth which are directly beneath the vertical rays of the sun are heated most intensely. Therefore the air over these regions, being rendered light by heating, rises, and a belt of low pressure is thus formed. Only at the equinoxes does this belt of high temperature, low pressure, and light winds or calms, coincide with the equator. In the northern summer it moves north with the sun; in the northern winter it travels south with the sun, being always over what is called theheat equator. Into this belt of low pressure air from north and south, where the pressure is greater, tends to rush in, and we have thus formed the constant or “trade” winds, which, owing to the deflection produced by the earth’s rotation, appear as the north-east trades in the northern hemisphere and the south-east in the south. These winds are dry winds, because they blow from colder to warmer latitudes, and they accompanythe equatorial low-pressure belt in its north and south movements.
In the northern summer the trade winds may extend northward to lat. 35° or even 40°, while in winter their northern limit is 10° to 15° further south. A glance at the map, then, will show that in summer the Mediterranean area is within or near the sphere of action of the dry trade winds, which are continental, sweeping into the region after having blown over land surfaces.
We must next consider the atmospheric movements in the region to the north of the trade wind belt. An area of more or less permanent low pressure, best marked in winter, exists in the North Atlantic, in about 60° N. lat., and draws the air into it in the direction known as counterclockwise, that is, in the direction opposite to that of the hands of the clock. The result is the production of the winds which appear off the coast of western Europe as the warm south-westerly winds of winter, while they appear off the coast of North America as cold northerly winds. In the southern hemisphere, where, as we have seen, there is less land to interfere with the development of the atmospheric circulation, these winds form the prevailing westerlies.
In the Atlantic these south-westerly winds obviously blow in a direction opposite to the north-east trades, whence the name of anti-trades often given to them. As they blow across the broad Atlantic they arrive off Europe saturated with moisture. As they come from lower latitudes they are warmth bringing. In winter these winds reach the Mediterranean area owing to the southern shift of the trades, and bring moisture with them; while in summer they lie more to the north, and though their moisture affects the coast of Portugal it does not reach the greater part of the Mediterranean area.
Within that area the northern limit of the rainless summer may be said, in a rough sense, to correspond with about the 40th parallel of latitude, though it varies according to local conditions in the different peninsulas. To the north of this line, therefore, the climate is more or less affected even in summer by the anti-trades.
It must not be supposed that the region of the trade winds and of the anti-trades lie side by side. Between the two there is a zone of variable winds, but in general terms we can explain the peculiarities of the Mediterranean rainfall by saying that the regionlies within or just at the edge of the dry trades in summer, and within the zone of the moist anti-trades in winter.
Let us next consider how the area is demarcated from the surrounding regions. There is of course no hard and fast line, but we can indicate in broad outline the meteorological limits. To take the absolutely rainless summer as the limit would cut out, as we have suggested above, the greater part of the northern shore of the Mediterranean, except the southern halves of all the great peninsulas. Quite generally, however, we may say that the northern limit of the Mediterranean region, in its western half, is defined by the occurrence of considerable summer rain. That is, it is bounded to the north by a region which is within reach of the rain-bringing anti-trades in summer as well as in winter, and which has a lower temperature than the Mediterranean region proper. To the east the region is limited by deserts, for the westerlies of winter can only carry their moisture a certain distance inwards, and though they are greatly assisted by the long, eastward-stretching, inland sea, yet there comes a time when all their load of moisture is lost, and desert conditions supervene.
To the south the desert again forms the boundary, though here for a different cause. North Africa behind the Atlas is permanently within the trade-wind belt, that is, it is permanently subjected to the action of drying winds, and its rainfall is therefore small or nil. Similarly in California the southern limit of the Mediterranean zone of climate is the desert region of Arizona, Mexico, and the north of Lower California. A similar band of desert separates the Mediterranean zone from the tropical region of summer rain in the other places where the Mediterranean type occurs.
This may be summed up as follows:—Defining the Mediterranean climate only by its rainfall, we may say that it prevails over lands both to the north and south of that sea, and these have all or most of their rainfall in winter, when the winds, though typically westerly, are often stormy and rendered variable by local conditions. In the summer there may be no rain at all, or, to the north, small amounts. To the north the region passes gradually into that colder zone where rain occurs abundantly both in summer and winter, while to the east and south the rainfall diminishes greatly, and there is a gradual transition to desert conditions. To thewest the boundary of the region is theoretically the ocean, but the western coastline owes to its peculiar position a more abundant precipitation, which makes the vegetation of,e. g., Portugal present quite a different appearance from that of southern Italy or Algiers. These peculiarities of rainfall the region owes to its position between two great wind systems, of which one gains the mastery in winter and the other in summer.
So far in this discussion we have spoken only of the distribution of the rainfall throughout the year, but there are other features of the Mediterranean climate which are almost as important in considering the effects of the climate on the life of the region. These are the amount of the rainfall, and the temperature.
Beginning with general points, it is very important to notice that the rainfall throughout the area as a whole is relatively scanty, except where special conditions,e. g.great elevation, or local rain-bringing winds, increase it. Translated into terms of plant life this means that continuous forests of the type so characteristic of the greater part of Europe till man interfered, are relatively rare within the limits of the Mediterranean region.Looking at the same fact from the human standpoint we may say that the rainfall is often so scanty that irrigation is necessary before man can prosper. These two facts, that Mediterranean man had not to clear forests before he planted and sowed, as the Teutons were obliged to do, and that he had often to bring water artificially before his crops would grow, have been of supreme importance in the evolution of Mediterranean civilisation. Even at this stage it is interesting to note that France in this, as in many other respects, has shared in two civilisations, for her territory to the south shows Mediterranean characters, and elsewhere resembles the cool temperate zone of Europe.
The next general point of importance is that of temperature. As was to be expected from its latitude the basin of the Mediterranean is a relatively warm region. Local conditions, and especially the presence of a great mass of water, make the winter exceptionally mild, while the summers, though not excessively hot they are considerably cooler than those of similar latitudes in Asia, are yet warm and sunny. The result is that, given water artificially supplied, or givencrops which can take water from the deeper layers of the soil, the region is productive, the destructive frost of the north not being a menace. This relative easiness of life in the more favoured parts of the region has been of great importance in its history.
We may give next some actual figures to illustrate what has been said about temperature and rainfall. Let us begin with rainfall, and in order to have a basis of comparison we may first note that Edinburgh has a mean annual rainfall of about 28 inches, and London one of about 25 inches. In other words, when the total amount of rain which falls in any one year is estimated for many years in either of these places, these totals added together and divided by the number of years of observation, the quotient is the figure given. The figures show that the rainfall in London is less than that in Edinburgh, while in Paris it is less than in either.
Passing now to consider the Mediterranean area we find that, speaking generally, the rainfall diminishes, for the reasons already explained, in passing from west to east, and in passing from north to south. Thus Gibraltar, at one end of the basin has a fall of 32"per annum, as compared with one of 15" at Athens near the other extremity. Genoa in the north has the heavy fall of 51", while Biskra in Algiers has only 8".
There are many local variations, due to local causes, and in comparing the falls with those of Edinburgh and London we must remember that the higher temperatures mean much greater evaporation. Sunny Naples has about 4" more rain in the year than Edinburgh, and has 7" more than foggy London, but yet has not a wet climate.
For temperatures a few figures may suffice. In London the mean January temperature is 39° F., while it is only 36° F. at Paris. In Nice the mean January temperature is 45°, which is about the same as that of Athens, and rather less than that of Naples. In January, then, the temperature of Nice is only 6° higher than that of London. In July the mean temperature at London is 62°, as against 73° at Nice and over 80° at Athens. In other words, owing to our mild winters and cool summers, there is far more difference between British and Mediterranean temperatures in summer than in winter. In the Mediterranean region itself the difference between thetemperatures of summer and winter increases as we pass eastwards, so that it is especially to the west that characteristically Mediterranean conditions occur,i. e.mild, frost-free winters, and summers which for the latitude are not excessively hot. This feature also has been of importance in the development of the Mediterranean civilisations.
We have treated the climate of the Mediterranean area in some detail, as an example of the methods and results of modern climatology. We may note much more briefly the characteristics of one or two other climatic provinces.
Mediterranean influences, expressed in winter rains, are continued eastward into Mesopotamia and even into Persia, the rain always becoming scantier, and desert conditions tending to supervene. Still further east, however, we come to a region where the rainfall is abundant, and where the population is once more dense. These are the monsoon countries, including India and China, where the usually plentiful rainfall again permits the land to nourish man abundantly.
Excluding Africa south of the Sahara from consideration, we may indeed say that theOld World has two regions of abundant rainfall and dense population, the one to the west and the other to the south-east, separated from each other by warm and cold deserts. Each of these two regions has given rise to its own civilisation, each has produced its own types of cultivated plants and domestic animals, and the root differences between the two must be regarded as largely the result of climatic conditions.
The monsoon countries are so named because of the regular seasonal reversal of the winds, which blow from land to sea in winter and from sea to land in summer, affording an example of a land and sea breeze on the gigantic scale. The result is that, subject to local modifications, the summer winds are moisture-bringing, and the winter winds are dry. Whereas, then, in the Mediterranean the heat of summer is largely wasted, from the agriculturist’s point of view, on account of the scarcity of the water necessary for growth, in monsoon regions, unless the rain fail, as it sometimes does, the hot season is the moist season, and, therefore, other things being equal, growth must be faster here than in the Mediterranean area.The monsoon countries extend over a great stretch of latitude, and therefore temperature conditions vary greatly, while the great variety of surface-relief produces here abnormally heavy rainfall, and there desert conditions. The essential contrast with the Mediterranean type is, however, the summer rainfall.
Taking the globe as a whole we find that summer rainfall is more common than winter, and in addition to occurring in monsoon regions, it tends to occur in tropical regions generally. As we approach the equator from the tropics we find that the total fall increases, and tends to show two maxima, which occur when the sun is overhead,i. e.at the equinoxes. For our particular purpose, however, the climatic conditions in tropical and equatorial regions generally, though of great importance to the climatologist, are not of great interest, for except in monsoon countries the hot parts of the earth do not show the most highly developed human societies.
Let us turn next to that part of Europe which is outside the reach of Mediterranean influences. Here we find that the rain is distributed throughout the year, and is usuallyabundant, though it decreases in passing eastwards from the seaboard. Temperatures are naturally lower than in the Mediterranean basin, and winter frost plays an important part in determining the choice of cultivated plants. As the figures which we have already quoted for London and Paris suggest, the winter cold increases on passing eastward. Paris is colder in winter than London, though it lies south of it. Vienna is again colder than Paris. But the increase in winter cold is compensated for by an increase in the summer heat. In other words, as the distance from the sea increases in Europe the climate becomes drier and more extreme.
This observation naturally leads up to a consideration of the effect of the proximity of the sea upon climate. Water heats more slowly than land, but also cools more slowly, and therefore the proximity of large masses of water has, speaking generally, a moderating influence upon climate, producing the so-called maritime climate. In the case of the British Isles this effect is very marked, because the ocean to the west of us is unusually warm, and the circulation of the atmosphere is such that the prevailing winds of winterblow towards us from the warmer parts of this ocean, while the fact that the summer winds often have a northerly component helps to keep the summer temperatures down.
The peculiar conditions of the British Islands illustrate the fact that climate does not depend upon latitude alone, but may be greatly modified by local conditions, especially by the distribution of land and water, and the direction of the wind.
Let us now sum up what has been said in regard to the main types of climate found in Europe. Round the Mediterranean basin we have an area with mild winters and warm summers, where the rain tends to fall during the winter season, making summer a period of drought. This climate extends beyond the limits of Europe into Northern Africa and Western Asia, and is separated from the regions of tropical climate, which have no winter and have rains at the hottest season, by a belt of desert.
The western seaboard of Europe has a maritime climate, the sea tempering the winter, but diminishing the summer heat. The prevailing winds are westerly, and the rainfall is typically abundant and distributedthroughout the year. On passing inwards this type of climate changes into the continental type, with cold winters and hot summers, and diminishing rainfall. Though precipitation occurs at all seasons of the year, it tends to be greatest in summer, giving,e. g.in parts of the Balkan States, a type eminently suited to the cereal maize, which needs more summer rain than wheat.
If we bear in mind that North America is a large continent, and Europe a very small one, and that while Europe has no eastern seaboard, it is the eastern seaboard of America which faces Europe, we may realise that the climates of North America show a remarkable analogy to the European. On the western side we have in British Columbia and California respectively the same two types of maritime climate which occur in Europe, that is, British Columbia has a mild equable climate with abundant and equally distributed rainfall, and California has a Mediterranean climate.
At the eastern side the conditions are a little different, and show us that the mere presence of the sea is not sufficient to produce a “maritime” climate. The prevailing windsin eastern North America are off the shore; they cannot therefore carry oceanic influences landwards. To the north the winds tend to have a northerly component, and cold currents of water also stream out of the Arctic and chill eastern North America. The result is that we find that Labrador, though lying in the latitude of Great Britain, has a very severe climate. Further south the conditions are of the “continental” character even on the seaboard, the winters being very cold and the summers hot. Rainfall is equally distributed throughout the year, but on passing inland it diminishes in amount and tends to be limited to the warm season. The diminution would be much more obvious than it actually is were it not that the existence of the large Gulf of Mexico, and also the size of the North American continent, give rise in the south to a monsoon effect, which greatly increases the rainfall of the south-eastern corner of the States. Further to the west, in the lee of the great barrier of the Rocky Mountains, the rainfall is slight.
Incidentally, we may notice that the eastern seaboard of the great Eurasian continent also has a more extreme climate than thewestern, offering in this respect an analogy to the conditions which prevail on the eastern and western halves of temperate North America. The cause in both cases is the same—the direction of the prevailing winds.
We cannot close this chapter without some reference to weather, a subject of more geographical importance than is generally realised. In speaking of climate we have used figures which were invariablymeans,i. e.have been obtained by averaging a great number of observations. But where a great number of mean figures are used in a discussion, it is always found that the different averages are based upon varying numbers of observations, and are therefore not strictly comparable with one another. There is always a risk that such figures may mask facts of real geographical importance. No doubt some of the difficulties will disappear with the progress of meteorological science, which will enable the geographer only to select figures which are strictly comparable. Meantime, however, observations for long periods are rare, and the meteorologist must be content to take the figures which are available. For this reason as well as for others, itis advisable to add to the somewhat abstract study of means, that is, of climate, some note upon the actual conditions, that is, upon weather.