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 markedly _discordant_, 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 term _gradin de confluence_ which 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.

[Ill.u.s.tration: 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 debris. It now usually forms a pasture or alp. The dotted line connecting CC shows the probable form of the pre-glacial valley; CD, the rocky wall of the existing U-shaped valley on whose floor the river now flows.]

a.s.sociated with the hanging valleys of Alpine 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 (see figs. 6 and 7). 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 the walk 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.

[Ill.u.s.tration: 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 or _alp_ the 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 which flow 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 debris, which makes fertile soil. The fertile soil, which is often irrigated by milky water from existing glaciers, combined with the effect of alt.i.tude 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 has a succession of steep "pitches," to use a mountaineering term, alternating with easy if wet walks across the floors of the successive cirques.

[Ill.u.s.tration: 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 that what 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 attempt to 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 the _Geographical Journal_ for 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 debris 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 old valley, 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, in _e. 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 acc.u.mulation 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 do the 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.

[Ill.u.s.tration: 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 together by 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 between the 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 pa.s.ses 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 pa.s.ses, 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 pa.s.ses there are almost innumerable ways of crossing the Alps on foot, and the presence either of Hospices or of small inns on many of the smaller pa.s.ses shows that they are constantly used at the present time, in spite of railway tunnels and carriage roads elsewhere. Even a pa.s.s relatively so difficult as the Theodule, 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 pa.s.ses, 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 that they, 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 pa.s.ses 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 pa.s.s 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 pa.s.s. Readers of Mark Twain's _A Tramp Abroad_ will remember his interesting description of the crossing of the pa.s.s, 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 short but 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 pa.s.sing 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 pa.s.s, 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 ma.s.s 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 pa.s.s. 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 pa.s.s 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 pa.s.ses are believed to have been produced in this way, and therefore we must add to the peculiarities of recently-glaciated countries, the fact that pa.s.ses 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 pa.s.ses, 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 debris. 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 often communicate with each other by pa.s.ses, 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 pa.s.sive 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 consideration of 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 pa.s.sed 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 lat.i.tudes 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 j.a.pan 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 from China, while the beautiful _Sophora j.a.ponica_, so frequently planted in towns, comes, as its name indicates, from j.a.pan, and the various species of those beautiful flowering trees known as Catalpa are either American or Asiatic. The western plane (_Plata.n.u.s 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 pa.s.sed 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 shall indicate 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.

CHAPTER IV

CLIMATE AND WEATHER

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 quant.i.tive 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 a.s.sume 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 is likely 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 term _climate_.