Читать книгу The Desert World - Arthur Mangin - Страница 9

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THE Dunes form the extreme line of the Brittany coast for nearly two hundred miles, from the Adour to the Garonne. They are hills of white sand, as fine and soft as if it had been sifted through an hour-glass. Their outline, therefore, changes every hour. When the wind blows from the land, millions of tons of sand are hourly driven into the sea, to be washed up again on the beach and blown inland by the first Biscay gale. A water hurricane from the west will fill up with sand square miles of shallow lake, driving the displaced waters into the interior, dispersing them in shining pools among the “murmurous pines,” flooding and frequently destroying the scattered hamlets of the people, and inundating their fields of rye and millet.[11]

A FLOOD IN BRITTANY.

Their origin is due to the prevalence of the sea-winds on those points of the coast which are not protected by rock and cliff, and whose slopes of sand descend very gradually to the margin of the waves. Their formation is easily explained. The sand of which they are composed is a silicious material, reduced to minute grains, generally rounded, by trituration. These grains, nevertheless, are often too big and too heavy for the wind to take them up and scatter them afar, like the dust of the highways or the ashes of volcanoes. But at low tide the sand, dried by the sun’s rays and the action of the wind, offers to the latter a sufficient holdfast to be dragged up the slopes which descend seaward, and deposited at a certain distance. This process being constantly repeated, the heaps are daily increasing in dimensions.

It will easily be understood that this accumulation along the shore cannot have taken place where the force and direction of the sands experience periodical or capricious changes; for then the sands cast upon the beach by the winds of the north and west would be driven back into the sea by the winds of the south and east. This is noticeable in many places where the nature of the coast is favourable for the production of such a phenomenon. But on other shores—as on the Atlantic littoral of France—the winds which blow most frequently and most violently are from the west and south-west. And it is there we encounter the Dunes. Those of Gascony are by far the most remarkable. Northward, they extend as far as the Point de Grave, which shuts in the mouth of the Gironde; southward, to the bank of the Adour, and even further, to the cliffs of Béarn. Here the basin of Arcachon constitutes one vast hollow; and some openings exist, moreover, in the department of Landes, between that basin and the Adour, for the overflow of the waters which descend from the interior. To the north and south of the Teste de Buch the chain of sand-hills measures from 4400 to 6600 feet in width. At other points it is still wider; but it narrows towards its extremities, and both at the Point de Grave and near Bayonne does not exceed 450 yards.

Owing to their extreme shiftiness of soil, the Dunes can attain no considerable elevation. The sand deposited by the wind on the summit of the hill is always in a state of precarious equilibrium. It has a constant tendency to be precipitated down the other side; and the higher the summit the greater is this tendency, so that there comes at last a moment when no further accumulation in height is possible. The Dune may then extend its basis, may even increase twofold in dimensions, but it no longer rises.

Let us note, moreover, that owing to its density the sand cannot be carried even by the most violent winds into the higher regions of the atmosphere; and that the Dunes, when they have reached a certain elevation, oppose to them an insuperable obstacle. This circumstance would consequently have a salutary effect, and the accumulation of sand would be determined by a law of its own, if the Dunes, once formed, had time to cohere. But this is not the case. Incessantly does the wind undo or modify its work; and the loftiest hills being the most exposed to its violence, are quickly reduced to the common level. In general, the greatest elevation of the Dunes corresponds to their greatest breadth. Thus the culminating point of those of Gascony is found in the belt situated between the lakes of Cazau and Biscarosse, where the chain is from 7500 to 9000 yards across. Their average height is 180 feet to 200 feet above the sea-level; but some of the hills in the forest of Biscarosse attain an altitude of 320 feet. In the neighbourhood of the mouths of the Gironde and the Adour, where the chain is considerably narrowed, the height of the Dunes is only thirty to forty-five feet.

The reader must not suppose that the Dunes consist of a single series of sand-hills ranged along the shore. He will, however, have conjectured, from our statements respecting their width, that they really compose a chain of several more or less regular ridges. The hills are separated from one another by valleys, locally named laites or lettes. These valleys, where the pluvial waters flow and accumulate, exhibit a striking contrast, in their freshly-blooming verdure, to the naked, barren Dunes. The general aspect of the landscape may, therefore, be compared to that of the ocean. There is the same broken surface, the same extent of undulation, the billows of sand being upheaved by the wind like the billows of the sea, and sharing in their mobility. You must see, says a writer, in order to form an idea of those colossal masses of fine sand, which the wind incessantly skims, and which travel in this way towards the inland country: you must see their contours so softened that they look like mountains of plaster of Paris polished by the workman’s hand, and their surface so mobile that a little insect leaves upon it a conspicuous track; their slopes, at every degree of inclination; their everlasting sterility—not a blade of grass, not an atom of vegetation; their solitude, less imposing than that of the mountains, but still of a truly savage character. You must see, from the summit of one of these ridges, the ocean on your right hand, and on your left the extensive lakes which border the littoral; and, in the midst of this tumultuous sea of tawny sand, green grassy valleys, rich and fertile pastures, smiling oases of verdure, where herds of horses graze, and cows half-wild, guarded by shepherds scarcely less wild than they.[12]

The marked characteristic of the Dunes, as we have already said, is their mobility, which renders them a constant menace for the neighbouring populations. To the wind which creates them they owe their frequent changes and their inland movement. While the sea eats into the coast, assisted by the breezes which gradually sweep clear the ground before it, the Dunes extend, and drive before them the shallow lakes: these in their turn encroach upon the Landes, and until now man has been constrained to recoil, step by step, before his threefold enemy. It is in this phenomenon, rather than in the ungrateful soil of the Landes, that we must seek the cause of the curse which has seemed so long to rest upon this country-side. You must go back some twenty centuries to trace the origin of the Dunes of Gascony. Fourteen or fifteen hundred years ago the coast north of the Adour was inhabited, and comparatively flourishing. Mimizan was then a town and a sea-port, from which were exported the resinous products of the neighbouring forests. The Normans disembarked there on several occasions. Under its walls, in 506, was fought a great battle between the allied Goths and Ostrogoths on the one side, and the Béarnais, commanded by a bishop of Lescar, on the other. Both town and port to-day are buried under the sands. “Full fathom five” lie church and convent, and the busy street, the noisy mart, and the once peaceful home. The present village has nearly perished: the Dune was not three yards from the church when its progress was recently arrested. Other cities, laid down in old charts of the country, but of which not a trace remains, have in this manner disappeared, and entire forests have been ingulfed, now under the sands of the Dunes, now under the sands and waves of the sea.

Some parts of the chain have been rendered to a great extent immovable by the vegetation which has gradually covered them, and these have opposed a formidable obstacle to the encroachments of the sands. Yet here and there the barrier has been defied. For example, in the forest of Biscarosse the movable Dunes, actually sweeping over the ancient hills, have not only filled up the valleys, but ingulfed a great number of pines, and raised themselves several yards above the crest of the oldest trees, planted on the summit of the highest hills.

In whose favour, in this struggle of science against the elements, will the victory eventually be decided? The question is one which the future alone can resolve.[13]