Note 224 — Chapter III · (continued 3)

The snow drifts into the ravines and accumulates to incredible depths, and the water resulting from its dissolution and from the deluging rains which fall in spring, and sometimes in the summer, being confined by rocky walls on both sides, rises to a very great height, and of course acquires an immense velocity and transporting power in its rapid descent to its outlet from the mountain. In the winter of 1842—'3, the valley of the Doveria, along which the Simplon road passes, was filled with solid snowdrifts to the depth of a hundred feet above the carriage road, and the sledge track by which passengers and the mails were carried ran at that height.

Other things being equal, the transporting power of the water is greatest where its flow is most rapid. This is usually in the direction of the axis of the ravine. As the current pours out of the gorge and escapes from the lateral confinement of its walls, it spreads and divides itself into numerous smaller streams, which shoot out from the mouth of the valley, as from a centre, in different directions, like the ribs of a fan from the pivot, each carrying with it its quota of stones and gravel. The plain below the point of issue from the mountain is rapidly raised by newly formed torrents, the elevation depending on the inclination of the bed and the form and weight of the matter transported. Every flood both increases the height of this central point and extends the entire circumference of the deposit. The stream retaining most nearly the original direction moves with the greatest momentum, and consequently transports the solid matter with which it is charged to the greatest distance.

The untravelled reader will comprehend this the better when he is informed that the southern slope of the Alps generally rises suddenly out of the plain, with no intervening hill to break the abruptness of the transition, except those consisting of comparatively small heaps of its own debris brought down by ancient glaciers or recent torrents. The torrents do not wind down valleys gradually widening to the rivers or the sea, but leap at once from the flanks of the mountains upon the plains below. This arrangement of surfaces naturally facilitates the formation of vast deposits at their points of emergence, and the centre of the accumulation in the case of very small torrents is not unfrequently a hundred feet high, and sometimes very much more.

Torrents and the rivers that receive them transport mountain debris to almost incredible distances. Lorentz, in an official report on this subject, as quoted by Marschand from the Memoirs of the Agricultural Society of Lyons, says: "The felling of the woods produces torrents which cover the cultivated soil with pebbles and fragments of rock, and they do not confine their ravages to the vicinity of the mountains, but extend them into the fertile fields of Provence and other departments, to the distance of forty or fifty leagues."—Entwaldung der Gebirge, p. 17.