Читать книгу The Romance of Modern Geology. Describing in simple but exact language the making of the earth with some account of prehistoric animal life онлайн

47 страница из 54

But when the snow has taken the form of a glacier its record is left in more unmistakable characters. Imagine a great mass of snow and ice descending between the clefts of mountains to lower levels. As it slips slowly down its valley, like a very slow river—slower than a river of thickest mud or pitch or lava would be—earth, sand, mud, gravel, boulders, and masses of rock sometimes are washed down on its surface from the slopes on either side. Avalanches will occasionally bring other contributions. Nearly all this rubbish accumulates on the edges of the glacier nearest the slopes, and it is slowly borne to the journey's end on the glacier's shoulders. Some of it falls into rents or crevasses in the ice, and may be imprisoned there and carried down as an inside passenger, or it may reach the rocky floor over which the ice is sliding. Its progress then resembles that of the Irish gentleman who was travelling in a Sedan chair out of which the seat and the bottom had fallen, and who said that if 'twere not for the fashion of the thing he'd as lief walk. The rubbish borne onward on the surface of the glacier is known as moraine-stuff, and the mounds of it at the edge of the glacier are called lateral moraines. Where two glaciers unite like two rivers, their moraines, right-hand and left-hand, will join, and in the new glacier a new moraine will appear running down the middle, and so called a medial moraine. Where a glacier has many tributaries bearing a good deal of moraine-stuff, its surface may be like a bare plain so covered with stones that the ice beneath can hardly be discerned except here and there. At the end of the glacier where the ice melts, the heaps of stones, ever adding to their numbers, are deposited in heaps, to which are given the name of the terminal moraine.