GLACIER WORK. Glacier action at present is confined to high mountains and high latitudes. There are two classes, valley, or alpine, and con tinental. Of the latter Greenland and the Ant arctic furnish illustrations; and during the Gla cial period (q.v.) continental glaciers covered Northwestern Europe and Northeastern America.
Hence glacial action assumes wider importance than it would if considered solely from the standpoint of the work of present glaciers. The erosive action of glaciers seems to be very great where the ice movement is free along vallqs. The weight of the ice, pressing its grinding tools on the under rock and slowly dragging over it, grooves and polishes the rock and deepens as well as broadens the valleys. The results of this work are readily seen in a region from which vigorous ice action has disappeared. The ma terial dragged along by the ice is a mixture of large and small rock fragments in various stages of reduction by the grinding process. At the ice front, or when the ice melts away, this ma terial is released and, falling to the ground, accumulates as an unassorted mixture of mate rials, because the ice carried large and small fragments with equal facility. This glacier de posit is known as till or boulder clay. If the ice front stands long enough along a single line the accumulation of ice-borne debris forms a moraine. The melting of the ice releases much
water along the front and this water assorts a portion of the till, causing clay deposits in one place, sand and gravel in other places. By the glacier-borne floods large quantities of rock frag ments are carried far away from the ice front and deposited in the river valleys, and even borne to sea. Where glaciers enter the sea there is a direct contribution of material to the ocean; and by means of the icebergs which break from the glaciers some of the rock fragments are carried far to sea. The deposits made directly by the ice, and by water supplied by ice melting, cover Northeastern America and Northwestern Eu rope, forming the soil of those regions. These glacial deposits vary greatly in form and in texture according to the exact nature of the formation; and they vary also in depth. Many important effects have been produced by these deposits, especially on the drainage. The great number of lakes in Europe and America are mostly due to some form of glacial interference with drainage; and the gorges and waterfalls are due to the turning aside of streams by glacial de posits. See GLACIER; BOULDER CLAY; etc.