Glacier drainage. The terminal moraine is commonly breached by a considerable stream, which issues from beneath the ice by a tunnel whose portal has been enlarged to a beautiful archway by melting in the sun and the warm air ([Fig. 107]). The stream is gray with silt and loaded with sand and gravel washed from the ground moraine. “Glacier milk” the Swiss call this muddy water, the gray color of whose silt proves it rock flour freshly ground by the ice from the unoxidized sound rock of its bed, the mud of streams being yellowish when it is washed from the oxidized mantle of waste. Since glacial streams are well loaded with waste due to vigorous ice erosion, the valley in front of the glacier is commonly aggraded to a broad, flat floor. These outwash deposits are known as valley drift.
Fig. 106. Heavy Moraine about the Terminus of a Glacier in the Rocky Mountains of Canada
Account for the fact that the morainic ridge rises considerably above the surface of the ice
The sand brought out by streams from beneath a glacier differs from river sand in that it consists of freshly broken angular grains. Why?
The stream derives its water chiefly from the surface melting of the glacier. As the ice is touched by the rays of the morning sun in summer, water gathers in pools, and rills trickle and unite in brooklets which melt and cut shallow channels in the blue ice. The course of these streams is short. Soon they plunge into deep wells cut by their whirling waters where some crevasse has begun to open across their path. These wells lead into chambers and tunnels by which sooner or later their waters find way to the rock floor of the valley and there unite in a subglacial stream.
Fig. 107. Subglacial Stream Issuing from Tunnel in the Ice, Norway
The lower limit of glaciers. The glaciers of a region do not by any means end at a uniform height above sea level. Each terminates where its supply is balanced by melting. Those therefore which are fed by the largest and deepest névés and those also which are best protected from the sun by a northward exposure or by the depth of their inclosing valleys flow to lower levels than those whose supply is less and whose exposure to the sun is greater.