[Illustration: Fig. 109. Map of Malaspina Glacier, Alaska] A trunk glacier has four medial moraines. Of how many tributaries is it composed? Illustrate by diagram. State all the evidences which you have found that glaciers move. If a glacier melts back with occasional pauses up a valley, what records are left of its retreat? [Illustration: Fig. 110. Outwash Plain, the Delta of the Yahtse River, Alaska] Piedmont Glaciers =The Malaspina glacier.= Piedmont (foot of the mountain) glaciers are, as the name implies, ice fields formed at the foot of mountains by the confluence of valley glaciers. The Malaspina glacier of Alaska, the typical glacier of this kind, is seventy miles wide and stretches for thirty miles from the foot of the Mount Saint Elias range to the shore of the Pacific Ocean. The valley glaciers which unite and spread to form this lake of ice lie above the snow line and their moraines are concealed beneath névé. The central area of the Malaspina is also free from debris; but on the outer edge large quantities of englacial drift are exposed by surface melting and form a belt of morainic waste a few feet thick and several miles wide, covered in part with a luxuriant forest, beneath which the ice is in places one thousand feet in depth. The glacier here is practically stagnant, and lakes a few hundred yards across, which could not exist were the ice in motion and broken with crevasses, gather on their beds sorted waste from the moraine. The streams which drain the glacier have cut their courses in englacial and subglacial tunnels; none flow for any distance on the surface. The largest, the Yahtse River, issues from a high archway in the ice,--a muddy torrent one hundred feet wide and twenty feet deep, loaded with sand and stones which it deposits in a broad outwash plain (Fig. 110). Where the ice has retreated from the sea there is left a hummocky drift sheet with hollows filled with lakelets. These deposits help to explain similar hummocky regions of drift and similar plains of coarse, water-laid material often found in the drift-covered area of the northeastern United States. The Geological Work Of Glacier Ice The sluggish glacier must do its work in a different way from the agile river. The mountain stream is swift and small, and its channel occupies but a small portion of the valley. The glacier is slow and big; its rate of motion may be less than a millionth of that of running water over the same declivity, and its bulk is proportionately large and fills the valley to great depth. Moreover, glacier ice is a solid body plastic under slowly applied stresses, while the water of rivers is a nimble fluid.