[Illustration: Fig. 99. Longitudinal Section of a Portion of a Glacier, showing Traverse Crevasses] [Illustration: Fig. 100. Map view of Marginal Crevasses] [Illustration: Fig. 101. The Rhone Glacier, showing Radial Crevasses, the Alps] [Illustration: Fig. 102. Map View of the Junction of Two Branches of a Glacier The moraines are represented by broken lines] =Lateral and medial moraines.= The surface of a glacier is striped lengthwise by long dark bands of rock debris. Those in the center are called the medial moraines. The one on either margin is a lateral moraine, and is clearly formed of waste which has fallen on the edge of the ice from the valley slopes. A medial moraine cannot be formed in this way, since no rock fragments can fall so far out from the sides. But following it up the glacial stream, one finds that a medial moraine takes its beginning at the junction of the glacier and some tributary and is formed by the union of their two adjacent lateral moraines (Fig. 102). Each branch thus adds a medial moraine, and by counting the number of medial moraines of a trunk stream one may learn of how many branches it is composed. [Illustration: Fig. 103. Cross Section of a Glacier showing Lateral Moraines _l_, _l_, and Medial Moraines _m_, _m_] Surface moraines appear in the lower course of the glacier as ridges, which may reach the exceptional height of one hundred feet. The bulk of such a ridge is ice. It has been protected from the sun by the veneer of moraine stuff; while the glacier surface on either side has melted down at least the distance of the height of the ridge. In summer the lowering of the glacial surface by melting goes on rapidly. In Swiss glaciers it has been estimated that the average lowering of the surface by melting and evaporation amounts to ten feet a year. As a moraine ridge grows higher and more steep by the lowering of the surface of the surrounding ice, the stones of its cover tend to slip down its sides. Thus moraines broaden, until near the terminus of a glacier they may coalesce in a wide field of stony waste. [Illustration: Fig. 104. Glacier with Medial Moraines, the Alps Is the ice moving from or towards the observer?] =Englacial drift.= This name is applied to whatever debris is carried within the glacier. It consists of rock waste fallen on the névé and there buried by accumulations of snow, and of that engulfed in the glacier where crevasses have opened beneath a surface moraine. As the surface of the glacier is lowered by melting, more or less englacial drift is brought again to open air, and near the terminus it may help to bury the ice from view beneath a sheet of debris.