Fig. 422.—Fragments of rock of different sizes, to bring out their different effects upon the melting of the glacier surface.
At the foot of the glacier the relatively angular rock débris, which has been carried upon the surface, and the soled and polished englacial material from near the bottom, are alike deposited in a common marginal ridge known as the terminal or end moraine ([plate 21 B]).
Plate 21.
A. View of the Harvard Glacier, Alaska, showing the characteristic terraces (after U. S. Grant).
B. The terminal moraine at the foot of a mountain glacier (after George Kinney).
Selective melting upon the glacier surface.—The white surface of the glacier generally reflects a large proportion of the sun’s rays which reach it, and its more rapid melting is largely accomplished through the agency of rock fragments spread upon its surface. Such fragments, however, promote or retard the melting process in inverse proportion to their size up to a certain limit, and above that size their action is always to protect the glacier from the sun. This nice adjustment to the size of the rock fragments will be clear from examination of [Fig. 422], for rock is a poor conductor of heat, and in even the longest summer day a thin outer layer only is appreciably warmed. Large rock blocks, grouped in the medial and lateral moraines, hold back the process of lowering the glacier surface during the summer, so that late in the season these moraines stand fifty feet or more above the glacier as armored ice ridges.