The surface of the glacier does not long retain the shining whiteness of the snow from which it is derived. It is flanked by mountains which are washed by rain, dislocated by frost, riven by lightning, traversed by avalanches, and swept by storms. The lighter debris is scattered by the winds far and wide over the glacier, sullying the purity of its surface. Loose shingle rattles at intervals down the sides of the mountains, and falls upon the ice where it touches the rocks. Large rocks are continually let loose, which come jumping from ledge to ledge, the cohesion of some being proof against the shocks which they experience; while others, when they hit the rocks, burst like bomb-shells, and shower their fragments upon the ice. [Sidenote: LATERAL MORAINES.] Thus the glacier is incessantly loaded along its borders with the ruins of the mountains which limit it; and it is evident that the quantity of rock and rubbish thus cast upon the glacier depends upon the character of the adjacent mountains. Where the summits are bare and friable, we may expect copious showers; where they are resistant, and particularly where they are protected by a covering of ice and snow, the quantity will be small. As the glacier moves downward, it carries with it the load deposited upon it. Long ridges of debris thus flank the glacier, and these ridges are called _lateral moraines_. Where two tributary glaciers join to form a trunk-glacier, their adjacent lateral moraines are laid side by side at the place of confluence, thus constituting a ridge which runs along the middle of the trunk-glacier, and which is called a _medial moraine_. The rocks and debris carried down by the glacier are finally deposited at its lower extremity, forming there a _terminal moraine_. [Sidenote: MEDIAL AND TERMINAL MORAINES.] It need hardly be stated that the number of medial moraines is only limited by the number of branch glaciers. If a glacier have but two branches, it will have only one medial moraine; if it have three branches, it will have two medial moraines; if _n_ branches, it will have _n_-1 medial moraines. The number of medial moraines, in short, is always _one less_ than the number of branches. A glance at the annexed figure will reveal the manner in which the lateral moraines of the Mer de Glace unite to form medial ones. (See Fig. 19.) [Illustration: MORAINES OF THE MER DE GLACE. Fig. 19. _To face p. 264_.]