The marginal moraines.—Study of both the Greenland and Antarctic glaciers has shown that if we disregard the smaller and short-period migrations of the ice front, the general later movement has been a retrograde one—we live in a receding hemicycle of glaciation. The earlier Greenland glacier has now receded so as to expose large areas of the former glacier pavement. In places this pavement is largely bare, indicating a relatively rapid retirement of the ice front, but at all points at which the ice margin was halted there is now found a ridge of unassorted rock materials which were dropped by the ice as it melted ([Fig. 306]). Such ridges, composed of the unassorted materials described as till, come to have a festooned arrangement largely concentric to the ice margin, and are the marginal or terminal moraines (see [Fig. 336], [p. 312]). Marginal moraines, if of large dimensions, usually have a hummocky surface, and are apt to be composed of rock fragments of a wide range of size from rock flour (clay) to large bowlders ([plate 17 A]), which may represent many types since they have been plucked by the glacier or gathered in at its surface from many widely separated localities.
Fig. 307.—Small lake impounded between the ice front and a moraine which it has recently built. Greenland (after von Drygalski).
As the glacier front retires from the moraine which it has built up, the water which emerges from beneath the ice is impounded behind the new dam so as to form a lake of crescentic outline ([Fig. 307]). Such lakes are particularly short-lived, for the reason that the water finds an outlet over the lowest point in the crest of the moraine and easily cuts a gorge through the loose materials, thus draining the lake ([Fig. 308]). Thereafter, the escaping water flows in a braided stream across the late lake bottom and thence at the bottom of the gorge through the moraine.
Fig. 308.—View of a drained lake bottom between the moraine-covered ice front in the foreground and an abandoned marginal moraine in the middle distance. The water flows from the ice front in a braided stream and passes out through the moraine in a narrow gorge. Variegated glacier, Alaska (after Lawrence Martin).
The outwash plain or apron.—The water which descends from the glacier surface in the glacier wells or mills, eventually arrives at the bottom, where it follows a sinuous course within a tunnel melted out in the ice. Much of this water may issue at the ice front beneath the coarse rock materials which are found there, and so be discovered with the ear rather than by the eye. The water within the tunnels not flowing with a free surface but being confined as though it were in a pipe, may, however, reach the glacier margin under a hydrostatic pressure sufficient to carry it up rising grades. Inasmuch as it is heavily charged with rock débris and is suddenly checked upon arriving at the front it deposits its burden about the ice margin so as to build up plains of assorted sands and gravels, and over this surface it flows in ever shifting serpentine channels of braided type ([Fig. 308]). Such plains of glacier outwash are described as outwash plains or outwash aprons.
Rising as it does under hydrostatic pressure the water issuing at the glacier front may find its way upward in some of the crevasses and so emerge at a level considerably above the glacial floor. It may thus come about that the outwash plain is built up about the nose of the glacier so as partially to bury it from sight. When now the ice front begins a rapid retirement, a depression or fosse ([Fig. 309] and [Fig. 339], [p. 314]) is left behind the outwash plain and in front of the moraine which is built up at the next halting place.
Fig. 309.—Diagrams to show the manner of formation and the structure of an outwash plain, and the position of the fosse between this and the moraine.