In addition to the visible streams upon the surface of the Greenland ice, there are others which flow beneath and can be heard by putting the ear to the surface. All surface streams eventually encounter the marginal crevasses and plunge down in foaming cascades, producing the well known “glacier wells” or “glacier mills.” The progress of the water is now throughout in tunnels within the ice until it again makes its appearance at the glacier margin.
FIG. 306.—Marginal moraine now forming at the edge of Greenland inland ice, showing a smooth rock pavement outside it. A small lake with a partial covering of lake ice occupies a hollow of this pavement (after von Drygalski).]
=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.
