[Illustration: +Fig.+ 267.—Diagram to illustrate the taking of débris from a hill-top. It also illustrates how englacial débris may become superglacial as the result of surface ablation.] [Illustration: +Fig.+ 268.—Taking débris from a protuberance of the bed.] Superglacial débris may obviously become englacial by falling into crevasses or by being carried down by descending waters. Either superglacial or englacial débris may become basal by the same means. From their form and position, there is less ice-free land in immediate association with ice-caps than with valley glaciers. Furthermore, the ice-free land about the borders of an ice-cap is less likely to be in the form of cliffs above it. As a result, the surfaces of ice-caps are comparatively clean, except at their edges where the ice is thin. [Illustration: +Fig.+ 269.—Side view of end of a glacier on the south side of Olriks Bay, North Greenland.] [Illustration: +Fig.+ 270.—Closer view of a part of the ice shown in Fig. 269.] Englacial material may become superglacial by surface ablation. In this case the drift does not rise, but melting brings the surface of the ice down to its level. This occurs chiefly at the end or edge of the ice, where the surface melting is greatest. Englacial débris, especially that near the bottom, may also become basal by the melting of the bottom of the ice. Englacial material plucked or rasped from an elevation over which the ice has passed is liable to be disposed in a longitudinal belt in the ice in the lee of the elevation itself. By surface ablation this material may reach the surface at some point below its source, and be disposed as a medial moraine. Such a moraine has an origin very different from that of a medial moraine formed by the junction of two lateral moraines of superglacial origin. Much less in the natural order of things is the transfer of material from a basal to an englacial and from an englacial to a superglacial position by upward movement of the débris itself. Such transfer is remarkable because the specific gravity of rock is from two and a half to three times as great as that of ice, so that its normal tendency is to sink. [Illustration: +Fig.+ 271.—Surface terminal moraines due to upturning. Edge of the ice-sheet, North Greenland.] In arctic glaciers, and probably in others, some material which has been basal becomes englacial by being sheared forward over ice in front of it. So far as observed this takes place chiefly where the ice in front of the plane of shearing lies at a lower level than that behind, as where the surface of an upland falls off into a valley, or where a boss of rock shelters the ice in its lee from the thrust of the overriding ice (Fig. 268). [Illustration: +Fig.+ 272.—Diagram illustrating the upturning of the layers of ice at the end of an arctic glacier as seen in end-section. The bottom line represents sea level.]